### Author Topic: Manual gauge for calibration  (Read 807 times)

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#### dport

• Senior Contributor
• Posts: 170
##### Manual gauge for calibration
« on: July 13, 2018, 08:55:50 AM »
Any suggestions on a manual rain gauge to help calibrate my VP2?  I'm not really interested in the cocorahs for \$38 as I really only plan to use it a handful of times to nail the calibration.  Surely there is a somewhat accurate solution for under \$15.

So many gauges on amazon though.  Couldn't tell you which are accurate and which are not.  Reviews are all over the place.

#### CW2274

• Forecaster
• Posts: 4162
##### Re: Manual gauge for calibration
« Reply #1 on: July 13, 2018, 03:54:48 PM »
I used a gallon milk jug with a small hole poked in the bottom corner and use putty to open or close the orifice to control the rate of flow. I also used a scale accurate to the gram to weight the water for the desired amount (1ml of water weighs exactly 1gram). Being gravity fed, the rate obviously starts high, then slows as it empties. The rate I used starts about 4-4.5" per hour, then drops to less than 1" by the time it empties.
If you're unfamiliar, the amount of water equaling 1" of rain for the VP2 is 544ml.

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #2 on: July 13, 2018, 04:30:04 PM »
I used a gallon milk jug with a small hole poked in the bottom corner and use putty to open or close the orifice to control the rate of flow. I also used a scale accurate to the gram to weight the water for the desired amount (1ml of water weighs exactly 1gram). Being gravity fed, the rate obviously starts high, then slows as it empties. The rate I used starts about 4-4.5" per hour, then drops to less than 1" by the time it empties.
If you're unfamiliar, the amount of water equaling 1" of rain for the VP2 is 544ml.

Great, good to know.  This will be a little project for me tomorrow.  Will measure out 544ml into a milk jug and slowly release into VP2.  Should take 20-30 minutes if I use your desired rain rate.  Let's hope it comes out close to 1 inch!

#### CW2274

• Forecaster
• Posts: 4162
##### Re: Manual gauge for calibration
« Reply #3 on: July 13, 2018, 04:51:09 PM »
I used a gallon milk jug with a small hole poked in the bottom corner and use putty to open or close the orifice to control the rate of flow. I also used a scale accurate to the gram to weight the water for the desired amount (1ml of water weighs exactly 1gram). Being gravity fed, the rate obviously starts high, then slows as it empties. The rate I used starts about 4-4.5" per hour, then drops to less than 1" by the time it empties.
If you're unfamiliar, the amount of water equaling 1" of rain for the VP2 is 544ml.

Great, good to know.  This will be a little project for me tomorrow.  Will measure out 544ml into a milk jug and slowly release into VP2.  Should take 20-30 minutes if I use your desired rain rate.  Let's hope it comes out close to 1 inch!
You can use the adjusting screws or if you have something like WL, you can adjust for the error there too. Always duplicate the test to make sure you've indeed got it where you want it (I did it ad nauseam).
Oh, don't forget to disable your PWS's internet connection while testing....

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #4 on: July 13, 2018, 04:59:20 PM »
I used a gallon milk jug with a small hole poked in the bottom corner and use putty to open or close the orifice to control the rate of flow. I also used a scale accurate to the gram to weight the water for the desired amount (1ml of water weighs exactly 1gram). Being gravity fed, the rate obviously starts high, then slows as it empties. The rate I used starts about 4-4.5" per hour, then drops to less than 1" by the time it empties.
If you're unfamiliar, the amount of water equaling 1" of rain for the VP2 is 544ml.

Great, good to know.  This will be a little project for me tomorrow.  Will measure out 544ml into a milk jug and slowly release into VP2.  Should take 20-30 minutes if I use your desired rain rate.  Let's hope it comes out close to 1 inch!
You can use the adjusting screws or if you have something like WL, you can adjust for the error there too. Always duplicate the test to make sure you've indeed got it where you want it (I did it ad nauseam).
Oh, don't forget to disable your PWS's internet connection while testing....

Yeah I was actually just thinking about that.  I use the WifiLogger.  I can easily disable uploads to WU, CWOP, etc, however, I am unsure how to disable the actual internal logging of my logger.  Don't want to take it out of the console as I think I'll have to do a full set up again.

