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Weather Station Hardware => Davis Instruments Weather Stations => Topic started by: WheatonRon on March 24, 2016, 02:20:37 PM
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I realize this topic has been covered periodically in this forum, but I thought a new topic was warranted since spring is arriving and the rains too! I had an 11 year old Davis Vantage Pro 2 that was showing its age, including reporting very unreliable rain amounts. I replaced the ISS (Davis Part 6322) a few weeks ago with a brand new one I purchased from Scaled Instruments that I thought would fix everything. A storm has just concluded leaving a significant rain fall. The Davis gauge shows .93 in the last 24 hours whereas my CoCoRaHS gauge sitting about 50 feet away on the same fence at the same level from the ground shows 1.18. That is a big difference--I would expect a minor difference, say, .01-.07, but .25? There are no obstructions above either gauge. Anybody have a similar issue with a new ISS from Davis or can explain such a large difference?
See my later post (May 19, 2016) in this thread where I discuss that my VP2 got completely clogged with over 2 inches of rain "stuck" in the rain collector!
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I have my 2 year old VP2 & CoCoRaHS gauge (I spelled it right for once) mounted on the same post.
My rainfall is generally only off by a few tenths. It was much more with my old VP2 as time went by.
Is your VP2 bucket level with no obstructions in the cone? Being level makes a big difference with them.
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There are no obstructions in the rain collector. The new ISS has a level indicator built it--a nice feature, I guess I should check that, but I don't think it is far from being perfect.
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Unfortunately one of the downfalls of tippers is that they tend to under report during heavier downpours. Just the nature of the beast. I would stick with your cocorahs for your "official" measurement.
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I thought NOAA et al used tippers for the most part, no? My $500+ Vantage Pro 2 station rain fall measurements are less accurate than a $30 plastic gauge? I guess I won't be using a saying I have used for years, you get what you pay for in life!
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Cool glad you got a CoCoRaHS Ron. I think many are a little surprised when they do. I recommend everyone have a 4" Cocorahs if for nothing else, its a great backup for storm of the century when everything fails.
What I did was adjust the data accordingly. I've had perfect calibrations on tipping buckets, sometimes I'm dead on with the Cocorahs and other times 15-25% off. Rainfall rate has a lot to do with it, the higher rate the more loss with tipping bucket. Its the nature of the beast, some are going to the 8" Rainwise that seems to work better but I can honestly say haven't done a full 1 year comparison like I did with the Davis.
Damn CW are you reading my mind :eek:, I typed all this out so posting anyway. :lol:
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Thanks Jim. I thought NOAA et al used tippers for the most part, no?
Honestly, I'm not sure what ASOS's use for the truly official measurement.
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ASOS uses a software correction is my understanding. Some of you smart guys out need to write something for the Davis... :idea:
https://mesonet.agron.iastate.edu/ASOS/precipnote.phtml
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In the article.
The ASOS uses a tipping bucket method to measure precipitation. This means that a single tip of the bucket records 0.01 inch of precipitation with the data logger. Under intense rainfall rates, various physical things happen causing the tipping mechanism to not be able to keep up with the flowing water rate. The ASOS Users Manual notes in section 3.4.2 that a correction is applied to the measured accumulation on a minute by minute basis. The equation shown in the manual is as follows:
Well, straight from the horse's mouth. ;)
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WMO tested 19 gauges:
The results of the Intercomparison showed that the tipping-bucket rain gauges that were equipped with proper correction software provided good quality rainfall intensity measurements. The gauges where no correction was applied had larger errors. Here is a graph showing final results. The gauges hugging the 0-line were software corrected.
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If I read this right, tipping bucket gauges will always be less reliable than CoCoRaHS type gauges unless corrected by software. Do you or anyone else (Davis support maybe) know whether Davis' Weatherlink software makes such corrections before uploads to CWOP or WU? My guess is no in that the console shows the same measurements that exists in Weatherlink--unless such correction is made in the data logger, but I doubt it.
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I don't know of any correction software for Davis or any PWS software for that matter.
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If I read this right, tipping bucket gauges will always be less reliable than CoCoRaHS type gauges unless corrected by software.
Always? No. Most of the time? Probably. If the rain rate is in the wheel house of your tippers calibration sweet spot, then it should be about as accurate as Mr. Coco.
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Well, think about it a minute... if your tipping bucket tips at .01, you'll always be .0099 Lower than what you might compare it to. :twisted: You're not plaing with a 'full spoon'. And if it stops raining, and evaporates, you've lost .0099 " of Precip. This could allow a layer of volcanic ash or meteorite dust to accrete in the spoon, it'll tip "faster" :twisted: If a bird parks his butt on the CCR gauge, you'll miss what runs off his tail...
If you've had an itzy bitsy critter lay some silk on the tipping pivot, it'll 'reset' slower, and be "less than"...
Yeah, miniscule... silly. But you're working apples and oranges here, Man with two watches never know what time it really is.
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Well, think about it a minute... if your tipping bucket tips at .01, you'll always be .0099 Lower than what you might compare it to. :twisted: You're not plaing with a 'full spoon'. And if it stops raining, and evaporates, you've lost .0099 " of Precip. This could allow a layer of volcanic ash or meteorite dust to accrete in the spoon, it'll tip "faster" :twisted: If a bird parks his butt on the CCR gauge, you'll miss what runs off his tail...
If you've had an itzy bitsy critter lay some silk on the tipping pivot, it'll 'reset' slower, and be "less than"...
Yeah, miniscule... silly. But you're working apples and oranges here, Man with two watches never know what time it really is.
I hear ya and get your point, all the time. Many strive to be as accurate as the equipment will allow and I painstakingly am one of them. Tippers will never be as accurate over the long haul, but are "usually" better than a sharp stick in the eye.
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At the end of the day, it is unfortunate to say a $30 rain gauge is more accurate than a $500+ Davis station! When I first got into weather as a hobby a decade or so ago, I thought spending the dollars on a Vantage Pro 2 would put me at the top of enthusiasts. Now I must conclude that the rain measurements being reported by my Pro 2 are only "directionally correct" and if rainfall is a critical measurement (generally not) I should rely on Mr. CoCo!
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At the end of the day, it is unfortunate to say a $30 rain gauge is more accurate than a $500 Davis station! When I first got into weather as a hobby a decade or so ago, I thought spending the dollars on a Vantage Pro 2 would put me at the top of enthusiasts. Now I must conclude that the rain measurements being reported by my Pro 2 are only "directionally correct"!
You'd be really hard pressed to find a temp/hum sensor as accurate as what Davis brings to the table that can TX up 1000' and can run perpetually by itself, and then report to you on your couch every 3 seconds at most while you enjoy your popcorn, for thirty bucks.
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I think it would be interesting to conduct a personal study using that correction equation, or some variation of it, to compare between the VP2 gauge and the CoCoRaHS gauge to see what the results are. With this research, perhaps some software developers could allow a correction equation to be used for rainfall accumulations.
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Great idea! Maybe that is why Davis has been slow (as molasses) in releasing the long anticipated Vantage Pro 3!
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:shock: Maybe! \:D/
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At the end of the day, it is unfortunate to say a $30 rain gauge is more accurate than a $500+ Davis station!
You are, essentially, comparing apples and oranges. The Stratus gauge is a static, volumetric device. It relies on the manufacturing tolerances of the molds that make it, levelness, and the mounting location. The Davis gauge is a dynamic, mass based device. The weight of the water tips it, not the volume. It relies on the levelness of the base, the adjustment of the calibration screws, the friction and wear of the pivot points and axle, and the cleanliness of the tippers (and probably something else I have missed).
Comparing them together requires that they both be mounted at the same elevation and as close to each other as possible. Mine are within about 9" of each other on opposite sides of a 6x6 post. And my Stratus sometimes disagrees with my copper NWS Standard Gauge, too.
If you really want to drive yourself nuts, deploy a few 4" Stratus gauges over an area of, say 100-200 feet and see how well they match storm to storm. :)
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Well put, boss.
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I think we have completely covered the pros and cons in this thread. I learned a lot and am better for this knowledge and most importantly, I still sleep reasonably well at night! Thanks to all who contributed to this topic. As the anchors on NBC news used to say, Goodnight Chet, Goodnight David! Happy Easter to all!
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I was thinking about an accurate software. You would need to know where each gauge was calibrated and come up with some scale which varies according to rain/rate and for best accuracy make changes on the fly like every minute.
What I've done is calibrate my Texas Electronic to 3" per hour rate. Example just the other day, the snowfall was much slower rate than the 3" calibrated so I ended up with .51, but actual moisture was .48 so +.03 over actual. The software would correct this and vise versa if the rain/rate was higher make a correction.
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I realize this topic has been covered periodically in this forum, but I thought a new topic was warranted since spring is arriving and the rains too! I had an 11 year old Davis Vantage Pro 2 that was showing its age, including reporting very unreliable rain amounts. I replaced the ISS (Davis Part 6322) a few weeks ago with a brand new one I purchased from Scaled Instruments that I thought would fix everything. A storm has just concluded leaving a significant rain fall. The Davis gauge shows .93 in the last 24 hours whereas my CoCoRaHS gauge sitting about 50 feet away on the same fence at the same level from the ground shows 1.18. That is a big difference--I would expect a minor difference, say, .01-.07, but .25? There are no obstructions above either gauge. Anybody have a similar issue with a new ISS from Davis or can explain such a large difference?
Am surprised that no one directly commented on the above highlighted comment in WheatonRon's original post. Having two gauges 50-feet apart could and most likely will result in having significant differences in rainfall readings. My CoCoRaHS and VP2 gauges are about five feet apart and I normally see differences of about .02 to .05 inches during a storm. If the wind is blowing hard or we have heavy rainfall rates, then the difference is generally greater. I just live with it and don't fuss about the variances. It is what it is...
Here in the high desert area of northern Nevada we're just happy to have precipitation, no matter how much, or little, it is!
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The wind was not really a factor. I will check to ensure the VP2 is level and if so, I will try another heavy rain and see what happens.
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Agree with Don in that the best comparison is having the gauges as close as possible. Preferably back to back.
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Being close is good but careful you don't get splash from one gauge to the other.
The more I've calibrated TB's with a calibrator the more I understand the limitations of tipping buckets. The best accuracy is within the sweet zone of calibration. Outside this range the error factor increases without software correction.
The siphon type TB handles higher rain rates better with smaller error factors even with 20" per /hr rates but they don't do well with slower rates <1" per hour. My understanding you won't get a first tip until .02 on the siphon gauge which is unacceptable for me and has reports of being erratic at slow rates.
