Author Topic: New guy here, and got a question about calibrating the rain gauge.  (Read 12188 times)

0 Members and 1 Guest are viewing this topic.

Offline Vettedrmr

  • Member
  • *
  • Posts: 15
Greetings all from down in (for the last few days at least!) rainy N. Texas!

We just got our first measurable rain since early January this last week, and I've noticed the rain gauge on my VP2 is off considerably when compared to a graduated glass tube.

First rain event was 2.20" on the VP2, the glass tube was 2.9.  2nd event a day later was 0.77" on the VP2, 1.0 on the tube.  The correction factor on the first event works out to 1.32, the second was 1.30, so that's pretty consistent

The VP2 is level in both planes, and was clear of any obstruction when I checked it a few weeks ago (roof mounted).  The glass tube is mounted about 75' away.

I did a search on the forum and didn't find anything.

How do I go about calibrating my rain gauge?

TIA, and have a good one,
Mike

Offline tomcj2

  • Forecaster
  • *****
  • Posts: 848
    • CanbyWeather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #1 on: March 14, 2009, 09:19:20 AM »
There are small screws under the buckets.  A full turn represents 6% change.  Full details at the Davis site, and there are many posts about it here.  If you cannot locate more information post again, or PM me.

Davis VP2 (6163), WL 5.9.0..  VWS 14.01 p25, Panasonic HM371A camera. WU & W4U KORCANBY3, CoCoRaHS OR-CC-27

Offline d_l

  • Forecaster
  • *****
  • Posts: 1700
  • Slide Mtn - Mt Rose
    • Thomas Creek Estates neighborhood weather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #2 on: March 14, 2009, 09:20:17 AM »
Welcome to the forum.

You say the ISS is roof mounted, but is the graduated tube also mounted at the same height?

If it isn't, see how gauge catch varies with height: http://www.onerain.com/includes/pdf/whitepaper/InconsistentRainGageRecords.pdf
--Dave--

Wireless VP2 w/ solar, 24hr FARS, Heater, (Envoy-WLIP)*3-Meteohub, plus custom VP2 @ 26', WL 6.0.4, WU & W4U=KNVRENO37 NetcamXL

People always talk about the weather, but they never do anything about it.  Not me.  I'm gonna measure it.  https://www.tceweather.com

Offline Vettedrmr

  • Member
  • *
  • Posts: 15
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #3 on: March 14, 2009, 09:39:59 AM »
Welcome to the forum.

You say the ISS is roof mounted, but is the graduated tube also mounted at the same height?

If it isn't, see how gauge catch varies with height: http://www.onerain.com/includes/pdf/whitepaper/InconsistentRainGageRecords.pdf

Interesting article.  No, the glass gauge is at about 5 feet, the roof mounted is probably 25 ft.  According to that article, that should be about a 5% variance.  I'm getting about 30%.

TomCJ2, thanks for the info.  I went to the Davis site first, but obviously I missed it.

Have a good one,
Mike

Offline d_l

  • Forecaster
  • *****
  • Posts: 1700
  • Slide Mtn - Mt Rose
    • Thomas Creek Estates neighborhood weather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #4 on: March 14, 2009, 09:58:59 AM »
Mike, remember that graph is for an unspecified wind speed condition.  Really there would be a whole series of curves, one for each wind speed.  So if the wind was blowing harder for your rains than that graph's wind speed, then the undercatch percentage would be greater for your gauges.

Keep that difference in mind if you want to do a correction factor.  Many people mount the ISS portion of the VP2 at a lower height and add an anemometer transmitter kit for the anemometer to avoid having to make a calibration adjustment for wind undercatchment.

