Author Topic: Davis Tipping Bucket correction?  (Read 625 times)

0 Members and 1 Guest are viewing this topic.

Offline Old Tele man

  • Singing in the rain...
  • Forecaster
  • *****
  • Posts: 1363
Davis Tipping Bucket correction?
« on: July 06, 2018, 03:03:44 PM »
Has anybody here compared their rain CoCoRaHS readings with their Davis tipping bucket readings to see if the ASOS tipping bucket correction formula (below) is close of not?

C = A(1 + 0.60A)

where:
C = calculated rainfall amount
A = measured amount from tipping bucket

• source: page 20 at https://www.weather.gov/media/asos/aum-toc.pdf
« Last Edit: July 06, 2018, 05:20:53 PM by Old Tele man »
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX1 Precision Digital Barograph
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline klschmidt

  • Member
  • *
  • Posts: 19
Re: Davis Tipping Bucket correction?
« Reply #1 on: July 06, 2018, 03:59:43 PM »
The article mentions data from the HTB. Would I be understanding correctly that the formula is only used when the heater is in operation?

I do not have a rain gauge heater, but the formula would overcorrect the factory error that I have observed. Both of my Davis tipping buckets have recorded 20% low from the factory. In a hypothetical rain event where CoCoRaHS recorded one inch and Davis recorded eight tenths, the formula would correct .80" to 1.184".

Why Davis does not calibrate the tipping bucket gauges correctly from the factory has been a mystery to me.

Offline Old Tele man

  • Singing in the rain...
  • Forecaster
  • *****
  • Posts: 1363
Re: Davis Tipping Bucket correction?
« Reply #2 on: July 06, 2018, 04:14:35 PM »
The article mentions data from the HTB. Would I be understanding correctly that the formula is only used when the heater is in operation?

I do not have a rain gauge heater, but the formula would overcorrect the factory error that I have observed. Both of my Davis tipping buckets have recorded 20% low from the factory. In a hypothetical rain event where CoCoRaHS recorded one inch and Davis recorded eight tenths, the formula would correct .80" to 1.184".

Why Davis does not calibrate the tipping bucket gauges correctly from the factory has been a mystery to me.
Yeah, I sorta suspected it wasn't exactly applicable 'as-is' but wondered *IF* something (equation-wise) MIGHT be applicable once the numbers were collected and crunched?

For instance, using your values, this is the first-cut approximation that I get:

C = A(1 + 0.3125*A); so: C = 0.80"(1 + 0.3125*0.80") = 0.998 ≈ 1.00"

that's a constant about half of what ASOS uses.
« Last Edit: July 06, 2018, 04:35:59 PM by Old Tele man »
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX1 Precision Digital Barograph
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline klschmidt

  • Member
  • *
  • Posts: 19
Re: Davis Tipping Bucket correction?
« Reply #3 on: July 06, 2018, 05:00:19 PM »
I cannot think of a logical explanation for the Davis gauges being incorrect from the factory, but once calibrated correctly, I've been very pleased with the accuracy. Mine is often within .01"-.02" of my CoCoRaHS readings. Last rain: .30" vs .30", June 15-16 storm: 1.47" vs 1.46"...

So, still thinking about the ASOS formula... If a heater is in use, more ice would take more time to thaw. More evaporation would occur and more correction would be needed. In a .50" rain, the ASOS formula is really close to Davis tipping bucket error. The fact that my Davis (after calibration) reads correctly in both small and large rain events would suggest that if one does not wish to adjust the tipping bucket leveling screws, a simpler formula could be used (Davis reading * 1.25). With precipitation falling in liquid form, the total rain amount should have no effect on the calibration (no more than one teaspoon in the gauge at any given time).

Online ValentineWeather

  • Forecaster
  • *****
  • Posts: 5472
Re: Davis Tipping Bucket correction?
« Reply #4 on: July 06, 2018, 05:13:44 PM »
Could be the evaporation plays into the heater ASOS formula. How much heat does the ASOS unit generate etc. I can confirm evaporation is an issue for me. Anywhere from 10-60% depending on snow rate and how cold it is. The wetter and warmer the snow the less loss. It's bad enough I just rely on manual and plug the correct numbers in after the snowfall.

