Topic: Davis Vantage Pro2 rain guage calibration

[sbj04769]

According to my measurements:

Rain collector:

   6.5 inches in diameter = 33.183072 square inches in area.


1 gallon = 128 ounces = 231 cubic inches = 3785 ml

231 cubic inches = 3785 ml

1 cubic inch = 16.387 ml

250 ml = 15.26 cubic inches

15.26 cubic inches = 33.183 square inches x 0.46 inches

Therefore:

250 ml of water should result in 0.46 of water collected by the rain gauge.

I have a volumetric flask that accurately measures 250 ml. I pour this (quickly of slowly) into my new and unused Vantage Pro rain gauge (I just cut the retaining strap) three times and it recorded 0.35 inches of rain.

I was told (I think) by Davis that the Vantage Pro2 rain gauge needs 5.44 ml to tip. I measured each side of the tipping bucket and one side tipped at 4.82 ml and the other tipped at 5.92 (average 5.37 ml)

If that is correct, my 250 ml pour volume should tip 46 (45.95 times) giving me 0.46 inches of rain, which is what I calculated above, not 0.35 inches

I repeatedly measure 0.35 inches of rainfall with a 250 ml pour volume, corresponding to 7.14 ml per tip.

I have several of these units and am looking of a set volume to pour in and a corresponding number. I have been told by Davis that I could send it into have it checked but it just came from them!

Could anyone clear this up for me?

Thanks

Steve

[SLOweather]

Could anyone clear this up for me?

Sure...

Anecdotally, VP2s are never very close when new. It takes so long to get one accurate, that I suppose they just do it with calibrated weights or something and call it good enough.

I have a NovaLynx calibration setup with a separate electric counter I use to adjust my rain gauges. It makes it very easy to do several runs while I'm doing other things.

There is a leveling screw under each half of the bucket. One turn = ~6% change.

My 951ml bottle and the smallest orifice on the calibrator take about 35 minutes to run through the gauge. That should be 174 tips, and is a rain rate of about 3.5" per hour. That's faster than I'd like it to go, but that's using the smallest orifice that came with the calibrator.

I would encourage you NOT to try to adjust each bucket individually with 5.44 ml of water. Been there, done that, very frustrating.

If I get within a tip or 2, experience has shown me that it's good enough. Chasing the last tip is madness.

One other key point is to do successive runs with as constant a flow rate as possible.

[ocala]

Where the tipper makes contact there is an adjustment screw. If you look closely it has a +/- sign on it. In your case since its measuring low you want to turn it to the + side.
Best way to do it is mark the adjustment screw with a marker before you start. Turn it a quarter turn on each side then pour your water. Also instead of pouring the water in you may want to get a plastic cup and put a small hole in the bottom. After each test adjust accordingly.
Edit.
Oops, I see Chris beat me to it.

[Flag]

I pour this (quickly of slowly) into my new and unused Vantage Pro rain gauge

"Pouring" water into a tipping rain gauge is subject to many variations all of which will generally give inconsistent and incorrect results. calibrating a tipping rain gauage takes some time.

[kray1000]

My 951ml bottle and the smallest orifice on the calibrator take about 35 minutes to run through the gauge. That should be 174 tips, and is a rain rate of about 3.5" per hour. That's faster than I'd like it to go, but that's using the smallest orifice that came with the calibrator.

According to my instructions, the 1/32" orifice equates to 2" per hour.

Chris, in a previous thread on this subject I think you mentioned trying to take one of the larger orifices and modifying it to produce a slower flow rate.  Any luck on that front?

[sbj04769]

OK, I'm listening. I poured exactly 150 ml through my new rain gauge by putting it into a plastic bottle and poking a hole in the bottom of the bottle. My rain rate was from 0.95 to 1.62 inches per hour. I calculate with the above numbers that I should have 0.276 inches of rain. My bucket measured 0.25 inches of rain accumulated.

Interesting.

