Topic: VP2 Rain Gauge way off?

[WXWIZARD]

Could you provide a link where I can purchase an 8 inch davis tipping bucket rain gauge.  My davis tipper and all the ones I could find on google search are around 6.5 inch diameter.

I have something similar in setup, 3 gauges all together properly setup with 13 hour rain, not much wind but steady rain for great gauge comparison.  This was my biggest discrepancy by far, I also keep records. Smaller rains the difference was less but I feel this is a better indication how far these gauges actually vary. Results are compared to 8" NWS style standard which also recorded the most rain, not surprisingly. 
 
Gauges:
8" Dia. manual NWS style: 3.95"
8" Dia. Davis Tipping:3.50 (-12.7%)
4" Dia. Cocorahs: 3.81 (-3.7%)

[ggsteve]

I am happy to say that after several attempts I finally solved a particularly vexing and hard-to-see spider web problem in my Rainwise gauge and it was only .01" different from my Cocorahs gauge over our 25 hour rainstorm yesterday, 3.84" to 3.85".

[ValentineWeather]

Could you provide a link where I can purchase an 8 inch davis tipping bucket rain gauge.  My davis tipper and all the ones I could find on google search are around 6.5 inch diameter.

I have something similar in setup, 3 gauges all together properly setup with 13 hour rain, not much wind but steady rain for great gauge comparison.  This was my biggest discrepancy by far, I also keep records. Smaller rains the difference was less but I feel this is a better indication how far these gauges actually vary. Results are compared to 8" NWS style standard which also recorded the most rain, not surprisingly. 
 
Gauges:
8" Dia. manual NWS style: 3.95"
6.5" Dia. Davis Tipping:3.50 (-12.7%)
4" Dia. Cocorahs: 3.81 (-3.7%)

You are correct the Davis is 6.5" diameter. Interesting I always thought they were standard 8" but the way they taper ends up being smaller.

[ValentineWeather]

I am happy to say that after several attempts I finally solved a particularly vexing and hard-to-see spider web problem in my Rainwise gauge and it was only .01" different from my Cocorahs gauge over our 25 hour rainstorm yesterday, 3.84" to 3.85".

I'm impressed.

[ocala]

Let me get this straight.   You never tinkered with the calibration on the gauge and it had been working fine until recently?

You are using a plastic rain gauge to compare the electronic one.

Try this before having too many worries,  buy another plastic rain gauge, but not of the same model .   Put it out by the other two and wait for the next rain.   compare  the three.

I would also check on the VP2, about splashing.  check for crap in the funnel that could be holding water rather than having it hit the pan.

Make sure that if you set the item in the software (calibration) that you set it to the right number, such as you have one tip being 0.1mm or you have one tip being 0.01 inc.   that is a big difference and most often the error.

Finally if you are doing everything right, Contact Davis right a way as it is under warranty?

Can you explain more about the splashing? There is nothing around it...it's pole mounted. I have 3 rain gauges of different brands they all said about the same thing.

What i don't get is it has been accurate for the past year and a half untill this big rain fall.

In the past year and a half have you had any other heavy rain events like this? If so what were the results?

No we never get that much rain that quick.

That sort of what I thought. As was mentioned in this thread heavy rains usually cause the Davis to under report. When it rains that hard in short time the bucket actually fills up a little bit, causing a steady stream to flow out. The more water in the bucket the harder the stream becomes. When the tipping gauge dumps there is still water pouring out of the hole that is not caught before the tipper goes back to the other side. That's where you begin to lose the tips that add up to your total measurement. I have actually stood next to mine during heavy rain and you can here it dump about every 3 seconds. Now add up the duration of the heavy rain and you can see how it under reports.
I did make the hole in the bucket slightly smaller to slow down the stream and during the heavy rain events  the difference was not as much as in the past.
FWIW I don't use the Davis gauge in keeping records. I use the totals from the Cocorahs gauge. Of course you could argue that gauge isn't as precise as others but that's another subject. 

[miraculon]

I'm starting to think you get what you pay for. High dollar tipping buckets cost around $350 without heaters and test results from Miraculon's Novalynx showed rain was recording in the +10% range over his Davis. I thought the Novalynx was just out of calibration but have second thoughts now. Even the 4" diameter plastic gauge under performed by 4%.

I found the NovaLynx to be within 3% or so with my home brew calibrator. I decided to leave it alone for now. Sorry about the columns. Beaker 1,2,3 are how I came to 946mL which is the amount specified in NovaLynx's calibration document.