Dave
--Dave--

Wireless VP2 w/ solar, 24hr FARS, Heater, (Envoy-WLIP)*3-Meteohub, plus custom VP2 @ 26', WL 6.0.4, WU & W4U=KNVRENO37 NetcamXL

People always talk about the weather, but they never do anything about it.  Not me.  I'm gonna measure it.  https://www.tceweather.com

Offline 92merc

  • BismarckWeather.net
  • Forecaster
  • *****
  • Posts: 1308
  • BismarckWeather.net
    • BismarckWeather.net
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #5 on: March 14, 2009, 10:47:39 AM »
Keep that difference in mind if you want to do a correction factor.  Many people mount the ISS portion of the VP2 at a lower height and add an anemometer transmitter kit for the anemometer to avoid having to make a calibration adjustment for wind undercatchment.


I have a VP1, but noticed similar issues.  It depended on which direction the wind was coming from.  Now that I moved to a new house out in the country, wind is a bigger issue.  I've brought my ISS down and used the anemometer kit.  I have a rain gauge like the one CoCoRahs recommends mounted on the same post.  After some leveling issues were taken care of, the 2 are spot on with each other.

I'd say you can't use that 5% mentioned in the article as gospel.  It will all depend on your type of winds in your area and the kind of house you have the ISS mounted on.  If you leave the ISS up on the roof and try "calibrating", you'll be chasing ghosts that you'll never catch.  Either bring it down, or leave it alone.
https://www.BismarckWeather.net
Davis VP2, Cumulus, WeatherDisplay, Blitzortung, Saratoga Scripts, NOAA Stream via PI

Offline George Richardson

  • WxElement panel
  • Forecaster
  • *****
  • Posts: 1391
    • Smith Mountain Lake Weather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #6 on: March 14, 2009, 11:00:37 AM »
Hey Mike,
Check http://tobaccovilleweather.com/about.htm for the thousand word picture. 92merc's recommendation is spot on, but if you do decide to adjust, I would urge you not to go more than the 1 revolution 6% at a time.
George

Offline Vettedrmr

  • Member
  • *
  • Posts: 15
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #7 on: March 14, 2009, 09:57:36 PM »
Thanks folks, I found the diagram on Davis' website showing where the adjuster screws are.  This last rain event the winds were fairly light and constant, averaging around 7 mph.  I understand how wind variations can make fairly significant differences, but I also thought the measurement differences for the last two rain events were pretty close to each other.

I'm also not planning on taking the screws out 5 full turns (a 30% delta), but was thinking about going 2 turns and see what happens.  Only problem is it might be another 6-8 weeks before we get any more rain!  ](*,)

Have a good one,
Mike

Offline mmorris

  • Forecaster
  • *****
  • Posts: 766
  • Hope your day is full of sunshine
    • Weather and Racin
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #8 on: March 14, 2009, 10:20:07 PM »
This is just my thought on this. I believe that Davis put those adjustment screw in the rain gauge for them to adjust the rain gauge. I payed them for equipment that was suppose to be right when I received it.(one drop of water is one drop of water) I think I would send it back and tell them to look at it. Or get it NIST Davis site says it $50.00 for the cert. 
>>Miles<<  By from Portage Lakes, OH.
Been using VWS since 1996 Ver# 14.01P43
Wireless Vantage Pro2Plus Serial Data Logger, Anemo Tran Kit
Win XP, Firefox, WXSIM, Cumulus, NexStorm, Yawcam, VVP, BadBlue Web server, Quake Catcher Net
Follow me on twitter
Vietnam era Veteran USAF bb loader
Quadruple Bypass survivor

Offline ocala

  • Forecaster
  • *****
  • Posts: 4383
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #9 on: March 14, 2009, 10:34:00 PM »
Put an inch of rain in that tube. Pour that into a styrofoam cup and put a pin hole at the bottom. Put that cup in the gauge and see what it reads.
Adjust the screws up to increase the reading.
Adjust the screws down to decrease the reading.