Novalynx I think has a great calibration rate for tipping buckets. All Novalynx tipping gauges from factory are calibrated at 4" per hour rate and it seems to work for my area.
I rarely hit 4" rate except thunderstorms for couple minutes it might peak from 4-7" hr. rate, but it always seems to work out. Usually within .03 for each inch rain sometimes better.... It's magic. 
Randy

Offline Mattk

  • Forecaster
  • *****
  • Posts: 935
Re: Davis Tipping Bucket correction?
« Reply #5 on: July 06, 2018, 06:57:57 PM »
Re snow/ice/sleet environments and if snow/ice sleet was really important to measure than ultrasonic would be the logical choice?

Offline jgentry

  • Forecaster
  • *****
  • Posts: 351
Re: Davis Tipping Bucket correction?
« Reply #6 on: July 07, 2018, 10:43:59 PM »
I cannot think of a logical explanation for the Davis gauges being incorrect from the factory, but once calibrated correctly, I've been very pleased with the accuracy. Mine is often within .01"-.02" of my CoCoRaHS readings. Last rain: .30" vs .30", June 15-16 storm: 1.47" vs 1.46"...

So, still thinking about the ASOS formula... If a heater is in use, more ice would take more time to thaw. More evaporation would occur and more correction would be needed. In a .50" rain, the ASOS formula is really close to Davis tipping bucket error. The fact that my Davis (after calibration) reads correctly in both small and large rain events would suggest that if one does not wish to adjust the tipping bucket leveling screws, a simpler formula could be used (Davis reading * 1.25). With precipitation falling in liquid form, the total rain amount should have no effect on the calibration (no more than one teaspoon in the gauge at any given time).

What do you use to calibrate your Davis gauge? Never could get a Davis gauge working that good. I use a RW 111 gauge (and had to adjust the screws since my setup doesn’t allow the gauge to be perfectly level) and the RW gauge works like a charm.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ocala

  • Forecaster
  • *****
  • Posts: 3486
    • http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KFLBELLE2
Re: Davis Tipping Bucket correction?
« Reply #7 on: July 08, 2018, 06:23:03 AM »
I cannot think of a logical explanation for the Davis gauges being incorrect from the factory, but once calibrated correctly, I've been very pleased with the accuracy. Mine is often within .01"-.02" of my CoCoRaHS readings. Last rain: .30" vs .30", June 15-16 storm: 1.47" vs 1.46"...

So, still thinking about the ASOS formula... If a heater is in use, more ice would take more time to thaw. More evaporation would occur and more correction would be needed. In a .50" rain, the ASOS formula is really close to Davis tipping bucket error. The fact that my Davis (after calibration) reads correctly in both small and large rain events would suggest that if one does not wish to adjust the tipping bucket leveling screws, a simpler formula could be used (Davis reading * 1.25). With precipitation falling in liquid form, the total rain amount should have no effect on the calibration (no more than one teaspoon in the gauge at any given time).
That's great that your Davis is accurate in even heavy rains. In light rain mine are pretty close but in heavy downpours the discrepancy can be pretty large.

Offline Old Tele man

  • Singing in the rain...
  • Forecaster
  • *****
  • Posts: 1363
Re: Davis Tipping Bucket correction?
« Reply #8 on: July 08, 2018, 01:20:25 PM »
Collecting side-by-side Tipping-bucket and CoCoRaHS numbers will "tell the tail" by enumerating their differences.
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX1 Precision Digital Barograph
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline dport

  • Senior Contributor
  • ****
  • Posts: 169
Re: Davis Tipping Bucket correction?
« Reply #9 on: July 08, 2018, 02:58:50 PM »
I cannot think of a logical explanation for the Davis gauges being incorrect from the factory, but once calibrated correctly, I've been very pleased with the accuracy. Mine is often within .01"-.02" of my CoCoRaHS readings. Last rain: .30" vs .30", June 15-16 storm: 1.47" vs 1.46"...

So, still thinking about the ASOS formula... If a heater is in use, more ice would take more time to thaw. More evaporation would occur and more correction would be needed. In a .50" rain, the ASOS formula is really close to Davis tipping bucket error. The fact that my Davis (after calibration) reads correctly in both small and large rain events would suggest that if one does not wish to adjust the tipping bucket leveling screws, a simpler formula could be used (Davis reading * 1.25). With precipitation falling in liquid form, the total rain amount should have no effect on the calibration (no more than one teaspoon in the gauge at any given time).
That's great that your Davis is accurate in even heavy rains. In light rain mine are pretty close but in heavy downpours the discrepancy can be pretty large.