Realistically the best rain gauge is the manual, preferably the US standard 8" diameter but the 4" will do being it only runs $35. Unfortunately they don't plug into the Davis ISS. :-(
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Don't feel bad, I got the same manual gauge a few weeks ago and my VP2 is not even close. The Davis is under reporting rain by 70% to 80%, this is with at least 0.15" of rain each day I compared. All the obvious things are not the problem, I just think it wasn't properly calibrated when leaving the factory, adjustment screws were basically screwed all the way down.
I've adjusted the screws, got the buckets tipping with an equal amount (roughly), and the last rain was Davis 0.18", manual 0.20", so it's a lot closer than it was. Going to rain tonight/tomorrow so I'll see how it does after that last reading.
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As I discussed in a separate thread, my 11 year old ISS was significantly under reporting rain fall. As a result, I purchased a new ISS and although the rain fall measurement is much better with the new ISS, it still seems off and may not be calibrated right. I checked whether the station is level or not and it is. Hopefully another storm will indicate the issue was isolated and not a calibration problem. That sounds hard to fix.
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As I discussed in a separate thread, my 11 year old ISS was significantly under reporting rain fall. As a result, I purchased a new ISS and although the rain fall measurement is much better with the new ISS, it still seems off and may not be calibrated right. I checked whether the station is level or not and it is. Hopefully another storm will indicate the issue was isolated and not a calibration problem. That sounds hard to fix.
It's been a somewhat easy fix for me, yesterday/today the rain total was Davis 0.16, manual 0.17.
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Good job Scotty, surprised the gauge was sent out without the stops adjusted.
The Novalynx 6" gauge buckets are designed to tip 0.01" of water = 4.63 ml calibrated at 4" per hr rate. Using eye dropper you can adjust the stops and come close. The Davis tippers if gauge is true 6" should be very close.
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Just had a two day snow event and had the heater in the VP2 rain collector on the whole time. Friday into Saturday was 6.6" of snow, Saturday into Sunday was 7.1". Measurements were made at 7:00 am for CoCoRaHS. Following are the SWEs.
Day CCRHS VP2 Snow
Sat 7:00 0.70" 0.63" 6.6"
Sun 7:00 0.59" 0.06" 7.1"
Mark
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Just had a two day snow event and had the heater in the VP2 rain collector on the whole time. Friday into Saturday was 6.6" of snow, Saturday into Sunday was 7.1". Measurements were made at 7:00 am for CoCoRaHS. Following are the SWEs.
Day CCRHS VP2 Snow
Sat 7:00 0.70" 0.63" 6.6"
Sun 7:00 0.59" 0.06" 7.1"
Mark
Ouch! that .06 hurts... :-(
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Just had a two day snow event and had the heater in the VP2 rain collector on the whole time. Friday into Saturday was 6.6" of snow, Saturday into Sunday was 7.1". Measurements were made at 7:00 am for CoCoRaHS. Following are the SWEs.
Day CCRHS VP2 Snow
Sat 7:00 0.70" 0.63" 6.6"
Sun 7:00 0.59" 0.06" 7.1"
Mark
Please confirm that Sunday was not a typo! Surely, you meant .60 and not .06? If truly only .06, Davis should announce a recall!
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The heater seems to evaporate snowfall that is light. I have seen something similar with mine. If the snowfall is heavy, and the heater can keep up with it, then the error isn't as bad.
I have toyed with the idea of using some kind of proportional control with a low PWM duty cycle for light snow conditions.
When I first built my heater (a virtual "clone" of the Davis one), I had a 60°F bi-metal thermostat on the heater board. I might go back to it, since I seemed to have better results for the light snow conditions. I currently have a much higher temperature control inside, closer to the Davis one. This would be an easier thing to do than devising a new control system. I also bought a 75°F control which I haven't tried yet.
Greg H.
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The heater seems to evaporate snowfall that is light. I have seen something similar with mine. If the snowfall is heavy, and the heater can keep up with it, then the error isn't as bad.
Greg H.
Even my fancy heater control in the Texas Electronics struggles with dry light snowfall. Nature of the beast I've decided. The only reliable way is manually.
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Okay, I jumped the gun. :oops: I had no reason to believe anything had changed over night since we had had minimal wind. Just shoveled the deck so I could get to rain collector and it was about a third full of water. ](*,) ](*,)Debris filter gone and opening plugged. Removed part of the plug and now have registered 0.28" for last hour and gauge is empty. I am sure the reading is low, as it registered a rate of over 10"/hr. Need to get a new debris filter. I think I will just cover the rain collector next winter and rely on my CCRHS readings.
Mark
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Figured that I would post my problem in this thread.
I purchased a Davis Vantage Pro 2 in early October as most reports indicated it was a superior package.
The rain measurements have never been right. Typically 50--65% off other weather stations in my area.
I decided to take on the problem after winter and the last snow. I have not purchased a CoCoRaHS rain collector yet to compare.
I have found numerous reports of problems with rain measurements on different web portals with the Davis Vantage Pro 2.
So I called Davis last week and was informed to try the following prior to sending back for possible repairs.
Did everything that was suggested.
Further I did the slow drip of a 1/4 cup of water that was suggested. The clicks that I heard, did match what was reported on the console.
Anyone else ever have this issue with a new station and reporting rain.
Troubleshooting steps:
1 Make sure there is no debris in the Rain Collector cone, preventing water from draining through to the tipping buckets.
2 Try pouring a small amount of water (1/4 cup) into the rain collector cone very slowly (should take at least 30 seconds). Listen for and count the number of times you hear the tipping buckets tip. You should see .01 in. of rain in the Daily Rain value on the console display for each tip of the buckets that you counted.
3 Try changing the Station ID # to something other than the default of Station # 1. Remove the solar panel cover from the transmitter. Locate the 4 DIP switches on the circuit board to the right of the battery chamber – almost to the right edge of the circuit board. Using a ballpoint pen or similar small pointed object, flip the # 1 DIP switch Up/On. You will now be transmitting as Station # 5.
3a Put the console in the Setup mode (DONE button and minus ( - ) button at the same time. You should see RECEIVING FROM… at the bottom of the display screen. Wait until you see the number 5 appear above the line where it says Station No. and a slowly blinking X in the lower right-hand corner of the screen. It can take anywhere from a few seconds to several minutes to establish communication between the transmitter and the console/receiver. Once you see the X and the number 5, press and release the DONE button quickly. Don’t hold it at all. Just push it and let it go. You should see the number 1 above the line, by itself and ON (ISS) in the lower left-hand corner. Turn that OFF by pressing and releasing either the plus or minus button. Press and release the right arrow ( > ) button to get to Station No. 2. You should see the number 2 above the line and OFF below the line. Continue with the right arrow button, making sure that 3 & 4 both say OFF. When you get to number 5, it will say OFF. Turn that on by pressing and releasing the plus or minus button. It should say ON (ISS) below the line. Check 6,7 and 8 and make sure they all say OFF below the line. Press and release the DONE button quickly again and make sure RETRANSMIT is OFF.
That will eliminate the possibility that you may be picking up someone else’s station data.
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If you look closely you will see me solution.
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So I was in the huge Texas drenching last night.
I have found that my CoCoRaHs gauge measured 9.65 inches, while my VP2 measured 7.49 inches.
We had a couple hours of 1.6 to 2 inches an hour according to the VP2, but Mr CoCo can be compared well to other CoCos in the area.
I am happy the CoCoRaHS gauge is in my back yard
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So I was in the huge Texas drenching last night.
I have found that my CoCoRaHs gauge measured 9.65 inches, while my VP2 measured 7.49 inches.
We had a couple hours of 1.6 to 2 inches an hour according to the VP2, but Mr CoCo can be compared well to other CoCos in the area.
I am happy the CoCoRaHS gauge is in my back yard
Hope your okay, that's a lot of rain. Daughter and brother in the Houston area also.
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Thanks, we are doing well.
Shocked the kids slept through it all!
I was upset that my internet went down too, so no logging on CWOP or WU
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Try the cell service. My daughters okay also, haven't been able to speak with brother yet.
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I did switch to cell, but since my internet was down, Meteobridge would not send my data to the web.
Do you have any good ways to tackle that?
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The rain measurements have never been right. Typically 50--65% off other weather stations in my area.
I decided to take on the problem after winter and the last snow. I have not purchased a CoCoRaHS rain collector yet to compare.
You may have some local condition affecting your readings. Until you purchase a CoCoRaHS (or similar) and mount it directly next to the VP2, you're guessing.
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If you had phone tethering you could probably do it. Or a hot spot service possibly. Not 100% with meteobridge however.
I did switch to cell, but since my internet was down, Meteobridge would not send my data to the web.
Do you have any good ways to tackle that?
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I started this thread several weeks ago hoping my new VP2 ISS would more closely reflect the rainfall in my Mr. CoCo gauge. Since that time, very little rain fell here and very few additional posts were made to this thread. Last night we had a very gentle rain before midnight. I reported .23 in Mr. CoCo and .14 in the VP2. Early this morning we had a torrent of rain in a short period. Mr. CoCo reported .70 and the VP2 reported .45 during that time frame. Clearly, in heavy downpours, the tipper technology under reports rain. But I am surprised that during very light rain there was a fairly significant difference too. Mr. CoCo and the VP2 were near each other, level, and at the same height. Davis support provided me the technique to calibrate the gauge--determine error factor over three storms, average them, then turn the two screws at either end of the two buckets one full counterclockwise turn for each 6% error factor for under reporting rain. Has anybody calibrated his or her gauge using this technique and having success? Maybe a better answer as suggested previously in this thread is to purchase an add on gauge from Rainwise.
Sent from my iPad using Tapatalk
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Considering your Davis is under reporting by about 40% (in both light and heavy rain) you have to turn the screws 6 turns. Is there even that much adjustment available?
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Six turns -- exactly what Davis support told me, and yes, the screws purportedly are long enough enabling this adjustment with "room to spare." But I find that tweaking a bit harsh for a rain gauge manufactured by Davis in January 2016, and on my fence only about a month!
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From Experience :?, I would urge you not to adjust more than ONE (1) turn, then retest. I followed that method (think I made 3 turns) and got my VP2 way off in the other direction. Took nearly a year of back and forth but finally have mine consistently within about 1 - 2% difference to adjacent Stratus gauge.
Good Luck,
George
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Good suggestion. I thought 6 turns on an instrument made just a few months ago sounded too drastic.