I just got done borrowing a Nova Lynx calibration kit from Chris at Sloweather.
I came to the conclusion that the Davis gauge is best used as a rough estimate and shouldn't be used as your official total. Keep a tube close by for an actual reading.
The blues had a baby and they named it Rock & Roll

Offline mmorris

  • Forecaster
  • *****
  • Posts: 766
  • Hope your day is full of sunshine
    • Weather and Racin
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #10 on: March 14, 2009, 10:43:06 PM »
Put an inch of rain in that tube. Pour that into a styrofoam cup and put a pin hole at the bottom. Put that cup in the gauge and see what it reads.
Adjust the screws up to increase the reading.
Adjust the screws down to decrease the reading.

I just got done borrowing a Nova Lynx calibration kit from Chris at Sloweather.
I came to the conclusion that the Davis gauge is best used as a rough estimate and shouldn't be used as your official total. Keep a tube close by for an actual reading.

I was just going to say why wait for the rain if it that important. But you took care of that. 
>>Miles<<  By from Portage Lakes, OH.
Been using VWS since 1996 Ver# 14.01P43
Wireless Vantage Pro2Plus Serial Data Logger, Anemo Tran Kit
Win XP, Firefox, WXSIM, Cumulus, NexStorm, Yawcam, VVP, BadBlue Web server, Quake Catcher Net
Follow me on twitter
Vietnam era Veteran USAF bb loader
Quadruple Bypass survivor

Offline Vettedrmr

  • Member
  • *
  • Posts: 15
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #11 on: March 14, 2009, 10:44:03 PM »
Put an inch of rain in that tube. Pour that into a styrofoam cup and put a pin hole at the bottom. Put that cup in the gauge and see what it reads.

It's not that easy.  The Davis rain gauge has a different opening size than the glass tube (by a lot!).  For your system to work the glass tube would have to have the same diameter as the VP2's gauge.

I'm going to be keeping the glass tube around as a check for quite some time.

Thanks again, and have a good one,
Mike

Offline JOE

  • Forecaster
  • *****
  • Posts: 390
    • Centralia IL Weather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #12 on: March 14, 2009, 10:58:18 PM »
Put an inch of rain in that tube. Pour that into a styrofoam cup and put a pin hole at the bottom. Put that cup in the gauge and see what it reads.


That won't work.

The volumes are way different.  An inch of rain in a little diameter tube probably won't even cause a cup to tip.

A Davis gauge is what about 7.5 inches in diameter?  That's about 44 Sq inches in diameter.

A glass tube is maybe 3/4 in diameter.  That's only 0.44 Sq inches.

A 100 times difference.
Joe
www.centraliaweather.com
DW1376
KILCENTR2
K9RJN


Offline ocala

  • Forecaster
  • *****
  • Posts: 4383
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #13 on: March 14, 2009, 11:00:10 PM »
Put an inch of rain in that tube. Pour that into a styrofoam cup and put a pin hole at the bottom. Put that cup in the gauge and see what it reads.

It's not that easy.  The Davis rain gauge has a different opening size than the glass tube (by a lot!).  For your system to work the glass tube would have to have the same diameter as the VP2's gauge.

I'm going to be keeping the glass tube around as a check for quite some time.

Thanks again, and have a good one,
Mike

Duh :oops:
Had a brain fart on that one.
For the Davis Gauge 1 inch of rain equals 544ml or 18.39 ounces.
Sorry about that.
The blues had a baby and they named it Rock & Roll

Offline tomcj2

  • Forecaster
  • *****
  • Posts: 848
    • CanbyWeather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #14 on: March 14, 2009, 11:18:44 PM »
I did the math many months ago and came up with the same figure.  I intend to use 272.02 grams of water for 1/2 inch reading.  Presently I am getting correct readings by putting a "gain" into the console, but that has resulted in some reporting issues to other sites.  When we start our dry season I will calibrate it properly. 
I compare my Davis reading with the CoCoRaHS gauge every day, and have been surprised that they are normally within .01 inches.  The largest discrepancy was .03 inches on a day that we had more than 1 inch of rain

Addendum 03/15/2009.  After posting above how accurate the Davis gauge was I did the readings this morning.