How far off in heavy rains, say 1 inch plus storm (non-calibrated)?

Offline ocala

  • Forecaster
  • *****
  • Posts: 3486
    • http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KFLBELLE2
Re: Davis Tipping Bucket correction?
« Reply #10 on: July 08, 2018, 06:26:46 PM »
This is an extreme example but on May 21st of this year the cocorahs recorded 6.20 inches in about 4 hours. The Davis recorded 4.36. There were times during the deluge that the rainfall was extremely heavy. Other times not as bad. Generally it's about a 15 to 20 percent difference. Light rains it's almost even.

Offline Old Tele man

  • Singing in the rain...
  • Forecaster
  • *****
  • Posts: 1363
Re: Davis Tipping Bucket correction?
« Reply #11 on: July 08, 2018, 07:29:11 PM »
This is an extreme example but on May 21st of this year the cocorahs recorded 6.20 inches in about 4 hours. The Davis recorded 4.36. There were times during the deluge that the rainfall was extremely heavy. Other times not as bad. Generally it's about a 15 to 20 percent difference. Light rains it's almost even.
With those numbers, this is what the *constant (K)* becomes:

C = A(1 + K*A)...so: 6.20" = 4.36"(1 + K*4.36") when K = 0.096793 ≈ 0.097

that's about 1/6th of the ASOS number.
« Last Edit: July 08, 2018, 07:30:53 PM by Old Tele man »
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX1 Precision Digital Barograph
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline klschmidt

  • Member
  • *
  • Posts: 19
Re: Davis Tipping Bucket correction?
« Reply #12 on: July 09, 2018, 04:00:16 AM »
What do you use to calibrate your Davis gauge? Never could get a Davis gauge working that good. I use a RW 111 gauge (and had to adjust the screws since my setup doesn’t allow the gauge to be perfectly level) and the RW gauge works like a charm.
The tipping buckets retain a little bit of water, so I start by dumping water into the gauge until the bucket tips a couple times, then drip water very slowly until it tips once more. At the sound of this last clunk, I stop immediately. Then I slowly dump distilled water into the rain gauge from a volumetric flask. I have a 250ml volumetric flask, which should be .452" in a 6.553" diameter funnel. I adjust the leveling screws (making sure to keep them level with each other) until 250ml will cause 45 clunks of the bucket and still leave a few drips remaining. It takes about a teaspoon to tip the bucket, so time can be saved by pouring almost a teaspoon quickly, then pour really slowly until the tipping bucket is heard. I unplug the rain gauge to keep from sending test data to the internet. I'm not really sure why distilled water makes a difference; I get about two tips less with tap water.

I still don't understand why total rainfall amount would have any affect on accuracy. Except for the initial rainfall amount required to get the funnel and tipping bucket wet, it seems like any amount of rain could be measured accurately. What am I missing?

The effect of rain RATE on accuracy makes more sense to me. In really heavy rain, some water can fall into the full side of the bucket even after the bucket has started to tip. If the funnel is dripping (as opposed to a stream), this shouldn't happen. I think it becomes a stream at a rate of about two inches per hour. I tried dumping 250ml of water into the funnel in 37.2 seconds, and recorded 39 tips. There was some water left in the tipping bucket, so I call it 39.5. 39.5/45.2 would indicate that the gauge would record 87.4% the amount it should have. Would the the amount of water caught by the full bucket be a linear increase when compared with rain rate (after the first two inches per hour)? 39.5 tips in 37.2 seconds would be a recorded rain rate of 38.2 inches per hour (87.4% of the actual rain rate). Therefore, for my gauge, at rain rates higher than two inches per hour, one might use the formula:

C=(1+(R-2)/250)*A

R=displayed rain rate (in/hr)

To really be accurate a formula like this would need to be applied continuously. Most rain events I have witnessed start and end with much lower rain rates.

