Calibrating Davis VP2 rain gauge

As my VP2 is currently in bits awaiting a new temp/hum sensor I’m going to calibrate the rain gauge … possibly :?

Question - My gauge is set to record rainfall in mm. How much water would I need to dribble through the rain gauge to be equal to say 1mm of rain. Plan would be to dribble the equivalent of 20mm rain through with a bottle with pin hole in or similar. Anyway, can’t seem to work out how much water would equal 1mm of rain? Of course if I knew the amount for 1mm then I’d multiply by 20 for 20mm!!

TIA

Rico

  1. Measure the diameter of the mouth of the gauge in centimeters

  2. Calculate the area of the bucket opening:

    radius = diameter / 2

    area = 3.14159265 * (radius ^ 2)
    area = PI times the (radius squared)

  3. Multiply the area from #2 by the desired rainfall depth in centimeters.
    For 1 cm of rainfall (10mm), this is just the area calculated in step #2 (area times 1)
    This gives you the number of cubic centimeters (cc), also known as milliliters (ml),
    required to simulate a rainfall of the depth used

  4. Slowly drip the amount of water calculated in step #3 into the rain gauge

Steve

Good info here: http://www.nzweather.net/forum/topic/8480-true-calibration-of-rain-gauges/

Thanks to both of you :smiley: Steve, been racking my brains for the correct formula for working out volume of water but unfortunately it’s been far too many years since I did it in the class room #-o

Good info on the website. Read elsewhere of a device available for calibrating rain gauges that was basically a bottle with some method of controlling the flow of water. Seemed a good idea though expensive at over 100 dollars!

I am certain my Davis VP2 gauge is under reading and comparing it against a cylinder gauge during a rain event showed the manual gauge had 9% more than the Davis. So I am going to adjust the screws and fine-tune it over a period of time. If it is under-reading, by say 10%, this gives a loss of about 120mm annually in our climate 8O From what I have read across various forums including this one, this is not uncommon with the Davis gauge.

Anyway the adjustment sounded like it would be easy to carry out - except a 3/16" wrench is required, seems to be a very uncommon size and am having difficulty finding somewhere that will sell an individual one to me…don’t want to buy an expensive set when all I need is 1 wrench! #-o

I’ve had my new Davis VP2 going for about 10 days and it appears to be under reading in the rain gauge. I put in the mm conversion thingy and I’m wondering whether that was a good thing to do. I still have my old La Crosse 2308 operational and a standard old fashioned plastic farmers rain gauge. Over the last 24 hours the LaCross is reading 25.3 the manual rain gauge 27mm and the Davis 20.4mm. That’s a fair difference.
How did you get on making your adjustments? Would putting the gauge back to inch setting make it read higher or lower?

Has anyone else got any tweaking ideas for making it more accurate? The anemometer is mounted a meter above the ISS, with both having to face north (I’ve offset the ISS to face NW) I do worry a little that with both wind and rain, the cups may be deflecting rain away from the collector.

cheers

Tony C.

Hi, I just been adjusting the “screws” as they are called (but they aren’t actually screws, you need a small wrench to turn them) over the last couple of months whilst comparing to the manual cylinder gauge placed next to it

I’m happy to say they are usually very close with their readings now :smiley:

A comparison of the last few wet periods days:
Davis vs Manual
19.8 vs 19.1
14.0 vs 13.7
14.4 vs 14.4

I doubt switching to inches will make much of a difference at all.

Thanks Dave,

Any tips on how much adjustment to make (1 turn…2)? Presumably both screws are adjusted the same amount anti clockwise to make the tip less?
I’ve marked the screws and made a half turn upwards. A 6ba ring spanner works a treat. Now it’s wait for the next rain storm. Judging from the crap we’ve had this winter , it shouldn’t take long.

cheers

Tony C

If you take the rain cone off you’ll see next to the screws there is a guide as to how much to turn the screws. Turning each screw 360

I had a ring of small BA spanners and a 6BA fits well.
I’ve wound them both out half a turn and will try that. I guess there’s a fair amount of hit and hope (like my golf)!!

cheers

Tony C

One thing I would check is that it tips with the same amount of water on both sides. Mine was off quite a bit even thought it was level. Use a syringe to measure the amount of water. When you get them even, same amount of water both sides. When you are ready to calibrate, allways turn both screws the same amount.

Yes, I forgot to mention that. I did the same test on mine and found one side of the tipping bucket took noticeably more water to tip it. The screws are at quite different heights now…

Thanks for all those tips.
I should be able to get it right now.

cheers,

Hi…if someone could test…with a Davis VP2

how much reading will you have on the davis console with a 5 ml test tube or 10 ml (emptyed drop by drop with a dropper)?

thanks

Goodness!! Hadn’t realised I’d had my VP2 for nearly 6 years. Have to say that it’s performed faultlessly over the intervening years.
If you’ve read through the above notes, probably the most important aspect is to make sure both tippers tip with exactly the same amount of water. Then it’s a matter of making sure the readings equate with the amount going in. this might take you a month or two to get the accuracy you require. The initial factory setting shouldn’t be too far out.
Trial and error using an old fashioned plastic rain gauge for checking is the best way. My VP2 under reads about 1-2mm over a 25mm rain storm. I check it every few months to make sure the VP2 hasn’t had any mechanical hiccups. I’m reasonably happy with that. Anything that has moving parts and reed switches isn’t going to be 100% accurate all the time!

Just follow the calculation posted by N7XSQ at the start of this thread to get the volume in ml for 1 cm of rain, and then divide that amount by 5 or 10. But you won’t get a reliable result with such a small amount of water, you need to put enough water through the gauge for maybe a hundred tips / 1 inch / 2.5 cm and average the result.

My Davis V2 rain gauge was reading 20% less than a 4" manual gauge placed beside it. I checked the balance between the 2 buckets, and it was way out - 1 side tipped at about 48 drops, and the other about 83 drops! I had to turn one screw 3 whole turns before they were about equal - and then proceeded to tweak both together to get the calibration about right (4.28ml/tip).

It is on my to-do list to check my VP2 tipping bucket also, I will probably do this using a drip test. I have read a few forums that the VP2 can be very awkward to calibrate and may be best avoided unless absolutely necessary. I have also read (but can’t find the source at the moment) that the VP2 tipping bucket is biased for some reason, I haven’t found anything from Davis about this and what percentage it would need to be biased - 48-to-83 tips sounds high though.

I would be interested if anyone knows any more about this :?

Be nice if we had some rain to test it with, 39 days without so far.

teal.

I found that dripping water from an eye dropper into each side to compare was quite unreliable/variable - ie results did not have good repeatability (part of which is probably cause by losing count - any distractions and I was lost!). I then calibrated it using the drip method recommended by Davis using 43ml of water. However, that only represents 2mm of rainfall, and when each bucket holds 0.2mm - that’s an error of 10%.

I am now comparing it to a manual gauge mounted right up against the Davis during good rainfall (and it’s lucky we’ve had a bit of that recently). I have checked the calibration of the manual gauge, and it seems good (Davis Australia were not happy that their gauge was being checked by a mass-produced item :D). I am gradually winding the adjustment screws up so my Davis readings are scattered each side of the manual gauge reading (rather than all on the “less” side).