#### CW2274

• Forecaster
• Posts: 4162
##### Re: Manual gauge for calibration
« Reply #5 on: July 13, 2018, 05:05:25 PM »
I used a gallon milk jug with a small hole poked in the bottom corner and use putty to open or close the orifice to control the rate of flow. I also used a scale accurate to the gram to weight the water for the desired amount (1ml of water weighs exactly 1gram). Being gravity fed, the rate obviously starts high, then slows as it empties. The rate I used starts about 4-4.5" per hour, then drops to less than 1" by the time it empties.
If you're unfamiliar, the amount of water equaling 1" of rain for the VP2 is 544ml.

Great, good to know.  This will be a little project for me tomorrow.  Will measure out 544ml into a milk jug and slowly release into VP2.  Should take 20-30 minutes if I use your desired rain rate.  Let's hope it comes out close to 1 inch!
You can use the adjusting screws or if you have something like WL, you can adjust for the error there too. Always duplicate the test to make sure you've indeed got it where you want it (I did it ad nauseam).
Oh, don't forget to disable your PWS's internet connection while testing....

Yeah I was actually just thinking about that.  I use the WifiLogger.  I can easily disable uploads to WU, CWOP, etc, however, I am unsure how to disable the actual internal logging of my logger.  Don't want to take it out of the console as I think I'll have to do a full set up again.
You don't need to remove the logger, just delete the rain totals in the console or thru your software after testing.

#### ocala

• Forecaster
• Posts: 3546
##### Re: Manual gauge for calibration
« Reply #6 on: July 14, 2018, 06:49:21 AM »
I used a gallon milk jug with a small hole poked in the bottom corner and use putty to open or close the orifice to control the rate of flow. I also used a scale accurate to the gram to weight the water for the desired amount (1ml of water weighs exactly 1gram). Being gravity fed, the rate obviously starts high, then slows as it empties. The rate I used starts about 4-4.5" per hour, then drops to less than 1" by the time it empties.
If you're unfamiliar, the amount of water equaling 1" of rain for the VP2 is 544ml.

Great, good to know.  This will be a little project for me tomorrow.  Will measure out 544ml into a milk jug and slowly release into VP2.  Should take 20-30 minutes if I use your desired rain rate.  Let's hope it comes out close to 1 inch!
You can use the adjusting screws or if you have something like WL, you can adjust for the error there too. Always duplicate the test to make sure you've indeed got it where you want it (I did it ad nauseam).
Oh, don't forget to disable your PWS's internet connection while testing....
When I did this a while back duplicate tests would come back off by .01.
Just kept doing several tests until it was tweaked just right.

#### ValentineWeather

• Forecaster
• Posts: 5614
##### Re: Manual gauge for calibration
« Reply #7 on: July 14, 2018, 07:33:07 AM »
To calibrate you could do this without a calibrator.  Level gauge especially direction tippers line up . Get a medicine dropper and adjust tippers to tip 4.2 ml per tip. Factory is 4.282 ml per tip, it's 1 or 2" per hr rate.  I would shoot for more and adjust tippers starting at 4.2 maybe even 4.1 ml to tip buckets.

Doesn't help finding manual rain gauge but checking and tweaking calibration this may help someone.

Here is where I found the numbers.  http://www.emesystems.com/davis/documents/D7852%20Installation.pdf
« Last Edit: July 14, 2018, 07:38:17 AM by ValentineWeather »
Randy

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #8 on: July 14, 2018, 08:02:20 AM »
To calibrate you could do this without a calibrator.  Level gauge especially direction tippers line up . Get a medicine dropper and adjust tippers to tip 4.2 ml per tip. Factory is 4.282 ml per tip, it's 1 or 2" per hr rate.  I would shoot for more and adjust tippers starting at 4.2 maybe even 4.1 ml to tip buckets.

Doesn't help finding manual rain gauge but checking and tweaking calibration this may help someone.

Here is where I found the numbers.  http://www.emesystems.com/davis/documents/D7852%20Installation.pdf

Now I'm confused, CW2272 said that 544ml would equal 1 inch of rain in the VP2.  That would come out to 5.44ml per tip.  But you are saying 4.282ml is one tip?

It's early and I haven't had my coffee.  I must be doing math wrong.

#### johnd

• Forecaster
• Posts: 2962
##### Re: Manual gauge for calibration
« Reply #9 on: July 14, 2018, 08:41:55 AM »
Now I'm confused, CW2272 said that 544ml would equal 1 inch of rain in the VP2.  That would come out to 5.44ml per tip.  But you are saying 4.282ml is one tip?