Code: [Select]

Trial Beaker1 Beaker2 Beaker3 Last Tip Total Gauge Target Delta Percent
1 900 45 1 5.5         951.5 1.19 1.15 0.04 3%
2 900 45 1 7.5         953.5 1.19 1.15 0.04 3%
3 900 45 1 7.0         953.0 1.20 1.15 0.05 4%

Greg H.

[ValentineWeather]

I'm starting to think you get what you pay for. High dollar tipping buckets cost around $350 without heaters and test results from Miraculon's Novalynx showed rain was recording in the +10% range over his Davis. I thought the Novalynx was just out of calibration but have second thoughts now. Even the 4" diameter plastic gauge under performed by 4%.

I found the NovaLynx to be within 3% or so with my home brew calibrator. I decided to leave it alone for now. Sorry about the columns. Beaker 1,2,3 are how I came to 946mL which is the amount specified in NovaLynx's calibration document.

Code: [Select]

Trial Beaker1 Beaker2 Beaker3 Last Tip Total Gauge Target Delta Percent
1 900 45 1 5.5         951.5 1.19 1.15 0.04 3%
2 900 45 1 7.5         953.5 1.19 1.15 0.04 3%
3 900 45 1 7.0         953.0 1.20 1.15 0.05 4%

Greg H.

Thanks Greg, straying a little off subject to include other gauges and referring back to the older thread where 1.19" on Novalynx was recorded. Many if not most know the Davis tipping accuracy suffers with heavy rain. So if your calibrator is correct when you were comparing rain results with different gauges, subtracted the 3% from the 8" Novalynx results would of been 1.15.

As stated before but if you missed, my recent 4" all day rain results, became a real eye opener when I realized  even the 4" cocorahs was around 3.75 % low from standard 8", this was without any real heavy downpour where the shallow funnel taper on the 4" gauge would of suffered even more loss with splash out.
So my conclusion from my individual results, not knowing any other explanation (wind speeds were low) is the 4" is around 3-4% low with 50% smaller catch size vs 8", if you add any heavy rain with splash out under catch increases even more.
 
 (Going back to the previous thread same subject)
The 8" seems superior grabbing 50% larger catch size and doesn't suffer from splash out. My thoughts if you had a 8" standard next to the Novalynx it would of recorded around 1.15 also.
 
These were the results posted in that thread. The VP2 as we can see is really suffering with under count on heavy rain, caused by overloading the tipping buckets. The Novalynx is made to avoid this tipping bucket error but also expensive for hobbyist. The Novalynx (only wish list) with heater has +/- 1% error the 8" is around $850 with mounting base.  Then they have the bargain basement gauge discounted several hundred with +/- 2% accuracy also. 
 
CoCoRaHS Official=1.03"
Roving 4"=1.04"
VP2=0.91"
NovaLynx=1.19"

[David9723]

I'm starting to think you get what you pay for. High dollar tipping buckets cost around $350 without heaters and test results from Miraculon's Novalynx showed rain was recording in the +10% range over his Davis. I thought the Novalynx was just out of calibration but have second thoughts now. Even the 4" diameter plastic gauge under performed by 4%.

I found the NovaLynx to be within 3% or so with my home brew calibrator. I decided to leave it alone for now. Sorry about the columns. Beaker 1,2,3 are how I came to 946mL which is the amount specified in NovaLynx's calibration document.

Code: [Select]

Trial Beaker1 Beaker2 Beaker3 Last Tip Total Gauge Target Delta Percent
1 900 45 1 5.5         951.5 1.19 1.15 0.04 3%
2 900 45 1 7.5         953.5 1.19 1.15 0.04 3%
3 900 45 1 7.0         953.0 1.20 1.15 0.05 4%

Greg H.

Today we had a slow rain and both gauges where spot on!

[Mtn_man]

I have always felt my VP2 rain gauge was reading lower than actual.  My comparison is an inexpensive plastic rain gauge mounted on my deck railing. 

Other day the VP recorded 2.3" and the cheap plastic gauge was at 2.5"

[ValentineWeather]

I made the +3% adjustment on the VP2 bucket several days ago, storm total results. Correction on Cocorahs. I made a math error earlier, just caught error so re-posted.
This rain did involve wind up to 21 mph at 33'.
8" standard- 1.23
4" Cocorahs- 1.17
Davis tipping- 1.17

[WXWIZARD]

Storm totals for me.