CoCoRahs Gauge  0.79 in
Davis                   0.86 in

The gauges are about 20 feet apart, both 5 feet above ground
« Last Edit: March 15, 2009, 09:10:03 AM by tomcj2 »

Davis VP2 (6163), WL 5.9.0..  VWS 14.01 p25, Panasonic HM371A camera. WU & W4U KORCANBY3, CoCoRaHS OR-CC-27

Offline Vettedrmr

  • Member
  • *
  • Posts: 15
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #15 on: March 27, 2009, 09:32:53 AM »
UPDATE:  I moved the bucket screws out 1.5 turns (supposedly a 9% change), and earlier this week we finally got some measurable rain.

Before the adjustment my VP2 was about 30% low.  After this last rain the glass tube measure right at 0.4", the VP2 measured 0.33".  The calculator computes that as a 21% difference.  So, at the least the 6%/revolution seems to be pretty close.

I'm going to get some more measurements before I make another round of adjustments, but so far so good.

Have a good one,
Mike

Offline Vettedrmr

  • Member
  • *
  • Posts: 15
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #16 on: March 31, 2009, 05:28:59 PM »
Update #2:  We got a small shower come through last night, and the difference was .25 tube, .20 VP2, for a correction factor of 25%.  So it looks like I need to take about 2.5 turns out again.

In Davis' defense, I think my son was fiddling with the screws when we were getting everything set up initially (kids,  :oops:)

Have a good one,
Mike

Offline ocala

  • Forecaster
  • *****
  • Posts: 4383
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #17 on: March 31, 2009, 06:20:52 PM »
I'm still not confident in the Davis gauge.
Saturday we had a long slow rain. The Davis gauge registered .56. The cocorahs gauge said .47.
Today I had a 5 minute down pour. Davis gauge .32. Cocorahs gauge .32.
Go figure. You would figure the slow rain would be more accurate, instead it was just the opposite.

Hopefully your adjustments will pay off for you but  once they get close , leave it. Trying to get them exact is too hard.
Davis says the the gauge has an error margin of +/- 4%
The blues had a baby and they named it Rock & Roll

Offline Vettedrmr

  • Member
  • *
  • Posts: 15
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #18 on: March 31, 2009, 10:07:59 PM »
Yeah, if I can get it within +/- 5% I'll be happy.  30% error I'll work on.  I took 2 full turns out of the adjusters today; we'll see hopefully in the next month or so when it rains here.  :roll:

Have a good one,
Mike

Offline LFWX

  • Forecaster
  • *****
  • Posts: 1089
  • Liberty Township, Butler County, Ohio
    • Liberty Fairfield Weather Center
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #19 on: April 01, 2009, 01:14:25 AM »
Quote
In Davis' defense, I think my son was fiddling with the screws when we were getting everything set up initially

Oh, WOW!
That could mean one screw needs to be adjusted more than the other, have fun getting it adjusted.

Maybe you should get a dropper and see if it takes the same number of drops to get the bucket to tip each direction.
Station: Davis Vantage Pro2 Fan Aspirated
Software: Virtual Weather Station V14.00p64
www.LFWeatherCenter.com
CWOP: DW1039
CoCoRaHS: OH-BT-1
Weather Underground: KOHHAMIL7
Weather For You: DW1039
Midwestern Weather Network

Offline tomcj2

  • Forecaster
  • *****
  • Posts: 848
    • CanbyWeather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #20 on: April 01, 2009, 08:29:48 AM »
[
That could mean one screw needs to be adjusted more than the other, have fun getting it adjusted.



If the gauge is properly calibrated are:
 (1)the tops  both of the screws exactly the same height from the base.
 or
(2)is each side adjusted independently by weight required to tip?

If #1 is correct I was going to use a feeler gauge, or turn both screws all the way in, and then back them out the exact same number of turns.

I would then verify by using the eye dropper.

Any other suggestions?