Sent from my iPad using Tapatalk
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Quick update on my post #37.
We have had 2 very light rainfall events the past week in NW NJ.
So far, my reporting seems to be in the accepted zone of 4 weather stations in a 1/2 to 2 mile sq mile radius.
I have been spot on with one station and slightly below with a 2 others by a few tenths of an inch of qpf. Positive step imop.
Cleaned the entire station out this afternoon as the forecast is for around a 1/2 inch of rain late tonight through tomorrow.
I did notice that the bubble level on the ISS set up is 1/2 way in and out of the circle.
Also, I have yet to purchase the CoCoRaHS rain set up to compare.
More news after this event.
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The results are in with the most recent event.
My station recorded 0.13 inches of rain.
All other stations in the area were 0.33 to a high of 0.43 inches of rain.
At this point, i am frustrated as I paid decent money for the set up.
Not sure what my plan is exactly. Sorta of just want to toss it in the trash. :-x
I assume if I call Davis again, they will tell me that their systems rarely fail and rain totals vary across areas.
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My station is also under reporting rain when compared with Mr. CoCo. I will be trying to adjust my station by turning the screws under the two tipper buckets. Davis told me 1 counterclockwise turn for each 6% under reporting error. As suggested by George Richardson above, probably the best approach, which I am going to follow, is try one full turn and see what happens in the next few rainfalls. However, you really do need a reliable nearby source (a few feet away at most) to compare to and Mr. CoCo is perfect for that.
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I assume if I call Davis again, they will tell me that their systems rarely fail and rain totals vary across areas.
Don't know about the "rarely fail" part, but the "rain totals vary" statement is completely correct. You might as well compare high wind gust while you're at it.
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Also, I have yet to purchase the CoCoRaHS rain set up to compare.
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Using a medicine or eye dropper you can adjust the stops. The Davis 6" diameter should be close to Novalynx 6" which tips at 0.01" of water = 4.63 ml. If you fill dropper to 10 ml. and slowly drip into tipper it should tip at 5.73 water left in dropper.
This needs to be checked with gauge as level as possible.
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Using a medicine or eye dropper you can adjust the stops. The Davis 6" diameter should be close to Novalynx 6" which tips at 0.01" of water = 4.63 ml. If you fill dropper to 10 ml. and slowly drip into tipper it should tip at 5.73 water left in dropper.
This needs to be checked with gauge as level as possible.
Randy, the Davis is 6.5" for 5.44ml per tip.
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Using a medicine or eye dropper you can adjust the stops. The Davis 6" diameter should be close to Novalynx 6" which tips at 0.01" of water = 4.63 ml. If you fill dropper to 10 ml. and slowly drip into tipper it should tip at 5.73 water left in dropper.
This needs to be checked with gauge as level as possible.
Randy, the Davis is 6.5" for 5.44ml per tip.
(Thanks) Don't know why I was thinking 6". Same thing its an easy way to see how far off the stops are but use the 5.44ml. Someone reported a new unit came in unadjusted not long ago.
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Well I got my CoCo gauge mounted on a 4x4 treated pole yesterday. Pole is 24" in the ground. Top of gauge is 2' above ground and 5" above the pole which was cut at a 45 degree angle. This was just in time for a brief, single cloud, shower that came through.
CoCo said .05" , Davis said .02" Not off to a good start...
The CoCo is about 40 feet from the Davis currently, so this data is probably meaningless but there it is.
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My station is also under reporting rain when compared with Mr. CoCo. I will be trying to adjust my station by turning the screws under the two tipper buckets. Davis told me 1 counterclockwise turn for each 6% under reporting error. As suggested by George Richardson above, probably the best approach, which I am going to follow, is try one full turn and see what happens in the next few rainfalls. However, you really do need a reliable nearby source (a few feet away at most) to compare to and Mr. CoCo is perfect for that.
Curious if your adjustments screws were all the way down from the factory ?
I posted about mine before, was under-reporting 40%-60%, lost count of how many turns I made to get it adjusted. After a few adjustments I've got it very close to the manual gauge, within 0.01".
The other thing you have to do is get the buckets tipping with an equal amount of water, the buckets are asymmetrically weighted so one screw will be a little higher than the other when properly adjusted.
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Haven't got around to opening the ISS again to tweak the tippers yet. Will let you know what happens. Glad you were able to your rainfall gauge accurate, Scotty! But this surprises me. My new ISS was manufactured in January, four months ago. Why should we be having to recalibrate so soon? Damage in transit? :mad:
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At the end of the day, it is unfortunate to say a $30 rain gauge is more accurate than a $500+ Davis station! When I first got into weather as a hobby a decade or so ago, I thought spending the dollars on a Vantage Pro 2 would put me at the top of enthusiasts. Now I must conclude that the rain measurements being reported by my Pro 2 are only "directionally correct" and if rainfall is a critical measurement (generally not) I should rely on Mr. CoCo!
You are not paying for accuracy as much as you are paying for not having to go out in the middle of a torrential downpour to see how much it has/is raining, empty gauge, make recordings in your waterproof journal with your waterproof pen, etc.
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For the money that Davis charges they could get the darn tipping bucket calibrated properly BEFORE it gets into a consumers hands! I have two Davis rain gauges. One part of the 6163 ISS and a separate stand alone unit that has yet to be mounted. Once I get my pole installed and move the Davis ISS to that, AND get the separate Davis rain gauge mounted at the CoCoRaHS recommended height of 2', I will be able to do some comparisons between the two new Davis gauges, one at 5', and one at 2'. Those readings will be compared to Mr. CoCo. If they are off by a lot, (I understand about all the variables) then Davis is going to re-calibrate them, not me.
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I have been following this thread for quite a while and yesterday had a huge (for my location) precipitation event, actually an all-time record. It started at noon with enough pea sized hail to turn the ground white, then turned to rain. I fully expected a large difference between my VP2 and COCO rain collectors. This morning when I took my COCO reading it was 0.82 inches. I then checked the VP2 readings for the last 24 hour period and it was also 0.82 inches. The high rain rate for yesterday was 1.35 "/hr. I don't know why the two are identical, but maybe because the event started with a significant amount of hail that slowed the flow of heavy rain enough to get good readings from the tipping buckets. It is also possible that the COCO did not catch as much of the hail. I really don't know why, but they were identical.
Mark
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Once I get my pole installed and move the Davis ISS to that, AND get the separate Davis rain gauge mounted at the CoCoRaHS recommended height of 2', I will be able to do some comparisons between the two new Davis gauges, one at 5', and one at 2'. Those readings will be compared to Mr. CoCo.
We are all looking forward to your carefully-controlled comparisons.
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I replaced my 11 year old VP2 with a new ISS a few weeks ago. Subsequently, my location had a significant rain fall while I was away for a few days. Before leaving, I quickly looked at the rain collector and it was apparently clear. While away, I looked at my CWOP rainfall amounts and noted they were way off from nearby airports and fellow nearby reliable CWOP contributors. I was off by over 2 inches when I returned. Upon inspection, the rain collector was full of water. My guess it came from Davis with a partially blocked collector hole and some unknown substance completed the task of plugging the collector. Moral of the story? Ensure your new rain collector is clear of "stuff" including ensuring Davis original specs are present! For the rain collector, clean your new ISS with a pipe cleaner before relying on it!!!
This issue may have contributed to the difference I was experiencing (before this event), of under reporting rainfall when compared with my CoCo gauge discussed previously in this thread.
:-)
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Well hello from rain drenched North Houston.
I wanted to give you update on the COCO vs Davis rain debate.
Thursday 7am-Friday 7am
COCO - 7.28 inches of rain
Davis - 6.40 inches of rain
12.1% off
Friday 7am to Friday 8pm
COCO - 4.90 inches of rain
Davis - 4.03 inches of rain
17% off
So i know the argument, A man with two watches never know the time. However, this rain gauges are situated within 5 ft of each other with little difference in exposure to rain.
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Interesting. We had a bunch of rain this week and my results are as follows:
CoCo. 1.80 inches
DVP2. 2.10
Subsequent rainfall
Coco. .20
DVP2 .31
I think my VP2 was loose on my pole which may have resulted in more rain than my
Mr. CoCo. Will see after tonight after I tightened the pole supporting my PWS. Bottom line, there will always be differences, but they should be less than significant. In my case, the differences were more than significant.
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I have a Rainwise setup at one of my stations it follows the Cocorahs very closely.
Calibration means so much on these tipping bucket gauges. The Rainwise came calibrated at 5" per hour rate when I tested, instead of the 1" per hr. rate some tipping buckets are factory set.
My Texas Electronics was set at the 1" per hour rate and worked great for snow in the winter but can't keep up in a thunderstorm so I calibrated gauge at 3" per hr. It recorded 4.31" and the 8" standard was 4.32" during our record rain last week. The Cocorahs came up short at 4.21". This storm included hail which I'm sure bounced out of the Cocorahs.
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Agree. Since my VP2 ISS is brand new, I hope the difference can be attributed to me not properly securing my PWS to my fence post. I should not have to recalibrate a new ISS fresh from Davis via Scaled Instruments! I hope I can get the two gauges reasonably close in rainfall totals before I die. Which Rainwise gauge do you use?
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Agree. Since my VP2 ISS is brand new, I hope the difference can be attributed to me not properly securing my PWS to my fence post. I should not have to recalibrate a new ISS fresh from Davis via Scaled Instruments! I hope I can get the two gauges reasonably close in rainfall totals before I die. Which Rainwise gauge do you use?
This little jewel. Very well made I just remove the counter leave enough wire attached to counter for using for calibrator if needed and splice into the existing line from Davis gauge using same plug or get a new one if you don't want to cut into it. Believe those are 4 wire plugs.
http://www.rainwise.com/products/detail.php?ID=6697&Category=Rain_Gauges:Wired&pageNum_cart=/products/index.php
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Agree. Since my VP2 ISS is brand new, I hope the difference can be attributed to me not properly securing my PWS to my fence post. I should not have to recalibrate a new ISS fresh from Davis via Scaled Instruments! I hope I can get the two gauges reasonably close in rainfall totals before I die. Which Rainwise gauge do you use?
This little jewel. Very well made I just remove the counter leave enough wire attached to counter for using for calibrator if needed and splice into the existing line from Davis gauge using same plug or get a new one if you don't want to cut into it. Believe those are 4 wire plugs.
http://www.rainwise.com/products/detail.php?ID=6697&Category=Rain_Gauges:Wired&pageNum_cart=/products/index.php
Ron, this is the exact same tipper I told you about 2 months ago in a PM. I have it wired as above and reports just as the Davis would.