Perhaps I should do a rain rate to accuracy correlation....


Steve

[SLOweather]

According to my instructions, the 1/32" orifice equates to 2" per hour.

Do you have the NovaLynx calibrator as well? As I recall, mine did not include instructions for the Davis rain cone diameter, so I had to do my own calcs.

Chris, in a previous thread on this subject I think you mentioned trying to take one of the larger orifices and modifying it to produce a slower flow rate.  Any luck on that front?

[/quote]

Not yet. I think I didn't have a drill bit smaller than 1/32" when i was thinking about it.

[SLOweather]

Perhaps I should do a rain rate to accuracy correlation....

Calibrate, then correlate...

Chris

[DanS]

According to my measurements:

Rain collector:

   6.5 inches in diameter = 33.183072 square inches in area.


1 gallon = 128 ounces = 231 cubic inches = 3785 ml

231 cubic inches = 3785 ml

1 cubic inch = 16.387 ml

250 ml = 15.26 cubic inches

15.26 cubic inches = 33.183 square inches x 0.46 inches

Therefore:

250 ml of water should result in 0.46 of water collected by the rain gauge.

I have a volumetric flask that accurately measures 250 ml. I pour this (quickly of slowly) into my new and unused Vantage Pro rain gauge (I just cut the retaining strap) three times and it recorded 0.35 inches of rain.

If that is correct, my 250 ml pour volume should tip 46 (45.95 times) giving me 0.46 inches of rain, which is what I calculated above, not 0.35 inches

I repeatedly measure 0.35 inches of rainfall with a 250 ml pour volume, corresponding to 7.14 ml per tip.

I may be way off here but I'm seeing 543.7731ml = 1cu.in. for a 6.5" dia. gauge (33.183cu.in. = 543.78ml volume) .  So divide in half, 272ml to get .50" and you're seeing 250ml for .35 may not be so far off.

[SLOweather]

So, my calcs say you are 9.4% low (not enough tips).

You need more tips. Raise each screw 1 turn (6%) and rerun it, and see what happens.

OK, I'm listening. I poured exactly 150 ml through my new rain gauge by putting it into a plastic bottle and poking a hole in the bottom of the bottle. My rain rate was from 0.95 to 1.62 inches per hour. I calculate with the above numbers that I should have 0.276 inches of rain. My bucket measured 0.25 inches of rain accumulated.

Interesting.

Perhaps I should do a rain rate to accuracy correlation....


Steve

[sbj04769]

I may be way off here but I'm seeing 543.837ml = 1cu.in. for a 6.5" dia. gauge.  So divide in half, 272ml to get .50" and you're seeing 250ml for .35 may not be so far off.

250/272 = 0.919

0.919 * .5 = 0.4595 or about 0.46 inches. I think my calcs are correct.

Steve

[kray1000]

Do you have the NovaLynx calibrator as well? As I recall, mine did not include instructions for the Davis rain cone diameter, so I had to do my own calcs.

Refer to page 6 of the Novalynx 260-2595 Destruction Manual.  (Referring just to the part about the flow rate produced by the 1/32" orifice).

[ocala]

OK, I'm listening. I poured exactly 150 ml through my new rain gauge by putting it into a plastic bottle and poking a hole in the bottom of the bottle. My rain rate was from 0.95 to 1.62 inches per hour. I calculate with the above numbers that I should have 0.276 inches of rain. My bucket measured 0.25 inches of rain accumulated.

Interesting.

Perhaps I should do a rain rate to accuracy correlation....


Steve

Thats pretty close Steve.
If you take off the bucket and look at one of the tippers you may have almost enough in one of them to produce 1 more tip, giving you .26.
You can go crazy, as Chris said, trying to get it exact. Sounds like you're there.

[dvautier1941]

Hi
I just wanted to add my procedure for calibrating the Davis pro2.  it may be of help to somebody.

  http://www.dvautier.com/misc/davis.htm