Assuming you're in N America then 5.44ml per tip is correct - it's really not difficult to work out for yourself. Personally I hate the thought of calibrating with a medicine dropper for all sorts of reasons. Much better to allow a known large amount (eg 544g or ml) of - preferably distilled or rain (which would be kind of appropriate) - water to drip steadily through the gauge.
Prodata Weather Systems
Prodata's dedicated Davis EnviroMonitor website
UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

#### johnd

• Forecaster
• Posts: 2962
##### Re: Manual gauge for calibration
« Reply #10 on: July 14, 2018, 08:42:38 AM »

Now I'm confused, CW2272 said that 544ml would equal 1 inch of rain in the VP2.  That would come out to 5.44ml per tip.  But you are saying 4.282ml is one tip?

Assuming you're in N America then 5.44ml per tip is correct - it's really not difficult to work out for yourself. Personally I hate the thought of calibrating with a medicine dropper for all sorts of reasons. Much better IMO to allow a known large amount (eg 544g or ml) of - preferably distilled or rain (which would be kind of appropriate) - water to drip steadily through the gauge.
Prodata Weather Systems
Prodata's dedicated Davis EnviroMonitor website
UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

#### SLOweather

• Forecaster
• Posts: 3447
##### Re: Manual gauge for calibration
« Reply #11 on: July 14, 2018, 09:04:32 AM »
Having done a lot of Davis rain gauge calibrations a lot of different ways, including a burette in a lab stand, and an eyedropper, I concur with Johnd. Trying to do it with a dropper will just lead to a lot of frustration. In theory, it sounds great, in practice, not so much. I finally purchased a Novalynx calibrator.

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #12 on: July 14, 2018, 09:11:26 AM »
Thanks guys.  Think I'm going to experiment with 500ml in a water bottle today. That equals 0.92 inches in the VP2. Will poke a hole or two with a pin to start.

I'm not terribly worried about being 100% accurate, but I do want to be within the 4% spec that davis cites.

#### ValentineWeather

• Forecaster
• Posts: 5614
##### Re: Manual gauge for calibration
« Reply #13 on: July 14, 2018, 12:32:42 PM »
To calibrate you could do this without a calibrator.  Level gauge especially direction tippers line up . Get a medicine dropper and adjust tippers to tip 4.2 ml per tip. Factory is 4.282 ml per tip, it's 1 or 2" per hr rate.  I would shoot for more and adjust tippers starting at 4.2 maybe even 4.1 ml to tip buckets.

Doesn't help finding manual rain gauge but checking and tweaking calibration this may help someone.

Here is where I found the numbers.  http://www.emesystems.com/davis/documents/D7852%20Installation.pdf

Now I'm confused, CW2272 said that 544ml would equal 1 inch of rain in the VP2.  That would come out to 5.44ml per tip.  But you are saying 4.282ml is one tip?

It's early and I haven't had my coffee.  I must be doing math wrong.

Anyway I disagree with the notion dropper doesn't work well. I use the Novalix calibrator also but once I know what is needed the dropper method works every time. It's quick, easy and accurate.
« Last Edit: July 14, 2018, 07:18:12 PM by ValentineWeather »
Randy

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #14 on: July 14, 2018, 03:43:54 PM »
The results are in!  At least after two of my tests.  Both tests achieved an average rainfall rate of about 4 inches an hour using 500ml in a water bottle with a pin hole at the bottom.  So, we know that 500ml is supposed to equal 0.92 inches of rain in the VP2.  Here are the actual results

Test 1:  500ml = 0.89 inches
Test 2:  500ml = 0.90 inches

Some water may have been left in the tipper from test 1 to test 2, that could account for the extra hundredth.  Regardless, these numbers are technically within spec according to Davis.  I will also say that I performed this test during the middle of the day with bright sunshine, temp of 87, and dew point of 62.  Some evaporation may have occurred, but probably not enough to make a real different.

Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.

*Edit*, just opened unit to see the tipper.  It was about half full at the end of the 2nd test.  I think I was probably spot on at about 0.895 inches per test.  0.895/0.919 = 2.61%
« Last Edit: July 14, 2018, 04:01:15 PM by dport »

#### CW2274

• Forecaster
• Posts: 4162
##### Re: Manual gauge for calibration
« Reply #15 on: July 14, 2018, 04:11:54 PM »

Now I'm confused, CW2272 said that 544ml would equal 1 inch of rain in the VP2.  That would come out to 5.44ml per tip.  But you are saying 4.282ml is one tip?