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Thanks to both you (Jim) and Randy. Since the Blackhawks lost to St. Louis a few weeks ago, I have not been the same retired CPA as before. Now I must cheer for the PENS and wait til next year, but hopefully my rain gauge issue will be resolved by then. Fortunately, I am also a big Denver Broncos fan and they won the Super Bowl this year, so I am not totally wasted!
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The results are in with the most recent event.
My station recorded 0.13 inches of rain.
All other stations in the area were 0.33 to a high of 0.43 inches of rain.
At this point, i am frustrated as I paid decent money for the set up.
Not sure what my plan is exactly. Sorta of just want to toss it in the trash. :-x
I assume if I call Davis again, they will tell me that their systems rarely fail and rain totals vary across areas.
Update.
I bought a new tripod for the ISS/rain collector. More money for the hobby 8-)
My rain readings have improved drastically. Very minor events in NW NJ recently with precip. I have not hit the jackpot yet locally with rain, but the past 3 events have been in line with other weather stations in a 1-7 sq mile radius.No other station in the area is a Davis model. Gut check tells me that my ISS/rain collector was not level on the post that it was installed on at first.
I have been using a construction level to verify prior to the most recent events that the set up is confirmed level.
True test comes Sunday night into Monday. Rain is forecasted to be between a 1/2 inch to locally 2 with a tropical system that is expected to impact the east coast. Biggest rain event in months.
New problem is that I am getting plant stuff in the Davis rain collector cone.
I have the debris screen, but still getting particles falling into the cone. Some particles physically breach or clog the rain entry point.
I have been cleaning it every other day. Dust and other stuff seem to be taking hold on the tipping scale or collecting particles inside etc.
Do you men/women keep the shield in during a rain event? Wondering if this impact totals.
Plan to buy the CoCoRaHS rain collector next week to dig deep in the data.
Best.
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New problem is that I am getting plant stuff in the Davis rain collector cone.
I have the debris screen, but still getting particles falling into the cone. Some particles physically breach or clog the rain entry point.
I have been cleaning it every other day. Dust and other stuff seem to be taking hold on the tipping scale or collecting particles inside etc.
Do you men/women keep the shield in during a rain event? Wondering if this impact totals.
If you have the Davis screen, many have changed to something different like discussed here http://www.wxforum.net/index.php?topic=4888.msg214211;topicseen#msg214211
Paul
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This is what the recent 3 1/4 hour event looked like in rainfall rates. (4.32") total. Gives a indication how non siphon tipping bucket can struggle to keep up if calibrated, say 1" per/hr rate vs 3" per/hr rate. The faster the rate and tips the more loss between tips.
If you are calibrated at 3" rate and rain never exceeds 1" which is typical light/moderate rain you will read a few tips high vs reading a lot low when the rates reach 4-7" per/hr even if not steady over extended time period.
So basically non siphon tipping buckets aren't one size fits all.
Siphon tipping buckets are more accurate at above the 2" per/hr rate less accurate below this rate where many rain events occur in my area. This is why I don't own one.
The best solution is software correction like the NWS ASOS where it adjusts on speed of tips unfortunately the software I use doesn't have this correction either. ](*,)
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I am over reporting rain in my DVP2. 1.05 in the Davis and .65 in Mr. CoCo. It has to be an ISS level issue. I watched the console and in very light rain, the gauge went from .73 to .75. So the tipper must have gone back and forth so quickly that .74 never showed up. I believe the ISS transmits to the console every few seconds instead of real time thus supporting my hypothesis. Now I will do my best to get the ISS level to remedy the problem. Other than a level tool, any suggestions? Maybe after 11 years I have periodically put too much pressure on the pole and maybe the pole has bent.
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Its really easy to get early tips when mounted on a pole at 6+feet, when the slightest movement will cause bucket to tip about 3/4 full. The fact is all rain gauges and especially something like a tipping bucket needs a solid non moving mount about 3-4' off the ground. If its calibrated correctly and level accuracy can be within 1-4% of actual rainfall consistently.
My favorite for this is the stand alone gauges like the Rainwise I mentioned earlier.
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Well, as suggested by Jim and Randy above, I purchased a Rainwise 111 tipping raingauge (8 inches in diameter). While it is early in the game, I now have 3 rain measurement tools none of which currently agree with each other. The VP2, Mr. CoCo, and now the Rainwise. In the latest rain event, the VP2 had .18, Mr. CoCo had .12 and the Rainwise had .16. While directionally the same, all different. The rainfall was a slow, non windy event. Oh well, solving this puzzle may take a better part of the Summer, but at least we are having fun (I think) and our spouses don't complain except for the money being spent on new Weather toys--first a new ISS, then a daytime fan, then a SHT31, and now another tipping gauge that confirms neither Mr. CoCo nor my VP2 are "correct"! Any other suggestions for toys to try? Seriously, I believe the Rainwise will be more accurate over time than the VP2 and plan to use it to calibrate the VP2, or as suggested by Jim, replace the VP2 gauge with the Rainwise!
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I just came across this discussion and figured I'd weigh in. I've been using Davis Instruments products ever since I purchased a Weather Monitor II over 20 years ago. I never knew how far off the tipping bucket measurements were until ~10 years ago when I mounted a CoCoRaHs gauge on the opposite side of the pole from my Vantage Pro2 tipping bucket. I was surprised to discover how much the Davis was under-reporting. While I recognize that a tipping bucket can never be calibrated to match the CoCo in all situations, I do think it can be adjusted such that the average error over time is minimal.
I start by calibrating the tipping bucket to a known volume of water. This gets it close and ensures both sides are equally balanced. Based on my calculations, 5.4377 cc should result in one tip of the bucket. However, it's pretty tough to measure a small volume of water to a fraction of a cc. I choose to use either 54.38 cc for 0.1" or 271.89 cc for 0.5". If you don't want to stand there long enough to trickle in such a large volume of water, I discovered a very simple solution. Measure the correct volume of water and pour it into a plastic water bottle. Puncture a VERY small hole in the cap, turn the bottle upside down in the rain collector, and if necessary, poke another small hole above the water line to let air in. You can stand there and count tips with the rain collector disconnected, but with the larger volume of water you might be there awhile. Conversely, you can leave the rain collector connected so it transmits to the ISS, then walk away until the water has drained and check your console for the total. (Note: When I do it this way, I always edit the database records to remove 'false' precipitation. If you're uploading to CWOP or Weather Underground, you'll want to suspend the uploads during this process and clear the archive before restarting them. Based on the delta between the tipping bucket and the CoCo, you simply adjust the screws as necessary to account for the discrepancy (one turn for each 6%).
The next step involves comparing the two gauges over a period of time and with varying rainfall intensities. At low rainfall rates, the volumetric technique described above should result in very little error. But if you live in an area with frequent heavy thunderstorms, this technique will result in the tipping bucket under-reporting by a significant amount. I adjust mine such that the readings are fairly close during a moderate thunderstorm. This typically results in over-reporting rainfall during light rainfall events and under-reporting during heavy rainfall events. In terms of error percentage, I want the positive error during light rain to be a bit larger than the negative error during heavy rain. Why? Because my intent is for the total rainfall reported over a longer period of time to be close to correct. Here along the Gulf coast, I tend to receive more precipitation throughout the year during heavy rainfall events than I do during light rainfall events.
The bigger question here is why Davis can't do a better job of calibrating the tipping bucket to begin with. I've found them to routinely be off by at least 25%. In fact, I recently sent my ISS in for refurbishing and I had to start the calibration process all over again when it was returned. It can take quite a bit of time, and I still don't have it just right. During the recent Tax Day flood, the CoCo measured 12.4", while the Davis reported 10.87" (>12% error). I made some additional adjustments, and based on several checks since then it seems that I'm over-reporting by 2.8% (this from a mixture of light and heavy rainfall events). It looks like I need to turn 'em to the right just a bit.
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I just came across this discussion and figured I'd weigh in. I've been using Davis Instruments products ever since I purchased a Weather Monitor II over 20 years ago. I never knew how far off the tipping bucket measurements were until ~10 years ago when I mounted a CoCoRaHs gauge on the opposite side of the pole from my Vantage Pro2 tipping bucket. I was surprised to discover how much the Davis was under-reporting. While I recognize that a tipping bucket can never be calibrated to match the CoCo in all situations, I do think it can be adjusted such that the average error over time is minimal.
I start by calibrating the tipping bucket to a known volume of water. This gets it close and ensures both sides are equally balanced. Based on my calculations, 5.4377 cc should result in one tip of the bucket. However, it's pretty tough to measure a small volume of water to a fraction of a cc. I choose to use either 54.38 cc for 0.1" or 271.89 cc for 0.5". If you don't want to stand there long enough to trickle in such a large volume of water, I discovered a very simple solution. Measure the correct volume of water and pour it into a plastic water bottle. Puncture a VERY small hole in the cap, turn the bottle upside down in the rain collector, and if necessary, poke another small hole above the water line to let air in. You can stand there and count tips with the rain collector disconnected, but with the larger volume of water you might be there awhile. Conversely, you can leave the rain collector connected so it transmits to the ISS, then walk away until the water has drained and check your console for the total. (Note: When I do it this way, I always edit the database records to remove 'false' precipitation. If you're uploading to CWOP or Weather Underground, you'll want to suspend the uploads during this process and clear the archive before restarting them. Based on the delta between the tipping bucket and the CoCo, you simply adjust the screws as necessary to account for the discrepancy (one turn for each 6%).
The next step involves comparing the two gauges over a period of time and with varying rainfall intensities. At low rainfall rates, the volumetric technique described above should result in very little error. But if you live in an area with frequent heavy thunderstorms, this technique will result in the tipping bucket under-reporting by a significant amount. I adjust mine such that the readings are fairly close during a moderate thunderstorm. This typically results in over-reporting rainfall during light rainfall events and under-reporting during heavy rainfall events. In terms of error percentage, I want the positive error during light rain to be a bit larger than the negative error during heavy rain. Why? Because my intent is for the total rainfall reported over a longer period of time to be close to correct. Here along the Gulf coast, I tend to receive more precipitation throughout the year during heavy rainfall events than I do during light rainfall events.