Weather Service:  4.33"
VP2: 3.52"
Davis Stand Alone: 3.48"
Rainwise:  4.31"

All are mounted in a row at 3 feet agl except the VP2 which the rim of gauge is 5 feel agl.

My rainwise is 13 years old,  just had to replace the reed switch once in that time.  It has from day one been very close to the weather service gauge in all rain events,  from light to heavy intensity and with wind,  it has been a very impressive tipper over the years.

The davis gauges over the years are the most inconsistant gauges.  They are calibrated.  I run the NovaLynx calibrator 2 times a year.  I run the small oriface which gives around 3" per hour rate.  So how does 2 davis gauges perform so poorly compared to the weather service and rainwise?  Rainwise is within calibration as well.
 
When we calibrate the gauges the water is run thru with NO WIND effects,  which is why all my gauges are within calibration.  Now add some wind to the equation and the davis gauges perform very poorly.  So lets take a look at the design.  The weather service and rainwise gauges have straight sides,  wind screens that can be added to rain gauges have sides sloping in from top to bottom to divert wind down and away from the rim of gauges.  The davis tippers have sides that slope from bottom to top which would direct any amount of wind up to the rim and cause problems with catch more so than the straight gauge sides of the weather service or rainwise or any of the more expensive tippers,  add you can usually get a rainwise tipper cheaper than a davis.

I strongly think the big problem with davis gauges is the cone design that slopes in from bottom to top,  that would cause any amount of wind and more of the wind to be deflected up to the rim of the gauge to cause unwanted eddies which causes rain droplets to spin away from the rim and miss the gauge.

I have run the next biggest orifice with my novalynx calibrator which is about 8" an hour rate in the davis gauges and the gauge records more rain than it did the past storm and a lof of storms that have a lot less rain rate,  which is why I believe the wind combined with the design of the davis tippers sloping from bottom to top is the biggest problem.

I haven't but I bet if someone installed straight sides to the davis rain gauges they would perform a lot better 

[BigOkie]

Storm totals for me.

Weather Service:  4.33"
VP2: 3.52"
Davis Stand Alone: 3.48"
Rainwise:  4.31"

All are mounted in a row at 3 feet agl except the VP2 which the rim of gauge is 5 feel agl.

My rainwise is 13 years old,  just had to replace the reed switch once in that time.  It has from day one been very close to the weather service gauge in all rain events,  from light to heavy intensity and with wind,  it has been a very impressive tipper over the years.

The davis gauges over the years are the most inconsistant gauges.  They are calibrated.  I run the NovaLynx calibrator 2 times a year.  I run the small oriface which gives around 3" per hour rate.  So how does 2 davis gauges perform so poorly compared to the weather service and rainwise?  Rainwise is within calibration as well.
 
When we calibrate the gauges the water is run thru with NO WIND effects,  which is why all my gauges are within calibration.  Now add some wind to the equation and the davis gauges perform very poorly.  So lets take a look at the design.  The weather service and rainwise gauges have straight sides,  wind screens that can be added to rain gauges have sides sloping in from top to bottom to divert wind down and away from the rim of gauges.  The davis tippers have sides that slope from bottom to top which would direct any amount of wind up to the rim and cause problems with catch more so than the straight gauge sides of the weather service or rainwise or any of the more expensive tippers,  add you can usually get a rainwise tipper cheaper than a davis.

I strongly think the big problem with davis gauges is the cone design that slopes in from bottom to top,  that would cause any amount of wind and more of the wind to be deflected up to the rim of the gauge to cause unwanted eddies which causes rain droplets to spin away from the rim and miss the gauge.

I have run the next biggest orifice with my novalynx calibrator which is about 8" an hour rate in the davis gauges and the gauge records more rain than it did the past storm and a lof of storms that have a lot less rain rate,  which is why I believe the wind combined with the design of the davis tippers sloping from bottom to top is the biggest problem.

I haven't but I bet if someone installed straight sides to the davis rain gauges they would perform a lot better

Depending on how far away the weather service is from your station (and even then it may still be a moot point) is the big factor.  I live 3 miles from the closest weather station.  I NEVER use that rain amount as a gauge (no pun intended) for comparing rain amounts.  Storms and rain just have too many variables at that distance.  I have the newer version tube/outer tube manual rain gauge (the same ones CocoRahs users use), and the VP2 for me is never more than about .05" of an inch off in a big rain event.  Small rain events are almost always exact.

For a time I was a big stickler for getting ultra accurate measurements.  To the point I was becoming obsessive about it.  I finally let it go and enjoy the collection of the data now instead of worrying about it's ultra-accuracy.