Assuming you're in N America then 5.44ml per tip is correct - it's really not difficult to work out for yourself. Personally I hate the thought of calibrating with a medicine dropper for all sorts of reasons. Much better IMO to allow a known large amount (eg 544g or ml) of - preferably distilled or rain (which would be kind of appropriate) - water to drip steadily through the gauge.
Just for clarification, the dropper is merely to get both spoons to tip as close together at the specified amount required. Then you use a much larger amount to fine tune by moving both screws up or down the exact same amount to keep the spoons as close to the same volume as possible. This method can also help correct for a less than perfectly level placement.

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #16 on: July 14, 2018, 04:15:56 PM »
The results are in!  At least after two of my tests.  Both tests achieved an average rainfall rate of about 4 inches an hour using 500ml in a water bottle with a pin hole at the bottom.  So, we know that 500ml is supposed to equal 0.92 inches of rain in the VP2.  Here are the actual results

Test 1:  500ml = 0.89 inches
Test 2:  500ml = 0.90 inches

Some water may have been left in the tipper from test 1 to test 2, that could account for the extra hundredth.  Regardless, these numbers are technically within spec according to Davis.  I will also say that I performed this test during the middle of the day with bright sunshine, temp of 87, and dew point of 62.  Some evaporation may have occurred, but probably not enough to make a real different.

Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.

*Edit*, just opened unit to see the tipper.  It was about half full at the end of the 2nd test.  I think I was probably spot on at about 0.895 inches per test.  0.895/0.919 = 2.61%

I also kind of screwed myself with upload data.  I disabled all uploads while testing, then zeroed out the daily rainfall in the console after testing was complete.  Then, when I enabled uploads again it added the rainfall to CWOP, WU, etc.  Bummed.

#### CW2274

• Forecaster
• Posts: 4162
##### Re: Manual gauge for calibration
« Reply #17 on: July 14, 2018, 04:18:55 PM »
Some water may have been left in the tipper from test 1 to test 2
You should always ensure both spoons are empty before starting a new test if you want consistency.

#### CW2274

• Forecaster
• Posts: 4162
##### Re: Manual gauge for calibration
« Reply #18 on: July 14, 2018, 04:24:26 PM »
The results are in!  At least after two of my tests.  Both tests achieved an average rainfall rate of about 4 inches an hour using 500ml in a water bottle with a pin hole at the bottom.  So, we know that 500ml is supposed to equal 0.92 inches of rain in the VP2.  Here are the actual results

Test 1:  500ml = 0.89 inches
Test 2:  500ml = 0.90 inches

Some water may have been left in the tipper from test 1 to test 2, that could account for the extra hundredth.  Regardless, these numbers are technically within spec according to Davis.  I will also say that I performed this test during the middle of the day with bright sunshine, temp of 87, and dew point of 62.  Some evaporation may have occurred, but probably not enough to make a real different.

Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.

*Edit*, just opened unit to see the tipper.  It was about half full at the end of the 2nd test.  I think I was probably spot on at about 0.895 inches per test.  0.895/0.919 = 2.61%
I disabled all uploads while testing, then zeroed out the daily rainfall in the console after testing was complete.  Then, when I enabled uploads again it added the rainfall to CWOP, WU, etc.  Bummed.
Are you sure you cleared the data? That shouldn't have happened. You may want to try again. If you're successful, the CWOP data should clear by itself, WU dunno.

#### ValentineWeather

• Forecaster
• Posts: 5614
##### Re: Manual gauge for calibration
« Reply #19 on: July 14, 2018, 04:43:52 PM »

Now I'm confused, CW2272 said that 544ml would equal 1 inch of rain in the VP2.  That would come out to 5.44ml per tip.  But you are saying 4.282ml is one tip?

Assuming you're in N America then 5.44ml per tip is correct - it's really not difficult to work out for yourself. Personally I hate the thought of calibrating with a medicine dropper for all sorts of reasons. Much better IMO to allow a known large amount (eg 544g or ml) of - preferably distilled or rain (which would be kind of appropriate) - water to drip steadily through the gauge.
Just for clarification, the dropper is merely to get both spoons to tip as close together at the specified amount required. Then you use a much larger amount to fine tune by moving both screws up or down the exact same amount to keep the spoons as close to the same volume as possible. This method can also help correct for a less than perfectly level placement.