The bigger question here is why Davis can't do a better job of calibrating the tipping bucket to begin with. I've found them to routinely be off by at least 25%. In fact, I recently sent my ISS in for refurbishing and I had to start the calibration process all over again when it was returned. It can take quite a bit of time, and I still don't have it just right. During the recent Tax Day flood, the CoCo measured 12.4", while the Davis reported 10.87" (>12% error). I made some additional adjustments, and based on several checks since then it seems that I'm over-reporting by 2.8% (this from a mixture of light and heavy rainfall events). It looks like I need to turn 'em to the right just a bit.
Great discussion, Mike. See my further issues in a separate thread, "4" CoCoRaHS loss vs. 8" NWS standard" elsewhere in this Forum. In short, I believe you are correct when Davis "calibrates" its tippers!
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So I decided to do a comparison over the past week and a half with three rain gauges. One is a VP2 gauge on the roof of my house, another one is the 4" (CoCo) on a fence post and lastly the Torrent rain gauge (metric version) which has the reading on the gauges and is located on the deck close to the 4". I also have a torrent rain gauge that reads in inches but did not use that one for this test.
Here are the numbers:
June 10
Davis 10.4 mm
4" 10.0 mm
Torrent 10.5 mm
June 11
Davis 0.2 mm
4" Tr
Torrent 0.2 mm
June 13
Davis 2.6 mm
4" 2.4 mm
Torrent 1.7 mm
June 14
Davis 0.6 mm
4" 0.8 mm
Torrent 0.8 mm
June 15
Davis 0.4 mm
4" 0.4 mm
Torrent 0.3 mm
June 18
Davis 13.2 mm
4" 12.4 mm
Torrent 13.3 mm
June 19
Davis 0.2 mm
4" 0.2 mm
Torrent 0.2 mm
Total
Davis 27.6 mm / 1.09 in.
4" 26.2 mm / 1.03 in.
Torrent 27.0 mm / 1.06 in.
Overall I am pleased with the results, I did not think it would be as close as it was. It'll be interesting when we get some even heavier rainfalls.
One thing I like about the torrent rain gauge is that it measures in increments of 0.1 mm as opposed to the other two which are 0.2 mm. The inch version of the torrent actually measures every thousandth of an inch!!
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So I decided to do a comparison over the past week and a half with three rain gauges. One is a VP2 gauge on the roof of my house, another one is the 4" (CoCo) on a fence post and lastly the Torrent rain gauge (metric version) which has the reading on the gauges and is located on the deck close to the 4". I also have a torrent rain gauge that reads in inches but did not use that one for this test.
Here are the numbers:
June 10
Davis 10.4 mm
4" 10.0 mm
Torrent 10.5 mm
June 11
Davis 0.2 mm
4" Tr
Torrent 0.2 mm
June 13
Davis 2.6 mm
4" 2.4 mm
Torrent 1.7 mm
June 14
Davis 0.6 mm
4" 0.8 mm
Torrent 0.8 mm
June 15
Davis 0.4 mm
4" 0.4 mm
Torrent 0.3 mm
June 18
Davis 13.2 mm
4" 12.4 mm
Torrent 13.3 mm
June 19
Davis 0.2 mm
4" 0.2 mm
Torrent 0.2 mm
Total
Davis 27.6 mm / 1.09 in.
4" 26.2 mm / 1.03 in.
Torrent 27.0 mm / 1.06 in.
Overall I am pleased with the results, I did not think it would be as close as it was. It'll be interesting when we get some even heavier rainfalls.
One thing I like about the torrent rain gauge is that it measures in increments of 0.1 mm as opposed to the other two which are 0.2 mm. The inch version of the torrent actually measures every thousandth of an inch!!
I would view these differences as statistically the same! Good job.
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I mounted the the CoCoRaHS Monday and finally can compare it to the VP2 tipping bucked rain gauge. Yesterday, the CoCoRaHS measured 0.70 inches rain and the VP2 0.63 inches. The rain was intermittent and light to moderate with very little wind. However the VP2 was not perfectly level during this test. It is level now though and so the next test will be interesting.
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I replaced my tipping mechanism in my new VP2 ISS this week and early returns from the first rainfall are good. Mr. CoCo had .52, my VP2 had .50 and my Rainwise had .51. Statistically the same in my view! It appears my new ISS may have been a lemon or poorly calibrated leaving the Davis factory. Need several more rains to conclude whether I can rely on my preferred choice, the VP2. :grin:
If these results stay consistent, will send original tipper back to Davis for replacement or recalibration, and then it will become my backup. Purchased both from Scaled Instruments. Working with Ryan there is great.
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I replaced my tipping mechanism in my new VP2 ISS this week and early returns from the first rainfall are good. Mr. CoCo had .52, my VP2 had .50 and my Rainwise had .51. Statistically the same in my view! It appears my new ISS my have been a lemon or poorly calibrated leaving the Davis factory. Need several more rains to conclude whether I can rely on my preferred choice, the VP2. :grin:
If these results stay consistent, will send original tipper back to Davis for replacement or recalibration, and then it will become my backup. Purchased both from Scaled Instruments. Working with Ryan there is great.
Very nice! I'm still waiting.....rain's just east of here. Gotta see how my personally calibrated RW stacks up to my new Coco!
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I have a manual gauge and two tipping gauges, the Netatmo and the Davis Pro2. The Davis is definitely under-reading, after careful scrutiny and logging results. All three gauges are at the same height and within 2' of each other, the Netatmo always reads the exact same as the manual gauge. The Davis, however, is working out to be under by 1.5mm for every 10mm of rain I record. I'm not going to go down the route of pouring water in and turning the screws to calibrate it, but I was wondering if anyone had a rough estimate of how much I should turn each screw to get it closer? If not, i'll just trial and error it..
Cheers.
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I have a manual gauge and two tipping gauges, the Netatmo and the Davis Pro2. The Davis is definitely under-reading, after careful scrutiny and logging results. All three gauges are at the same height and within 2' of each other, the Netatmo always reads the exact same as the manual gauge. The Davis, however, is working out to be under by 1.5mm for every 10mm of rain I record. I'm not going to go down the route of pouring water in and turning the screws to calibrate it, but I was wondering if anyone had a rough estimate of how much I should turn each screw to get it closer? If not, i'll just trial and error it..
Cheers.
I believe it is 6% correction for each turn. Let me look through my emails from Davis support on this to verify.
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I have a manual gauge and two tipping gauges, the Netatmo and the Davis Pro2. The Davis is definitely under-reading, after careful scrutiny and logging results. All three gauges are at the same height and within 2' of each other, the Netatmo always reads the exact same as the manual gauge. The Davis, however, is working out to be under by 1.5mm for every 10mm of rain I record. I'm not going to go down the route of pouring water in and turning the screws to calibrate it, but I was wondering if anyone had a rough estimate of how much I should turn each screw to get it closer? If not, i'll just trial and error it..
Cheers.
I believe it is 6% correction for each turn. Let me look through my emails from Davis support on this to verify.
Cheers Ron - Muchly appreciated. :)
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I started this thread several weeks ago hoping my new VP2 ISS would more closely reflect the rainfall in my Mr. CoCo gauge. Since that time, very little rain fell here and very few additional posts were made to this thread. Last night we had a very gentle rain before midnight. I reported .23 in Mr. CoCo and .14 in the VP2. Early this morning we had a torrent of rain in a short period. Mr. CoCo reported .70 and the VP2 reported .45 during that time frame. Clearly, in heavy downpours, the tipper technology under reports rain. But I am surprised that during very light rain there was a fairly significant difference too. Mr. CoCo and the VP2 were near each other, level, and at the same height. Davis support provided me the technique to calibrate the gauge--determine error factor over three storms, average them, then turn the two screws at either end of the two buckets one full counterclockwise turn for each 6% error factor for under reporting rain.
I have a manual gauge and two tipping gauges, the Netatmo and the Davis Pro2. The Davis is definitely under-reading, after careful scrutiny and logging results. All three gauges are at the same height and within 2' of each other, the Netatmo always reads the exact same as the manual gauge. The Davis, however, is working out to be under by 1.5mm for every 10mm of rain I record. I'm not going to go down the route of pouring water in and turning the screws to calibrate it, but I was wondering if anyone had a rough estimate of how much I should turn each screw to get it closer? If not, i'll just trial and error it..
Cheers.
I believe it is 6% correction for each turn. Let me look through my emails from Davis support on this to verify.
Cheers Ron - Muchly appreciated. :)
See prior post I made in this thread copied above on how to calibrate the VP2 rain tool. As noted in my post discussing the error from April, I was getting rain under reporting errors from my VP2 and on separate occasions, large over reporting errors from my VP2 so I really didn't know how to calibrate the ISS! So I decided to replace the tipping mechanism in my new ISS that I purchased in February 2016. I concluded I had a Davis lemon! I did not want to send the ISS to Davis for warranty evaluation thus requiring my PWS to be offline several weeks. So far so good with the new tipper I got this month (June 2016). I am cautiously optimistic the new tipper will fix this bizarre rain reporting issue and Davis will replace the one I removed from my new ISS. It will become a spare for future "tweaks" backups or whatever. What is rather ironic about this situation is the reason I bought a new ISS in the first place! My 11 year old VP2 ISS was showing its age and was not properly reporting rainfall correctly (10+ years of accurate rain reporting, however) so I decided to get a new one! Oh well, the daytime fan in that old ISS still works showing Davis can make parts that last, if we can just get rain properly measured again! And yes, the new SHT 31 temperature-humidity sensor is outstanding! In addition, my 11 year old VP2 consoles (yes, having two consoles in the house is most useful-one dedicated to a computer for uploading to CWOP and WU; the other console where the family can easily view what the weather is doing outside) continue to work great which is probably why Davis is not interested in a VP3--the VP2 product keeps humming along with little competition to force the issue!
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Got my first shot at seeing how my cali went with the RW compared to the Coco. At bed time both were at 0.23 :-), but woke to .24 with the RW and still .23 for the Coco. Apparently a brief shower came through earlier and tipped what was left over the RW but didn't show in the Coco. Considering I did this cali with a milk carton, I'm pleased, but obviously still more to see. Heaviest rate was 1.5" per hour.
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Got my first shot at seeing how my cali went with the RW compared to the Coco. At bed time both were at 0.23 :-), but woke to .24 with the RW and still .23 for the Coco. Apparently a brief shower came through earlier and tipped what was left over the RW but didn't show in the Coco. Considering I did this cali with a milk carton, I'm pleased, but obviously still more to see. Heaviest rate was 1.5" per hour.
Pretty good so far. There is no such thing as the perfit tipping bucket calibration as I'm sure you know without correction software.
If the rain rate falls at 7" per hour and you had the tipping buckets humming dead on at 3" per hour it will come up short. I'm happy with 0-4% error.