Davis VP2 (6163), WL 5.9.0..  VWS 14.01 p25, Panasonic HM371A camera. WU & W4U KORCANBY3, CoCoRaHS OR-CC-27

Offline sam2004gp

  • Mount Crawford, Virginia
  • Forecaster
  • *****
  • Posts: 2865
  • Weeeeeeeee!!!!
    • Mount Crawford Weather, VA
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #21 on: April 01, 2009, 11:31:19 AM »
That sounds like the best way to me, but then I don't have a Davis.  The logic of it seems sound.
SAM --->>> http://www.mountcrawfordweather.org
OS WMR-968 with a Dedicated PWS Weather Computer running VWS v13.01 p09


Offline SlowModem

  • Weather at the speed of dialup!
  • Forecaster
  • *****
  • Posts: 6641
  • WX @ 26.4 kbs
    • Watts Bar Weather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #22 on: April 01, 2009, 05:59:14 PM »
Here's a fun math problem that will help you determine the accuracy of your Davis rain gauge: The rain collection area of the DAVIS VP2 rain gauge is approximately 33.2 in2 (A = pi * r2, so 33.2 in2 = (3.25)2 * pi), and the rain collection area of the Stratus gauge is approximately 12.56 in2 (2*2*3.14 = 12.56). If you can determine what amount of water in the Stratus graduated collector that is equivalent to 1" of water in the Davis rain gauge, you could then use that amount to calibrate your Davis rain gauge without having to do much guessing. I know, fun stuff.  :lol:

Bruce

Ummmmmmm.................Is the answer 3?

Greg Whitehead
Ten Mile, TN USA

Offline JOE

  • Forecaster
  • *****
  • Posts: 390
    • Centralia IL Weather
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #23 on: April 01, 2009, 06:00:59 PM »
Here's a fun math problem that will help you determine the accuracy of your Davis rain gauge: The rain collection area of the DAVIS VP2 rain gauge is approximately 33.2 in2 (A = pi * r2, so 33.2 in2 = (3.25)2 * pi), and the rain collection area of the Stratus gauge is approximately 12.56 in2 (2*2*3.14 = 12.56). If you can determine what amount of water in the Stratus graduated collector that is equivalent to 1" of water in the Davis rain gauge, you could then use that amount to calibrate your Davis rain gauge without having to do much guessing. I know, fun stuff.  :lol:

Bruce

Ummmmmmm.................Is the answer 3?




Blue?
Joe
www.centraliaweather.com
DW1376
KILCENTR2
K9RJN


Offline AZmonsooncats

  • Current weather conditions in North Phoenix, Ariz
  • Senior Contributor
  • ****
  • Posts: 248
  • Me & My Little Sis!!
    • Pepper Ridge North Valley  Weather and Lightning
Re: New guy here, and got a question about calibrating the rain gauge.
« Reply #24 on: April 01, 2009, 08:00:36 PM »
Interesting observation Ocala  #-o
I'm still not confident in the Davis gauge.
Saturday we had a long slow rain. The Davis gauge registered .56. The cocorahs gauge said .47.
Today I had a 5 minute down pour. Davis gauge .32. Cocorahs gauge .32.
Go figure. You would figure the slow rain would be more accurate, instead it was just the opposite.

Hopefully your adjustments will pay off for you but  once they get close , leave it. Trying to get them exact is too hard.
Davis says the the gauge has an error margin of +/- 4%

I have observered the same thing, slow rains have larger errors :roll:, I agree with you about the confidence in the gauge :-x when compared to the "4 inch manual gauge. I have not made any adjustments yet as lately it rains so little here, but when we do get a rain it usually 0.01 diff per 0.10 of rain but it varies and the hard rains the difference is smaller or the Davis measures slightly more, so which gauge is more accurate :!: ](*,) who really  :roll: knows????

Jeanette
"Some dreams are in the night time, And some seem like yesterday
But leaves turn brown and fade, Ships sail away
You long to say a thousand words…but Seasons Change."



CWOP:CW3794, MesoWest: C3794, Facebook: https://www.facebook.com/PepperRidgeNorthValleyWX/

 

anything