IMO, it's just not worth it.

[WXWIZARD]

The weather service gauge is in row with the other 3 gauges in my instrument compound which was at the very start of my posting.


[/quote]

Depending on how far away the weather service is from your station (and even then it may still be a moot point) is the big factor.  I live 3 miles from the closest weather station.  I NEVER use that rain amount as a gauge (no pun intended) for comparing rain amounts.  Storms and rain just have too many variables at that distance.  I have the newer version tube/outer tube manual rain gauge (the same ones CocoRahs users use), and the VP2 for me is never more than about .05" of an inch off in a big rain event.  Small rain events are almost always exact.

For a time I was a big stickler for getting ultra accurate measurements.  To the point I was becoming obsessive about it.  I finally let it go and enjoy the collection of the data now instead of worrying about it's ultra-accuracy.

IMO, it's just not worth it.
[/quote]

[ValentineWeather]

Storm totals for me.

Weather Service:  4.33"
VP2: 3.52"
Davis Stand Alone: 3.48"
Rainwise:  4.31"

All are mounted in a row at 3 feet agl except the VP2 which the rim of gauge is 5 feel agl.

My rainwise is 13 years old,  just had to replace the reed switch once in that time.  It has from day one been very close to the weather service gauge in all rain events,  from light to heavy intensity and with wind,  it has been a very impressive tipper over the years.

The davis gauges over the years are the most inconsistant gauges.  They are calibrated.  I run the NovaLynx calibrator 2 times a year.  I run the small oriface which gives around 3" per hour rate.  So how does 2 davis gauges perform so poorly compared to the weather service and rainwise?  Rainwise is within calibration as well.
 
When we calibrate the gauges the water is run thru with NO WIND effects,  which is why all my gauges are within calibration.  Now add some wind to the equation and the davis gauges perform very poorly.  So lets take a look at the design.  The weather service and rainwise gauges have straight sides,  wind screens that can be added to rain gauges have sides sloping in from top to bottom to divert wind down and away from the rim of gauges.  The davis tippers have sides that slope from bottom to top which would direct any amount of wind up to the rim and cause problems with catch more so than the straight gauge sides of the weather service or rainwise or any of the more expensive tippers,  add you can usually get a rainwise tipper cheaper than a davis.

I strongly think the big problem with davis gauges is the cone design that slopes in from bottom to top,  that would cause any amount of wind and more of the wind to be deflected up to the rim of the gauge to cause unwanted eddies which causes rain droplets to spin away from the rim and miss the gauge.

I have run the next biggest orifice with my novalynx calibrator which is about 8" an hour rate in the davis gauges and the gauge records more rain than it did the past storm and a lof of storms that have a lot less rain rate,  which is why I believe the wind combined with the design of the davis tippers sloping from bottom to top is the biggest problem.

I haven't but I bet if someone installed straight sides to the davis rain gauges they would perform a lot better

You are the second person saying the Rainwise did an impressive job recording a heavy rain event. ggsteve also had good results. The weather service gauge is 8" diameter same as the Rainwise correct? 

[ValentineWeather]

I'm going with the standalone Rainwise and will convert the Davis heater mod over into it.  I've ran into another thread different site also agreeing how well the 8" Rainwise handles heavy rain. They also mention how it matches the Cocorahs within couple hundredths even with very heavy rain.  Same person has added the Davis heater modification into the Rainwise gauge with success. Not sure how that will work until I get a close up view of the gauge itself. http://www.weather-watch.com/smf/index.php?topic=43640.0
Rainman Weather carries the standalone gauge with counter. Wire going into counter just needs converted to plug into Davis ISS.

[ggsteve]

Same person has added the Davis heater modification into the Rainwise gauge with success. Not sure how that will work until I get a close up view of the gauge itself. http://www.weather-watch.com/smf/index.php?topic=43640.0
Rainman Weather carries the standalone gauge with counter. Wire going into counter just needs converted to plug into Davis ISS.

I look forward to reading about your heater adaptation.  Last winter's precipitation reporting by my Rainwise was far from perfect.  Very early on we had an ice storm and I think it "sealed up" the rain gauge for some time (the gauge gets very little, if any direct sun at that time of year).  Much of the percipitation during winter was not reported, it probably sublimated rather than melting.

