Topic: CoCoRaHS vs. Davis Vantage Pro 2 Rainfall Measurements

[WheatonRon]

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!

[CNYWeather]

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.

[WheatonRon]

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.

[CW2274]

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.

[WheatonRon]

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!

[ValentineWeather]

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  , I typed all this out so posting anyway. 

[CW2274]

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.

[ValentineWeather]

ASOS uses a software correction is my understanding. Some of you smart guys out need to write something for the Davis...


https://mesonet.agron.iastate.edu/ASOS/precipnote.phtml

[CW2274]

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.

[ValentineWeather]

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.

[WheatonRon]

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.

[ValentineWeather]

I don't know of any correction software for Davis or any PWS software for that matter.

[CW2274]

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.

[Cutty Sark Sailor]

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.   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"  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.

[CW2274]

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.   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"  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.

[WheatonRon]

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!

[CW2274]

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.

[alexstaar]

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.

[WheatonRon]

Great idea! Maybe that is why Davis has been slow (as molasses) in releasing the long anticipated Vantage Pro 3!

[alexstaar]

  Maybe! 

[SLOweather]

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.

[CW2274]

Well put, boss.

[WheatonRon]

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!

[ValentineWeather]

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.

[W3DRM]

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!