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BTW I'm calibrated on my Texas Electronics at 6" per hour rate this is what Novalynx sends tipping buckets out from factory calibrated at.
With the Novalynx calibrator I use the 1/16" ORIFICE and get exactly 115 tips with 946ml weighed water on the 8" diameter tipping bucket. Some companies make 200mm tipping buckets and the 8" is 203mm.
Seems to work best for my area mainly because many thunderstorms are getting into the 7-8" per/hr rate and I'm still sometimes short.
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Got my first shot at seeing how my cali went with the RW compared to the Coco. At bed time both were at 0.23 :-), but woke to .24 with the RW and still .23 for the Coco. Apparently a brief shower came through earlier and tipped what was left over the RW but didn't show in the Coco. Considering I did this cali with a milk carton, I'm pleased, but obviously still more to see. Heaviest rate was 1.5" per hour.
Pretty good so far. There is no such thing as the perfit tipping bucket calibration as I'm sure you know without correction software.
If the rain rate falls at 7" per hour and you had the tipping buckets humming dead on at 3" per hour it will come up short. I'm happy with 0-4% error.
I settled with the flow starting at about 4" per hour, then as the carton emptied, the rate was about 1" per hour at the end. It rained somewhat hard yesterday but I'll be waiting on a good toad choker to really see how they compare. BTW, when that comes, I'll have the top of the Coco off. I may of had a tiny splash-out yesterday, but doubt it.
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I'll have the top of the Coco off. I may of had a tiny splash-out yesterday, but doubt it.
Great idea, I believe it happens but with 1.5" rate you were probably ok.
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I'll have the top of the Coco off. I may of had a tiny splash-out yesterday, but doubt it.
Great idea, I believe it happens but with 1.5" rate you were probably ok.
Concur.
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but I'll be waiting on a good toad choker to really see how they compare. BTW, when that comes, I'll have the top of the Coco off. I may of had a tiny splash-out yesterday, but doubt it.
What a great saying--a toad choker! Never in my 65+ years have I heard that expression!
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but I'll be waiting on a good toad choker to really see how they compare. BTW, when that comes, I'll have the top of the Coco off. I may of had a tiny splash-out yesterday, but doubt it.
What a great saying--a toad choker! Never in my 65+ years have I heard that expression!
Ha! Been using that one for years here. ;)
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but I'll be waiting on a good toad choker to really see how they compare. BTW, when that comes, I'll have the top of the Coco off. I may of had a tiny splash-out yesterday, but doubt it.
What a great saying--a toad choker! Never in my 65+ years have I heard that expression!
Ha! Been using that one for years here. ;)
Did you use that before moving to Arizona or is that a transplanted saying from where....?
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but I'll be waiting on a good toad choker to really see how they compare. BTW, when that comes, I'll have the top of the Coco off. I may of had a tiny splash-out yesterday, but doubt it.
What a great saying--a toad choker! Never in my 65+ years have I heard that expression!
Ha! Been using that one for years here. ;)
Did you use that before moving to Arizona or is that a transplanted saying from where....?
Quite sure I picked that up when I moved here to Tucson in '88. Probably comes from the emergence of the Colorado river toad, which coincides with the monsoon, and boy do they ever "come out".
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OK, just got my toad choker! At a 7.38" max rate, which came down in just under 20 minutes, the RW and Coco both had 0.55". I'm quite tickled with myself! \:D/
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OK, just got my toad choker! At a 7.38" max rate, which came down in just under 20 minutes, the RW and Coco both had 0.55". I'm quite tickled with myself! \:D/
Good calibration for summer. 8-)
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OK, just got my toad choker! At a 7.38" max rate, which came down in just under 20 minutes, the RW and Coco both had 0.55". I'm quite tickled with myself! \:D/
Good calibration for summer. 8-)
BTW, it came on so fast I had no time to remove the top of the Coco.
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I just checked. The average monthly rainfall in Tucson during the month of June is .28. You received twice the monthly average in one day? Are you gauges smoking dope? Seriously, it goes to show you that averages, over time, will have substantial variations!
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You know with all that weed crossing the border some eventually falls off the trucks. :grin:
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I just checked. The average monthly rainfall in Tucson during the month of June is .28. You received twice the monthly average in one day? Are you gauges smoking dope? Seriously, it goes to show you that averages, over time, will have substantial variations!
Monsoon came a bit early (about two weeks) this year.
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You know with all that weed crossing the border some eventually falls off the trucks. :grin:
Yep! UU UU
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You know with all that weed crossing the border some eventually falls off the trucks. :grin:
:-"
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Does it seem odd for the Pro2 rain gauge to go from under-reading to over-reading? In the past, it had a tendency to under-read by a few mm, even worse when the debris screen was in.
We haven't had much rain in July, but what we did have was fairly accurate against my manual gauge. However, today has been a wet day, currently the Pro2 reads at 24.6mm, something niggling in me about that, made me go and check the manual gauge. It is reading 18mm, they're at exactly the same height and 1 foot apart, to me, that is too much of a gap! My Netatmo gauge is about 12 feet (same height) from the other gauges, and it is reading within half a mm of the manual gauge. I just find it odd how it can switch around that easily - not impressed!
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I have long given up on trying to take the VP2 rain measurements as 100% accurate and official for my location. It always under-reports in heavy downpours so I just use my manual guage and only thing I use my VP2 for rain-wise is the rain rate which is really cool to watch.
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Does it seem odd for the Pro2 rain gauge to go from under-reading to over-reading? In the past, it had a tendency to under-read by a few mm, even worse when the debris screen was in.
We haven't had much rain in July, but what we did have was fairly accurate against my manual gauge. However, today has been a wet day, currently the Pro2 reads at 24.6mm, something niggling in me about that, made me go and check the manual gauge. It is reading 18mm, they're at exactly the same height and 1 foot apart, to me, that is too much of a gap! My Netatmo gauge is about 12 feet (same height) from the other gauges, and it is reading within half a mm of the manual gauge. I just find it odd how it can switch around that easily - not impressed!
I would agree--too big a difference. With the gauges that close together, only significant wind could justify such a difference. I too have been having differences between my VP2 and my manual gauge (CoCo). I have replaced my VP2 tipper, still having strange differences. Posters on this thread have suggested using a Rainwise 111 tipper instead of the standard VP2 tipper. I have such a tipper but forget to put it out before a significant rainfall to compare to my other two gauges. As others have pointed out, during heavy rainfalls, the VP2 seems to always under report which some believe is due to the diameter of the collector (about 6.5 inches) vs. the Rainwise that has an 8 inch diameter collector. Not sure why 1.5 inches makes a difference, but what we pay Davis, significant rainfall differences should not happen!
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I am very happy with my Davis rain collector, except during the winter. For the month of July, my total rainfall from the Davis was 0.83" and my CoCo was 0.84". This is close enough for me. Granted, my highest rainfall rate was only 1.25".
Mark
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I am very happy with my Davis rain collector, except during the winter. ... Granted, my highest rainfall rate was only 1.25".
Mark
That is the difference I believe. Your rainfall rate was not heavy thus the VP2 can and did keep up. Have you tried buying the Davis collector heater (Davis part no. 7720) for winter use? My guess is you don't have a nearby AC outlet that would facilitate this, correct?
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Does it seem odd for the Pro2 rain gauge to go from under-reading to over-reading? In the past, it had a tendency to under-read by a few mm, even worse when the debris screen was in.
We haven't had much rain in July, but what we did have was fairly accurate against my manual gauge. However, today has been a wet day, currently the Pro2 reads at 24.6mm, something niggling in me about that, made me go and check the manual gauge. It is reading 18mm, they're at exactly the same height and 1 foot apart, to me, that is too much of a gap! My Netatmo gauge is about 12 feet (same height) from the other gauges, and it is reading within half a mm of the manual gauge. I just find it odd how it can switch around that easily - not impressed!
the VP2 seems to always under report which some believe is due to the diameter of the collector (about 6.5 inches) vs. the Rainwise that has an 8 inch diameter collector. Not sure why 1.5 inches makes a difference,
Under reporting has nothing to do with collector size, other issues are at play.
8" is better than 6.5" because the "sample" size is bigger, which equates to a generally more accurate tipper, all things being equal.
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I have the heater, but if I leave it on, it way under reports, due to evaporation of the melted snow before it makes it to the tipping buckets, I believe. If I only turn it on after a snow, it is fairly accurate but usually not on the correct day. There are other problems with this if we get a big snow and then wind, it blows away the snow that has built up above the cone and it doesn't get measured.
This winter I plan to cover the collector and manually enter my CoCo values into my weather data base.
Mark
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I have the heater, but if I leave it on, it way under reports, due to evaporation of the melted snow before it makes it to the tipping buckets, I believe. If I only turn it on after a snow, it is fairly accurate but usually not on the correct day. There are other problems with this if we get a big snow and then wind, it blows away the snow that has built up above the cone and it doesn't get measured.
This winter I plan to cover the collector and manually enter my CoCo values into my weather data base.
Mark
I am impressed! Yes, manually entering in snowfall you melt from Mr. CoCo sounds like the way to go! In the wintertime, I just let the VP2 do its thing, recognizing that the precipitation measurement will be off a few days to allow for melting. However, I regularly post Mr. CoCo readings online using the melted snow from that gauge-- I guess I am too lazy to update my rain database in my Davis Weatherlink software!
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I took the funnel off today to have a look, and the tippers were quite dirty- a lot of residue over them. This could of been a hindrance as it may have made the tippers tip faster.. I have given them a clean and now I'll have to wait for some more rain. :)
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See page 20, ASOS, 3.4.2 - Precipitation Accumulation Algorithm: http://www.nws.noaa.gov/asos/pdfs/aum-toc.pdf (http://www.nws.noaa.gov/asos/pdfs/aum-toc.pdf)
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Good information in the immediate above post. See also this thread started today.
http://www.wxforum.net/index.php?topic=30175.new;topicseen#new
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I've finally adjusted both screws on my Davis gauge. I marked them and gave them one full turn. On Saturday, we had some pretty heavy rain for the first time in ages, and it was spot on! It matched the manual gauge, just 2 feet away, and matched my other automated gauge, at the same height but 15 feet away. However, during the early hours, there was a period of very light rain and drizzle, and it has gone back to under-reading again. I'm pretty fed up of this now, the consistency for this Davis equipment is extremely poor.
