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Davis Vantage Pro 2 - Rain measurement calibration

Discussion specific to Davis weather stations
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Darmain
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Davis Vantage Pro 2 - Rain measurement calibration

Post by Darmain »

Hi,

It's been a fair while since I've been on the forum. I've been running Cumulus for a fair while now and it is nothing short of Brilliant. I am hoping that there will be folk on here that can assist me with a problem I "believe" I have. I have been led to believe that my rain measurement unit is under reading. I therefore set about attempting to test it and to re-calibrate it if necessary. This is what I did.

The unit is configured for measurements in millimetres. I understand this to represent 1 mm of water depth on one square metre surface, which relates to one Litre of water.

I calculated the measurement factor for the gauge as follows. The circular bucket is 164 mm in diameter. I therefore calculated that the surface area of this catchment is PI * (164 / 2 )2, which gives an area of 21124 mm2. Now the standard area would be 1000000 mm2. Therefore the bucket represents 2.1124% of this. (I am really hoping I'm getting this right.)

So I therefore used this ratio to calculate the physical volume of water that the measurement system would determine to be 1 mm of rain, with is calculated as ( 1000 (ml) * ( 2.1124 / 1000) ) * 1 = 21.124 ml.

So to test this I measured 211 ml, or 10 mm worth, best I could and poured it very slowly through into the bucket, listening for the clicks of the seasaw. As the system is supposed to represent 0.2 mm of rain per click, this amount of water should give me 50 clicks.

After several repetitions, using two different measuring jugs, I got between 30 and 32 clicks. As this represents 6 mm to 6.4 mm I determined there was a problem. To back this up I regularly look at the GFS model and predict the amount of rainfall. I have noted that as a general rule the reading of the weather station was about half of what was predicted. Suddenly two and two were put together.

So far so good, until I attempted to correct the rain gauge calibration. This is done by raising the stops that determine the tipping point of the seasaw. The stops are now as high as they can go, without modification. Any higher and the seasaw probably won't operate properly. There is a ballast weight on the seasaw, which is fitted if you want the measurement in millimetres, rather than inches. I could start removing material from this to lighten it, and then use the screws to fine tune it. This is a big step and probably a one way one, so I wanted to be sure I have calculated by test volume correctly.

So the question is, have I got all my assumptions and sums right? Any assistance, before I start filing anything, would be very much appreciated.

Many thanks and best regards,

Dave
Darmain
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Joined: Wed 07 Mar 2012 10:52 pm
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by Darmain »

Further to this. I have since read here - http://athena.trixology.com/index.php?P ... 59#msg5359 - that 500 ml with give a reading of 23.4 mm.

So if I take ( 500 / 23.5) * 10, I get a value of 213.6 ml, which is very close to what I calculated.

I note that it is supposed that the bucket diameter is 165mm in diameter. Not according to my engineering ruler its not....

So, if 1 mm of rain is 1 mm depth in 1 square meter, what is 1 inch of rain?
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steve
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by steve »

Darmain wrote:So, if 1 mm of rain is 1 mm depth in 1 square meter, what is 1 inch of rain?
1mm of rain is exactly that, 1mm over any given area. In terms of volume, as you say above, it's equivalent to one litre per square metre. One inch of rain is one inch over any given area.

Assuming you poured your water in very slowly (no more than a drip at a time) it sounds like you have a problem with your gauge, and I don't advise trying to modify it to get it to read correctly. Sometimes the reed switch can get 'sticky', so some tips don't register, but you were actually counting the tips yourself anyway, so that can't be the issue. I can't suggest a cause for the problem, though. Does the 'seesaw' move freely?
Steve
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mcrossley
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by mcrossley »

I calibrated my gauge when I got it, and did my own imperial to metric conversion (I hadn't seen how cheap the genuine adapters were on ebay :roll: still I need one there and then) so I am fairly familiar with them. Like Steve says there is something wrong if you are having to adjust the screws that far. I'd take the tipper out (the 'axle' pin just slides out) and give it a good clean. Be careful not to damage the reed switch in the housing. Make sure everything is totally free and not catching when you put it back.

PS: When I removed my bucket I found it was out of balance quite a bit which was giving asymmetric tip rates.I had noticed that during my drip calibration tests, I was using the console and the rain rate went up and down for each left/right tip. So you may want to check that whilst it is out. Just hang it upside down and see if it is level.
Darmain
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by Darmain »

Thanks chaps. Oh course, it would be 1mm over any area, assuming uniform rain fall. So that leads me to conclude my sums are right and the station is wrong. My tests involved me listening for, and counting the bucket tips. I had the reed switch unplugged so to prevent my totals being corrupted, if the system was working accurately. With the screws fully up my 10mm of rain gives me 46 tips, so it's still too low. The bucket appears to be free moving. What bothers me is the apparent randomness of the metric conversion ballast weight. Just a lump of plastic. Tolerance never great. I will strip down the unit ( when the rain stops) and give it a service. Then try calibrating it again.
Many thanks,
Dave
Darmain
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by Darmain »

Hi, I believe I have the rain measuring device working accurately. I discovered the seesaw was out of balance and had to file a series of notches in the underside to reduce the material mass until it did balance. Then I had to lighten the ballast until it started giving the right count, or at least as close as I could. I'm getting 110 to 120 clicks for 500 ml of water, which would represent 117 clicks. As I'm pouring the water slowly in by hand then this seems good enough considering potential over filling of the seesaw bucket with the high flow rate.
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mcrossley
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by mcrossley »

Darmain wrote:I discovered the seesaw was out of balance and had to file a series of notches in the underside to reduce the material mass until it did balance.
Ha, I added some 'Bluetack' to mine!
Darmain
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by Darmain »

I thought of adding mass but then considered that it would slow the response of the seasaw and the more obvious risk of it falling off. The adjuster screws are still fully out but its reading right, or at least a great deal better than it was. Thanks for your help with this. It's a little annoying to discover that the equipment that we spent a lot of money buying is so out of calibration. Still, all fixed now. :D
griffo42
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by griffo42 »

I believe the rain readings reported by my VP2 are not accurate with the actual rainfall. Is the information in this topic relative to making reportings of rainfall accurate still current?
Keith
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mcrossley
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Re: Davis Vantage Pro 2 - Rain measurement calibration

Post by mcrossley »

Yes, unless you have the new tipping spoon rain gauge.
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