[ValentineWeather]

The 8" Rainwise will be a larger area to heat vs the Davis so I'm truthfully not sure if the 24 watt heater is enough heat. I'm thinking some kind of insulating blanket possibly around the gauge body itself will be needed. Using the reptile heater cord wrapped around body with insulation along with the Davis heater are my current thoughts. Some of the snows last winter were with single digit temperatures

[WXWIZARD]

Yes that's correct.  I've been a very happy Rainwise rain gauge owner for 13 years.  There's no consistancy with the Davis tippers,  Rainwise is consistant compared to the weather service gauge.  A lot blame rainfall rates and wind as the reasons the davis under reports but it's hard for me to say that when the rainwise and davis gauges are next to each other and next to the weather service gauge with the same rainfall rates and wind.  I believe the design of the davis gauge cone is the real issue.


[/quote]

You are the second person saying the Rainwise did an impressive job recording a heavy rain event. ggsteve also had good results. The weather service gauge is 8" diameter same as the Rainwise correct?
[/quote]

[BCJKiwi]

Been watching this thread with interest.
There seem to be two different reasons being given for the inaccuracy of the Davis under high rain/wind conditions;
1. the cone - size/taper/aperture
2. the tipper itself - being overloaded and not counting all the water

Considering that there are numerous diameters of cone sizes in use on a variety of equipment including "Standards" units typically from 4" thru 8" I looked at the tipper.

Viewing the info available on the Rainwise site, I could only find one grainy picture of the tipper. This was enough to point to very obvious and I would suggest significant differences in design.

The Davis tipper is deep and narrow with a curved bottom and a high wall between sides.
The Rainwise is wide and shallow with a flat bottom and a low wall between sides. The centre of gravity of the water mass generating the tip also appears to be further from the pivot.

This suggests to me that in the Rainwise;
a. The tipping point would be more precise,
b. the tip motion would be faster
c. the lower wall would mean less loss of water to the wrong side/splashing on the changeover to the other side.

Raises the question if a redesigned tipper along the lines of the Rainwise could be retrofitted to the Davis system and achieve better results.
A challenge to all you guys/gals who have test rigs and multiple sets of kit at your disposal. Unfortunately I don't else I would begin testing today!

[ValentineWeather]

Good input BCJKiwi something I never thought about the tipper itself, its as much if not more of a critical component as the bucket design.
I know Rainwise speaks highly of their tipper and its ability to shed water. I'm not sure who makes the Davis gauge, its been mentioned before the rain gauge wasn't an original Davis product but a retrofitted product, in fact someone had linked to the original company but at the time didn't think much of it so didn't save the link. 

[dalecoy]

..... about the tipper itself, its as much if not more of a critical component .......

It is technically impossible to have a tipping mechanism that measures perfectly at all flow rates.  That applies to "spoons", also.

Of course, some are better than others, and it can be optimized for a particular flow rate, etc. etc. 

Much less the question of end-of-precipitation partial quantities.

Oh, and how to handle dew, etc.

[BCJKiwi]

Good input BCJKiwi something I never thought about the tipper itself, its as much if not more of a critical component as the bucket design.
I know Rainwise speaks highly of their tipper and its ability to shed water. I'm not sure who makes the Davis gauge, its been mentioned before the rain gauge wasn't an original Davis product but a retrofitted product, in fact someone had linked to the original company but at the time didn't think much of it so didn't save the link. 

If you were concerned about the bucket and considered testing a modified tipper, then it would be trivial to add an 8" dia extension with vertical sides to the top of the VP2 bucket at the same time.

[ValentineWeather]

Good input BCJKiwi something I never thought about the tipper itself, its as much if not more of a critical component as the bucket design.
I know Rainwise speaks highly of their tipper and its ability to shed water. I'm not sure who makes the Davis gauge, its been mentioned before the rain gauge wasn't an original Davis product but a retrofitted product, in fact someone had linked to the original company but at the time didn't think much of it so didn't save the link. 

If you were concerned about the bucket and considered testing a modified tipper, then it would be trivial to add an 8" dia extension with vertical sides to the top of the VP2 bucket at the same time.

The only testing will be on the Rainwise to see if I can modify it with the Davis heater.  I've never been a fan of the VP2 bucket mounted 6' plus above the ground so have been using a stand alone mounted at 30". If the Rainwise works as being reported I'll be content with my Davis/Rainwise station.  Unfortunately our heavy rain season is over, so may not get any conclusive information until next May.

[kcidwx]

Rainwise tipper

[ValentineWeather]

Thanks for pics Jim. The tipping bucket size looks much larger vs the Davis. But with bucket diameter  8 vs 6.5" you would expect tipper to be larger.  I see a great spot for the heater.