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I've finally adjusted both screws on my Davis gauge. I marked them and gave them one full turn. On Saturday, we had some pretty heavy rain for the first time in ages, and it was spot on! It matched the manual gauge, just 2 feet away, and matched my other automated gauge, at the same height but 15 feet away. However, during the early hours, there was a period of very light rain and drizzle, and it has gone back to under-reading again. I'm pretty fed up of this now, the consistency for this Davis equipment is extremely poor.
I think that is why folks believe a VP3 should be issued that would include a larger rain base that would lead to more accurate reporting. In the interim, I guess we have to live with it and trust our manual gauges more! :-)
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My one Davis gauge still in service I software adjust +6% and its close to Cocorahs. Today it reported (.40 vs .41 Cocorahs).
My high end tipping bucket is more accurate but spent more time calibrating with professional equipment. Today .45 matching 2 manual gauges 8" NWS .45, Cocorahs .45.
Tipping buckets will never be dead on due to loss with tipping speed the higher the rain rate the more loss. They also suffer from light precipitation especially powdered snowfall even heated units evaporation occurs causing under reporting.
The manual NWS 8" is the standard I use as does the NWS when comparing.
Open (top off) 4" Cocorahs is just as accurate during heavy rain/hail events but due to evaporation needs measurement taken soon after rainfall event. Cocorahs with inner funnel and funnel top on tend to splash out under reporting 1-3% vs 8" during heavy rainfall events.
Cocorahs has acknowledged and mentioned in news letters this does occur its not made up. I use 2 Cocorahs gauges 1 open, 1 closed both mounted side by side the open always records more on heavy events.
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My one Davis gauge still in service I software adjust +6% and its close to Cocorahs. Today it reported (.40 vs .41 Cocorahs).
My high end tipping bucket is more accurate but spent more time calibrating with professional equipment. Today .45 matching 2 manual gauges 8" NWS .45, Cocorahs .45.
Tipping buckets will never be dead on due to loss with tipping speed the higher the rain rate the more loss. They also suffer from light precipitation especially powdered snowfall even heated units evaporation occurs causing under reporting.
The manual NWS 8" is the standard I use as does the NWS when comparing.
Open (top off) 4" Cocorahs is just as accurate during heavy rain/hail events but due to evaporation needs measurement taken soon after rainfall event. Cocorahs with inner funnel and funnel top on tend to splash out under reporting 1-3% vs 8" during heavy rainfall events.
Cocorahs has acknowledged and mentioned in news letters this does occur its not made up. I use 2 Cocorahs gauges 1 open, 1 closed both mounted side by side the open always records more on heavy events.
Oh no! You have now given me another reason to get a second CoCo gauge. That means I have two VP2s, a Rainwise 111, and with two CoCo gauges, that means I will have 5! I better not tell my wife, she may have me institionalized!
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Ron FYI the open is primarily used for heavy rainfall events if its not measured soon after it will actually read below the closed top gauge within hour or so. Amazing how fast the rain will evaporate so long prolonged rain the closed will still be the more accurate. I measure both gauges and just use the higher reading.
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I have them mounted like this...
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I have them mounted like this...
Very nice!
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Guess I was just lucky last month and so far this month.
CoCo VP2 Vue Max rate
Aug. 1.57" 1.56" 1.66" 1.96"/hr
Sept. 0.30" 0.30" 0.31" 0.76"/hr
Mark
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Guess I was just lucky last month and so far this month.
CoCo VP2 Vue
Aug. 1.57" 1.56" 1.66"
Sept. 0.30" 0.30" 0.31"
Mark
I would view these differences as statistically insignificant! Good job!
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My one Davis gauge still in service I software adjust +6% and its close to Cocorahs. Today it reported (.40 vs .41 Cocorahs).
My high end tipping bucket is more accurate but spent more time calibrating with professional equipment. Today .45 matching 2 manual gauges 8" NWS .45, Cocorahs .45.
Tipping buckets will never be dead on due to loss with tipping speed the higher the rain rate the more loss. They also suffer from light precipitation especially powdered snowfall even heated units evaporation occurs causing under reporting.
The manual NWS 8" is the standard I use as does the NWS when comparing.
Open (top off) 4" Cocorahs is just as accurate during heavy rain/hail events but due to evaporation needs measurement taken soon after rainfall event. Cocorahs with inner funnel and funnel top on tend to splash out under reporting 1-3% vs 8" during heavy rainfall events.
Cocorahs has acknowledged and mentioned in news letters this does occur its not made up. I use 2 Cocorahs gauges 1 open, 1 closed both mounted side by side the open always records more on heavy events.
Count me as schooled. I never knew that. I still have my original Coco although it is quite glazed over. Guess I should get a new one and see this for my self.
I am surprised it evaporates so quickly though. That's kind of a head scratcher. :???:
Going to fill that big tube with an inch of water today and leave it all day without the funnel and see how much evaporates.
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It's a case of accepting that i'm not going to get the consistency. The trouble with that is, my old OS WMR200 and WS-1001 held ranks nicely with my manual gauge, in heavy rain, or drizzle, this is why it is so frustrating. ](*,)
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Today we are under a red flag warning so I figure it will be a good day to check CoCo evaporation. Put my spare outer cylinder out with 1.00" of water in it and my spare inner cylinder with 1.00" of water with no funnel, right next to my CoCo gauge.
Current humidity @9:00 am 63% Temp 48.6° No clouds in sky
Mark
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Today we are under a red flag warning so I figure it will be a good day to check CoCo evaporation. Put my spare outer cylinder out with 1.00" of water in it and my spare inner cylinder with 1.00" of water with no funnel, right next to my CoCo gauge.
Be sure to post your results. Readers will be very interested!
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Had to start over with the inner cylnder :oops:, I spilled some bringing it in to weigh at 10:00. Inner cylinder now started with only 0.96". Hopefully with it less full, I won't spill again. I am using my Escali scale to weigh.
Outer cylinder lost 0.005" in the first hour. Currently 59.9 and 27% humidity
Mark
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Had to start over with the inner cylnder :oops:, I spilled some bringing it in to weigh at 10:00. Inner cylinder now started with only 0.96". Hopefully with it less full, I won't spill again. I am using my Escali scale to weigh.
Outer cylinder lost 0.005" in the first hour. Currently 59.9 and 27% humidity
Mark
I am going to test my CoCo gauge too. I am starting with .50 and will give it 24 hours and then remeasure and see how much evaporated from the gauge with the inner tube and funnel removed.
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Had to start over with the inner cylnder :oops:, I spilled some bringing it in to weigh at 10:00. Inner cylinder now started with only 0.96". Hopefully with it less full, I won't spill again. I am using my Escali scale to weigh.
Outer cylinder lost 0.005" in the first hour. Currently 59.9 and 27% humidity
Mark
I am going to test my CoCo gauge too. I am starting with .50 and will give it 24 hours and then remeasure and see how much evaporated from the gauge with the inner tube and funnel removed.
It will vary for sure depending on humidity/temp/and wind speed what rate it evaporates.
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Had to start over with the inner cylnder :oops:, I spilled some bringing it in to weigh at 10:00. Inner cylinder now started with only 0.96". Hopefully with it less full, I won't spill again. I am using my Escali scale to weigh.
Outer cylinder lost 0.005" in the first hour. Currently 59.9 and 27% humidity
Mark
I am going to test my CoCo gauge too. I am starting with .50 and will give it 24 hours and then remeasure and see how much evaporated from the gauge with the inner tube and funnel removed.
It will vary for sure depending on humidity/temp/and wind speed what rate it evaporates.
Yep, all very true.
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Had to start over with the inner cylnder :oops:, I spilled some bringing it in to weigh at 10:00. Inner cylinder now started with only 0.96". Hopefully with it less full, I won't spill again. I am using my Escali scale to weigh.
Outer cylinder lost 0.005" in the first hour. Currently 59.9 and 27% humidity
Mark
I am going to test my CoCo gauge too. I am starting with .50 and will give it 24 hours and then remeasure and see how much evaporated from the gauge with the inner tube and funnel removed.
Wow! Interesting experiment! I placed .50 of water in my CoCo gauge without the funnel and inner tube and after 20 hours evaporation brought the measurement down to .45! I had no idea. Yes it did get to 92 yesterday, but it was fairly humid (typical Chicago). Can you imagine in Arizona in mid July how much evaporation would have occurred?
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It seems that with the funnel in place, evaporation is is inhibited so some extent. We had rainfall, which I checked immediately afterwards. It was a hot sunny day, so I was curious whether it would be the same at the next morning 8AM observation. It was the same amount, some droplets had formed inside the measuring tube immediately above the water in the gauge.
Greg H.
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Here are the results of yesterday's test. Wish I had gotten the weights from 2:00 to 8:00, but had to play golf. :-) We have another fire warning day today, so I am going to run the test again, only with the funnel on and no inner cylinder.
I am very curious about this because I do numerous multi-day CoCo reports. Next week I will be leaving for at least 3 weeks. During the time I will be gone, it is likely that we will get snow and freezing overnight temperatures are a given. Based on what I saw yesterday, I am thinking I will forgo the snow capture and risk breaking the inner cylinder by leaving it in and the funnel on.
Mark
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Good test. Open cylinder .18" loss in 22 hours with bulk evaporating in first 12 hours shows how fast evaporation can occur even with a cool 70ish ° day.
Even the .025 loss in first 12 hours on closed inner cylinder shows get readings as soon as possible. Longer you wait the more likely you under report especially once the sun breaks out.
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Here are the results of yesterday's test. Wish I had gotten the weights from 2:00 to 8:00, but had to play golf. :-) We have another fire warning day today, so I am going to run the test again, only with the funnel on and no inner cylinder.
I am very curious about this because I do numerous multi-day CoCo reports. Next week I will be leaving for at least 3 weeks. During the time I will be gone, it is likely that we will get snow and freezing overnight temperatures are a given. Based on what I saw yesterday, I am thinking I will forgo the snow capture and risk breaking the inner cylinder by leaving it in and the funnel on.
Mark
Mark for using the open outer funnel while gone 3 weeks add mineral oil to help stop the evaporation.
Here is a link about it. http://cals.arizona.edu/climate/proj/rr/rg_info.htm
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Thanks Randy. I'll consider the mineral oil. I also want to clarify that the inner cylinder was open on top, but in my empty outer cylinder used for my daily CoCo report.
Mark
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Here's my results.
Put both cylinders out yesterday about noon.
Checked tonight around 6.
Duration-30 hours. Temperature both yesterday and today was about 90. Partly to mostly sunny.
DP about 70 and humidity about 70% with a light breeze.
