VP2 Rain calibration

I purchased a 4" manual rain gage to measure snow and mounted it on the opposite side of a 4x4 post that my VP2 rain gage is mounted on and at the same height. I’ve only had rain two days since but it appears the VP is underreporting by about 9%.

Did a couple of searches on the net and here but can’t find definite instructions on how to calibrate it using the bolts under the tipping buckets. I found a post by Bob from the wxqc list that talks about this but not clear instructions.

Embossed on the baseplate is a dial with markings. At the 12 o’clock position is a +|- then marks at 2 o’clock and 4 o’clock are the numbers 1 and 2. At 6 o’clock is a % sign. Finally at 10 o’clock and 8 o’clock are also the numbers 1 and 2. So I’m thinking the numbers are 1 and 2 % in either the plus or negative direction and one complete turn of the bolt would equal 6%. I am also thinking that raising the bolts (turning counterclockwise) will cause the buckets to tip more often causing the reading to increase.

Has anyone tried this and is my thinking correct? OR are there instructions that I haven’t found.

BTW, no spiders, bugs or dirt in the buckets that would appear to affect the readings. I also plan to take a few more readings before making any adjustments.

–Dave

Raising the bolts will reduce the amount of rain needed to make it tip. I would probably calibrate it with a measured amount of water.

I agree with you on both counts. In theory raising the bolts 1 1/2 turns should increase the readings by 9%.

–Dave

I might do this too before this tropical system moves down, as my VP is underreading too

However i’m thinking that it will underread again during heavy rainfall events… so maybe just the offset in WD will do the trick?

I’m sure the off-set in WD will do the trick. I also want the console to record the correct amount so that’s why I intend to make the adjustments. Not all that important because I rarely look at it anyway. Just more of a challenge than anything.

–Dave

I emailed Davis and got a reply two days later saying to call them because troubleshooting would be too difficult using email. I was just asking how not about troubleshooting but, oh well, I called them.

They were very friendly but really discouraged me from making adjustments to the rain gauge. He said that they feel that the plastic 4" manual gauges over report.

He did tell me how to do it and it’s what I thought. Use the embossed dial on the baseplate and turn in the direction indicated. But he did say one turn is 5% when the baseplate data says 6%.
He also said they calibrate by dripping 540ml of water into the gauge over a 45 minute period. So the bottom line is that before deciding the VP is under reading I’ll have to make a ‘dripper’.

However, any thoughts or experience that the manual gauges do over-read??

Dave

I recently bought a rain gauge (see here http://store.yahoo.com/cspoutdoors/alraingaug.html) to compare my VP2 amounts and am finding that they are very close. In the past 2 days I have had .31" according to the VP and the rain guage showed .32". I see no reason to attempt any adjustment of the VP rain bucket.

That’s the exact one I bought. I’ve had a consistent 9% under-read from the VP2 during 4 different rains.

–Dave

I’m going to test my VP1, it’s far from new now and I just reinstalled it after a hiatus. What I don’t have is a known to be accurate measure (and I don’t think Betty Crocker’s will cut it :slight_smile: ). How accurately are bottled water bottles filled I wonder? Be pretty easy to make a dripper by drilling a hole in the cap, and a small air hole in the bottom.

Save me the math, what amount of rain is 540 ml?

From what I am seeing here that gauge is accurate enough to use for adjusting the VP tip bucket.

[quote author=nikoshepherd

[quote author=nikoshepherd

Sorry, I meant inches of rain, I assumed that since they had given you that amount it equaled some round number in rainfall.

The 540ml is equal to 1" of rain.

–Dave

I’ll buy that. So a 500 ml bottle of water should be 0.93". I got less than that but I haven’t figured out how to make it drip slowly enough yet :frowning:

We talked about this once before and Bob came up with this calibrator. Maybe you can get some ideas from it. http://www.novalynx.com/260-2595.html

–Dave

Nice sig. :slight_smile: I had to turn them on to see what everyone was talking about.

[quote author=nikoshepherd

That would be perfect :slight_smile: but I don’t have one. Hypodermic tubing would work too. I used to have an office in a not so nice area of the city and there I could have picked up as many needles as I needed on the street between the parking lot and the office every morning #-o

Anyway I did manage to make a very slow dripper and dripped 500ml in 90 minutes. Unfortunately the result was exactly the same as the fast dripper, 0.76" vs the correct 0.93", which is way off 8O :frowning: so I’m going to have to think about this a bit. I’m wondering if the tipper shaft could be corroded, it’s stainless but here on the coast you soon learn that there are many grades of stainless and most of them will corrode in the salt fog.

Could you take the funnel off and drip directly into the buckets? That way you could see if any water is spilling out of the buckets before they tip.

–Dave