The dropper is really all you need if you know how much. That's the key and why the calibrator comes in handy you can select a volume of water knowing how many tips you expect.
Then check tippers and measure how much water they take to tip. Keep this information and share it Its all you need to recalibrate later.
Randy

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #20 on: July 14, 2018, 04:45:12 PM »
The results are in!  At least after two of my tests.  Both tests achieved an average rainfall rate of about 4 inches an hour using 500ml in a water bottle with a pin hole at the bottom.  So, we know that 500ml is supposed to equal 0.92 inches of rain in the VP2.  Here are the actual results

Test 1:  500ml = 0.89 inches
Test 2:  500ml = 0.90 inches

Some water may have been left in the tipper from test 1 to test 2, that could account for the extra hundredth.  Regardless, these numbers are technically within spec according to Davis.  I will also say that I performed this test during the middle of the day with bright sunshine, temp of 87, and dew point of 62.  Some evaporation may have occurred, but probably not enough to make a real different.

Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.

*Edit*, just opened unit to see the tipper.  It was about half full at the end of the 2nd test.  I think I was probably spot on at about 0.895 inches per test.  0.895/0.919 = 2.61%
I disabled all uploads while testing, then zeroed out the daily rainfall in the console after testing was complete.  Then, when I enabled uploads again it added the rainfall to CWOP, WU, etc.  Bummed.
Are you sure you cleared the data? That shouldn't have happened. You may want to try again. If you're successful, the CWOP data should clear by itself, WU dunno.

Ok once I cleared the "daily rain" AND "storm total" I am now reset to ZERO on WU and CWOP.  Unfortunately, pwsweather.com is still showing the rainfall.
« Last Edit: July 14, 2018, 04:47:23 PM by dport »

#### johnd

• Forecaster
• Posts: 2962
##### Re: Manual gauge for calibration
« Reply #21 on: July 14, 2018, 04:53:34 PM »
Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.

Definitely not would be my view. Best not to touch the factory settings at all IMO. It sounds like you're as close as you're ever going to get in practical terms to checking that the calibration of the gauge is properly within spec.
Prodata Weather Systems
Prodata's dedicated Davis EnviroMonitor website
UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #22 on: July 14, 2018, 05:40:33 PM »
Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.

Definitely not would be my view. Best not to touch the factory settings at all IMO. It sounds like you're as close as you're ever going to get in practical terms to checking that the calibration of the gauge is properly within spec.

Yeah I'm leaning towards your viewpoint.  I really wanted to test though because I read about some folks being 30-40% off from the factory. I'm so thankful mine is within spec.

Now, that said, I wonder if me being low by 2% has do to with actual rainfall not going into the bucket (but rather a water bottle with a pin hole).  Also to note is that I averaged 4 inches per hour rate during my test.  Perhaps if I used a rate of 2 inches per hour I would be at 0% error.  Who knows.

#### CW2274

• Forecaster
• Posts: 4162
##### Re: Manual gauge for calibration
« Reply #23 on: July 14, 2018, 05:47:51 PM »
Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.

Definitely not would be my view. Best not to touch the factory settings at all IMO. It sounds like you're as close as you're ever going to get in practical terms to checking that the calibration of the gauge is properly within spec.

Also to note is that I averaged 4 inches per hour rate during my test.  Perhaps if I used a rate of 2 inches per hour I would be at 0% error.  Who knows.
You averaged 4" per hour or started at 4? I start at 4 and by the time the jug is empty, it's about 1" per hr.

#### dport

• Senior Contributor
• Posts: 170
##### Re: Manual gauge for calibration
« Reply #24 on: July 14, 2018, 06:24:25 PM »
Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.

Definitely not would be my view. Best not to touch the factory settings at all IMO. It sounds like you're as close as you're ever going to get in practical terms to checking that the calibration of the gauge is properly within spec.

Also to note is that I averaged 4 inches per hour rate during my test.  Perhaps if I used a rate of 2 inches per hour I would be at 0% error.  Who knows.
You averaged 4" per hour or started at 4? I start at 4 and by the time the jug is empty, it's about 1" per hr.

I averaged about 4 inches an hour.  It started north of 6 inches an hour.  All said and done it took 16 minutes to dump all 500ml of water.  Averages to about 3.4 inches an hour if you do math. So, I take back my original 4 inch per hour mark

Most of the rain we get from May through August is in the form of thunderstorms with heavy rainfall rates.  I wanted it to be as accurate as possible for those instances.