Large tube started with .99. Ended up with .90. almost 10%
Smaller tube started with 1" and ended up with .98. So about a 2% loss.
Part of the difference was that the smaller tube was in the shade while the bigger one was in full sun.
Never expected that amount of loss from the larger tube.
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Here's my results.
Put both cylinders out yesterday about noon.
Checked tonight around 6.
Duration-30 hours. Temperature both yesterday and today was about 90. Partly to mostly sunny.
DP about 70 and humidity about 70% with a light breeze.
Large tube started with .99. Ended up with .90. almost 10%
Smaller tube started with 1" and ended up with .98. So about a 2% loss.
Part of the difference was that the smaller tube was in the shade while the bigger one was in full sun.
Never expected that amount of loss from the larger tube.
As noted above, I got a 10% evaporation loss from the large tube in about 20 hours with comparable DP, temp and humidity that you had. Moral of the story? A person will get evaporation loss when the inner tube and funnel are removed--the only question is how much.
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Put both cylinders out yesterday about noon.
Checked tonight around 6.
Duration-36 hours.
:?:
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Put both cylinders out yesterday about noon.
Checked tonight around 6.
Duration-36 hours.
:?:
oops. 30 hours. :oops:
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Put both cylinders out yesterday about noon.
Checked tonight around 6.
Duration-36 hours.
:?:
New math?
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I had another exact match between the CoCoRaHS gauge and the Davis. The common thread has been that these are rather heavy, straight-down rainfall events with little or no wind. (very light wind from the SW)
I do have a problem with a neighbor's tall shrub to the SE, that can have a rain shadow effect, but I have had lower catch on the Davis from other wind directions when it is windy.
I am becoming more convinced that the problem with the Davis rain collector is the tapered shape, rather than calibration or tipping mechanism issues in general. I know that spider webs, dirty "buckets", friction in the pivot, etc. have an effect, but we are talking about an inherent issue here.
Greg H.
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Interesting.
A straight bucket shape where the water simply drains into the hole instead of being forced into a smaller area via the tapered shape would reduce the volume somewhat.
Right?
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Yesterday's test of the outer cylinder with 1" of water and the funnel in place resulted in a loss of 0.01".
Mark
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I had another exact match between the CoCoRaHS gauge and the Davis. The common thread has been that these are rather heavy, straight-down rainfall events with little or no wind. (very light wind from the SW)
I do have a problem with a neighbor's tall shrub to the SE, that can have a rain shadow effect, but I have had lower catch on the Davis from other wind directions when it is windy.
I am becoming more convinced that the problem with the Davis rain collector is the tapered shape, rather than calibration or tipping mechanism issues in general. I know that spider webs, dirty "buckets", friction in the pivot, etc. have an effect, but we are talking about an inherent issue here.
Greg H.
Significant rainfall overnight. Total for CoCoRaHS was exactly an inch. (this doesn't happen very often...)
Wind was from the SE when rain was at the heaviest.
CoCoRaHS: 1.00"
VP2: 0.86"
NovaLynx 8" Tipping Bucket: 1.07"
The NovaLynx was 7% high and the VP2 was -14%.
Is this the VP2 cone shape, or the rain shadow? Which predominates? The local Marina VP2 recorded 0.84", which is roof-mounted in the clear of any obstructions.
One recent day with little/no wind not out of SE rain shadow matched CoCoRaHS, last night with SE wind negative discrepancy.
Greg H.
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Last 2 days rain
Davis VP2 2.89"
CoCoRaHS 2.98"
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I had another exact match between the CoCoRaHS gauge and the Davis. The common thread has been that these are rather heavy, straight-down rainfall events with little or no wind. (very light wind from the SW)
I do have a problem with a neighbor's tall shrub to the SE, that can have a rain shadow effect, but I have had lower catch on the Davis from other wind directions when it is windy.
I am becoming more convinced that the problem with the Davis rain collector is the tapered shape, rather than calibration or tipping mechanism issues in general. I know that spider webs, dirty "buckets", friction in the pivot, etc. have an effect, but we are talking about an inherent issue here.
Greg H.
Significant rainfall overnight. Total for CoCoRaHS was exactly an inch. (this doesn't happen very often...)
Wind was from the SE when rain was at the heaviest.
CoCoRaHS: 1.00"
VP2: 0.86"
NovaLynx 8" Tipping Bucket: 1.07"
The NovaLynx was 7% high and the VP2 was -14%.
Is this the VP2 cone shape, or the rain shadow? Which predominates? The local Marina VP2 recorded 0.84", which is roof-mounted in the clear of any obstructions.
One recent day with little/no wind not out of SE rain shadow matched CoCoRaHS, last night with SE wind negative discrepancy.
Greg H.
My Davis during heavy events is typically 15 to as high as 20% low compared to the manual CoCoRaHS gauge. Less so during nice steady rain with minimal wind. I'm surprised to see that your NovaLynx was higher, albeit slightly. I may have you confused with another forum member but at one time were you making comparisons with a RainWise TB? I might try adding one in the future.
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I had another exact match between the CoCoRaHS gauge and the Davis. The common thread has been that these are rather heavy, straight-down rainfall events with little or no wind. (very light wind from the SW)
I do have a problem with a neighbor's tall shrub to the SE, that can have a rain shadow effect, but I have had lower catch on the Davis from other wind directions when it is windy.
I am becoming more convinced that the problem with the Davis rain collector is the tapered shape, rather than calibration or tipping mechanism issues in general. I know that spider webs, dirty "buckets", friction in the pivot, etc. have an effect, but we are talking about an inherent issue here.
Greg H.
Significant rainfall overnight. Total for CoCoRaHS was exactly an inch. (this doesn't happen very often...)
Wind was from the SE when rain was at the heaviest.
CoCoRaHS: 1.00"
VP2: 0.86"
NovaLynx 8" Tipping Bucket: 1.07"
The NovaLynx was 7% high and the VP2 was -14%.
Is this the VP2 cone shape, or the rain shadow? Which predominates? The local Marina VP2 recorded 0.84", which is roof-mounted in the clear of any obstructions.
One recent day with little/no wind not out of SE rain shadow matched CoCoRaHS, last night with SE wind negative discrepancy.
Greg H.
My Davis during heavy events is typically 15 to as high as 20% low compared to the manual CoCoRaHS gauge. Less so during nice steady rain with minimal wind. I'm surprised to see that your NovaLynx was higher, albeit slightly. I may have you confused with another forum member but at one time were you making comparisons with a RainWise TB? I might try adding one in the future.
The Rainwise was probably ValentineWeather, I would guess.
There are two calibration targets for the NovaLynx gauge. Assuming a 946mL amount of water (measured on the approved CoCoRaHS Escali scale), these targets are either 1.15" or 1.19". I used a #72 gas orifice on my homebrew calibrator and got a consistent 1.19" over three runs. With the 1/16" orifice, I got 1.09" which is low against the 1.15" target.
Since this is so rate dependent, and I do get agreement within 0.01" of the CoCoRaHS gauge from time to time, I don't plan on changing the calibration again. I think that there are so many variables with rain rate, wind, geometry, etc. that it is impossible to get an exact match anyways.
Greg H.
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Great info. Thanks much.
I think that there are so many variables with rain rate, wind, geometry, etc. that it is impossible to get an exact match anyways.
Absolutely, especially under varying rainfall rates.
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I had another match between CoCoRaHS and the VP2 rain gauge during yesterdays brief but intense rain. (0.26")
Once again this was straight-down heavy rain that only lasted about 10 minutes.
Winds were stronger (22 mph) and from SW to NW direction.
I am now suspecting that my neighbors shrub is the main culprit, with wind to the SE. I have a small lot and I don't think that she is willing to remove the tall shrub, so I guess that I have to live with the situation.
By the way, the rain amounts for the local Marina (2,000 feet away) and my NovaLynx were 0.28" and 0.29" respectively.
At this time, I see no need to mess with the stop screws on the VP2.
Greg H.
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I have two Rainwise buckets. Way too little rain for any meaningful analysis but it is all I have to go on in this year of drought. This morning a front blew through. The Rainwise Rainew self-contained rain gauge and the Cocorahs gauge right next to it and at the same height both registered .06" of rain. I am concerned that the Rainwise Mk III bucket that I just moved from the peak of the roof to the eve only recorded .04" I hope that trend doesn't continue.
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I had to do another comparison of five different rain gauges (my other post is on page 4 i believe) after we had a day and some of heavy rain and little to no wind.
The five to compare:
VP2: rooftop
Vue: fencepost (8 feet above ground)
CoCoRaHS: lower but near Vue
Torrent: lower but also near Vue
Acurite 5 in 1: only 4 feet above grass
The rainfall amounts:
VP2: 70.4 mm (2.77 in.)
Vue: 69.1 mm (2.72 in.)
CoCo: 71.0 mm (2.80 in.)
Torrent: 80.0 mm (3.15 in.)
Acurite: 79.8 mm (3.14 in.)
A number of other tests always show the Torrent and Acurite show higher amounts during big rain events. The VP2 rooftop gauge is very close to the fence mounted Vue and CoCoRaHS which makes me happy of course. However when we had a heavy rain storm and very windy conditions last month the VP2 showed significantly less rain then the Vue and CoCoRaHS, as expected I suppose.
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I had to do another comparison of five different rain gauges (my other post is on page 4 i believe) after we had a day and some of heavy rain and little to no wind.
The five to compare:
VP2: rooftop
Vue: fencepost (8 feet above ground)
CoCoRaHS: lower but near Vue
Torrent: lower but also near Vue
Acurite 5 in 1: only 4 feet above grass
The rainfall amounts:
VP2: 70.4 mm (2.77 in.)
Vue: 69.1 mm (2.72 in.)
CoCo: 71.0 mm (2.80 in.)
Torrent: 80.0 mm (3.15 in.)
Acurite: 79.8 mm (3.14 in.)
A number of other tests always show the Torrent and Acurite show higher amounts during big rain events. The VP2 rooftop gauge is very close to the fence mounted Vue and CoCoRaHS which makes me happy of course. However when we had a heavy rain storm and very windy conditions last month the VP2 showed significantly less rain then the Vue and CoCoRaHS, as expected I suppose.
Other than your Acurite and Torrent tools, I would view these results as excellent! The differences between the CoCo gauge and the Davis products are statistically insignificant in my view. And yes, during heavy rainfalls, the tipper in the VP2 will always read less than the gold standard—the CoCo gauge—it can’t keep up with the volume of rain. Nature of the beast I suppose.