In a hypothetical rain event which the Davis recorded as .75", it is possible that half of the rain occured at a rate lower than two inches per hour, allowing the bucket to tip between drips. If the Davis recorded the other half of the rain at a rate of 20"/hr (about half as fast as my test, but really hard rain for my area), this formula would be applied to the .375" that fell fast: (1+(20-2)/250)*.375, so: 18/250=.072, so: 1.072*.375=.402, .402+.375 (the portion that fell slowly)=.777" (.027" error). Perhaps I am too quick to say "good enough".

If the funnel becomes filled with water (birds clog the outlet) and this water is allowed to drain at full speed, I've seen significant error. When such conditions have occured, I have blamed myself and accepted the fact that the rainfall was not recorded accurately.
« Last Edit: July 09, 2018, 04:15:28 AM by klschmidt »

Offline ocala

  • Forecaster
  • *****
  • Posts: 3486
    • http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KFLBELLE2
Re: Davis Tipping Bucket correction?
« Reply #13 on: July 09, 2018, 06:21:39 PM »
I think the key to getting a comparable reading to cocorahs is making the hole smaller. Less volume going through the outlet will increase accuracy. The problem with a smaller outlet is it will get clogged so you would need a strainer to sit inside the bucket. As for the smaller outlet, I guess just leave that up to trial and error to see what works best. I actually tried the stuff you use to seal cracks in concrete but eventually it washed away
Also your rain rate would be way off but I would rather get a more accurate total.

Offline Mattk

  • Forecaster
  • *****
  • Posts: 935
Re: Davis Tipping Bucket correction?
« Reply #14 on: July 09, 2018, 07:05:23 PM »
Davis do need to modify the water outlet of both the VP2 & Vue with a mesh/screen riser arrangement but this isn't going to change/affect the accuracy (as such) but will prevent blockages. The current VP2 screen and Vue insert don't do much in blockage prevention at all. In fact during very heavy large droplet rain the Vue spoon can be continuously bounced from direct hits through the aperture which is quite large without that silly little plug that birds just love to pinch.     

However the Davis can be calibrated quite well within specifications but of course no one setting will cover all especially abnormal rain events.

Offline jgentry

  • Forecaster
  • *****
  • Posts: 351
Re: Davis Tipping Bucket correction?
« Reply #15 on: July 09, 2018, 07:34:08 PM »
I think the RainWise rain gauge hit the jackpot overall in their design of the tipping bucket gauge. The key thing is that you have to have the gauge perfectly level in order for it to perform within specs. If not perfectly level like mine, all you would have to do is raise the screws until it’s a even reach for the scoops.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline klschmidt

  • Member
  • *
  • Posts: 19
Re: Davis Tipping Bucket correction?
« Reply #16 on: July 10, 2018, 12:21:25 PM »
I think the RainWise rain gauge hit the jackpot overall in their design of the tipping bucket gauge. The key thing is that you have to have the gauge perfectly level in order for it to perform within specs. If not perfectly level like mine, all you would have to do is raise the screws until it’s a even reach for the scoops.
With a Davis, "perfectly level" may need to be checked with a regular level instead of the little level implanted in the base.

I found this article which mentions the effect of rain gauge height on catch amount.  http://www.nws.noaa.gov/om/csd/pds/PCU6/IC6_2/tutorial1/Factors_exposure.htm

When comparing CoCoRaHS with Davis, are the rain collectors the same height?

Offline jgentry

  • Forecaster
  • *****
  • Posts: 351
Re: Davis Tipping Bucket correction?
« Reply #17 on: July 10, 2018, 12:35:35 PM »
I think the RainWise rain gauge hit the jackpot overall in their design of the tipping bucket gauge. The key thing is that you have to have the gauge perfectly level in order for it to perform within specs. If not perfectly level like mine, all you would have to do is raise the screws until it’s a even reach for the scoops.
With a Davis, "perfectly level" may need to be checked with a regular level instead of the little level implanted in the base.

I found this article which mentions the effect of rain gauge height on catch amount.  http://www.nws.noaa.gov/om/csd/pds/PCU6/IC6_2/tutorial1/Factors_exposure.htm

When comparing CoCoRaHS with Davis, are the rain collectors the same height?

When I was using the Davis gauge, yes. My Davis stations didn’t  have a “bubble level” inside like the newer models. I will say that if someone wants to get the most accuracy from a Davis gauge, they will need to install it perfectly level and on a very sturdy pole/post. You do not want the Davis gauge to be vibrating. 
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358