Vantage Pro2 Rain Calibration Screws

[KeithC]

Does anyone have any experience with calibrating the Pro2 rain measurements by way of the adjustment screws?

I'm not going to try to pour measured amounts of water to simulate rain, that sounds way too unpredictable. I just want to raise these two screws 2 full turns (12% increase) in order to more closely match the measurements of my 2 standard basic graduated cylinder type gauges. They both read 3" for a recent all-day rain event while my Pro2 read 2.51".

After adjusting I'll wait for another rain day to assess and re-adjust if necessary.

I don't want to over-adjust because some of the discrepancy could very well be the result of a faulty reed switch (Vantage is sending me a new one). But my initial test for a reed switch showed no problem with counting 12 bucket tips.

Am I getting myself into a big adjustment mess by doing it this way?

[Steve]

Keith,

I'm not sure where I got it. Probably from the WXForum. Here's a set of instructions to adjut your rain gauge using the measured drip method. The reason for multiple tests is to simulate different rates of rain. The tipping type have trouble keeping up with heavy rain, so might under-report (well, under-report more, as tipping type have a reputation for under-reporting vs a NWS type collector.) Two full turns sounds like a huge adjustment. 12% of what? You don't want it to get so high that it is too near the tipping point, as wind blowing through the gauge could then tip it.

Pour 27.9 ounces of water into a plastic jug with a small hole poked in the
bottom allowing a slow drip (to spend say, two hours draining) into the
funnel, and calibrate it to read 1".  Poke a few more holes in the bottom to
increase the rate, and retest.  Repeat until the jug is drained in say, 15
minutes.

[KeithC]

I like that plan. Thanks Steve!

You already said you don't remember where you got it, but I wonder how the number 27.9 ounces is arrived at.  Vantage's formula?

Question: then what do I adjust to 1"? The screws I'm assuming. Or the console reading maybe, somehow?

To answer your question, it's 12% of the under-measurement (2.51"). So if I adjust the screws two full turns I would theoretically adjust upward the measured rain from 2.51" to 2.81".

[Steve]

Keith,

I had that clipping in my Davis Instruments folder, so I at least know it related to that. Since I have the VP2, it was most likely specific to it, too. Beyond that, I don't remember where it came from...

Yes, the screws would be adjusted to agree with the one inch of "rain" you've poured in. Raise the screws if under-reading, lower if over-reading. I remember reading that you should listen to the clicks during tipping. Most likely one bucket will tip sooner than the other. To increase your reading, raise the side that has the longer wait (time to fill) to balance them out. Lower the shorter side if your gauge is over-reading.

Make sense? Click............click..click..........click..click......etc. Both sides should take the same amount of time to fill and tip.

I see your logic on the 12%, but I'm not sure if I agree with the amount to adjust for it. 12% of the 100 tips it takes to read 1.00" is 12 tips. You'll need to adjust the screws upward so that the theoretical 88 tips you are getting now increases to 100 tips. I don't know where I going with this, but two full turns seems like a LOT for such a fine adjustment. Each side only has to increase 6 tips per inch to get to your target. I'm not sure, but I thought I remember people talking about making 1/4 turn adjustments.

I'm rambling....

[KeithC]

You've been a big help. Thanks again.

Just to clarify the 12% thing let me show you how I understand it (please correct me if I'm wrong).

In my example my Pro2 read 2.51" when it should have read 3.00". So the correction needs to be as follows: 2.51 divided by 3.00 = 84%, which means I'll need a 16% correction. Each full turn of the screws counterclockwise = 6% upward correction. Therefore I'll need slightly less than 3 full turns of both adjustment screws (I was planning to start with 2 turns then wait for another storm to see some results).

Both screws are factory set at the exact same height and it is important to put a small mark on the screws so both screws can be turned the exact same amount. Supposedly it is not considered excessive to make such adjustments, relatively common in fact.

[mcrossley]

I adjusted mine much more than that. It did not have the metric conversion kit, so I added a bit of weight myself, and used the screws for the rest of the adjustment. I used a known volume of water (I forget the value, but it is easy to calculate from the funnel collection area as publish by Davis).

When doing my drip test calibrations I noticed the rain rate was going up and down - a check of the buckets showed they were far from balanced when suspended freely upside down from the pivot. I balanced them by adding a bit of weight to one bucket. After that the rain rate was consistent with tips on each side.

After the drip calibration it is in almost perfect agreement with a nearby VP2 - though the days when we have consistent rain over a wide area are not that frequent.

[KeithC]

Mark,

Just curious, is your Pro2 reading close to the other Pro2 during both heavy and light rain?

[mcrossley]

In light and medium it is pretty close, heavy tends to be very localised events so comparison is all but impossible. Looking at the monthly totals, mine first...

Code: [Select]

May  62mm  64mm
Jun  58mm  62mm
Jul 100mm 107mm
Aug  73mm  63mm
You can see the greatest difference was in August where most of the rain total occurred in heavy downpours. I think the calibration of the other station has not been touched from new.

[KeithC]

Mark,

You're observations, or maybe assessments is a better word, are spot on. Thank you.