Davis Vantage Pro - Recording rainfall

[David Smith]

Davis Vantage Pro - model of 10 yr vintage.

 

I have recently recently replaced the reed switch, before I never had any concern about the rain fall measurements, since I felt it did not record the rain fully.

 

My tests reveal that for every swing the recorder is measuring 0.1mm, I thought this should be 0.2mm.  The model is a UK version.

I put 0.2litres slowly into the vessel, the rainfall came as 2.7mm (and I counted 27 movements).  This should be around 9mm of rain.

 

Firstly any idea why it is only counting every movement as 0.1mm and not 0.2mm (I can only assume it has always done this).

 

 

[Mapantz]

You'll need to use the calibration stop screw on the gauge, i can't remember the direction it is but you can experiment moving it clockwise/anticlockwise and then pour some water in gently, keep an eye on the console to see what registers. 

Edited November 27, 2013 by Mapantz

[4wd]

I found the calibration screws fiddly, and nothing much seemed to happen with just one turn or whatever.You can set an offset in the console if you are sure it's recording half then +100%After mucking with mine on three separate occasions I still think it reads low and have put a +10% offset in.I have two other rain gauges and two other stations nearby to compare with (one was only a mile away but they moved away now)It seems commonly accepted VP2 reads on the low side.

[Mapantz]

I found the calibration screws fiddly, and nothing much seemed to happen with just one turn or whatever.You can set an offset in the console if you are sure it's recording half then +100%After mucking with mine on three separate occasions I still think it reads low and have put a +10% offset in.I have two other rain gauges and two other stations nearby to compare with (one was only a mile away but they moved away now)It seems commonly accepted VP2 reads on the low side.

One flat will only account for 1% difference in calibration, so a turn of 360 degrees will be 6%, you'll need some considerable adjustment for it to calibrate.To the OP, there should be two screws to fiddle with, if you're unsure, i suggest emailing Davis, they're not bad ar corrospondance.

[David Smith]

Thanks for the responses, much appreciated.  I have solved the first problem, during the summer I had to put the standard settings back in. On mine for the rain bucket, it has three calibrations ie 0.01inch, 0.1mm and 0.2mm.  I had selected 0.1mm in error.

 

I not convinced I have the metric measurement adapter, if this is the case then it will be measuring in 0.01 inch increments ie 0.254mm. Which will make it closer to the correct value in  my calibration exercise.

[reef]

Glad to see you got it sorted.

 

I cant speak for the Davis VP but certainly my Davis Vantage Pro 2 was exceptionally poor when it came to accuracy of recording rainfall out of the box, in fact it read some 25% too low. Even after calibration I still too this day run my manual gauge alongside it just to be sure.

[David Smith]

Thanks Reef.

I have now selected 0.01 in in the setup as I'm sure I don't have the metric adapter.  From my previous calibration this should be reasonably close.

But I can't remember what setup I selected 10 yrs ago, on the original setup.

If the rainfall setup is on 0.2mm bucket and it is actually calibrated for 0.01 in, that will result in 25% under estimating of the actual rainfall.

[johnholmes]

I have a similar problem and keep meaning to do a check with a manual home made guage but never get round to it. I am generally about 25% down on the Davis with two sites within 3 miles of me, each in back gardens not that different to mine. I have actually visited one of them. Bit late now after 7 years!

[reef]

The best way to check is to very slowly drip 544ml of water into the gauge and see what reading it gives you. A tip on the Davis rain gauge (0.2mm) is 5.44ml so 544ml should give you exactly 20.0mm. 

 

I weighed 544ml of water on scales (should be around 544g) and then filled a plastic container and pin-pricked the bottom to slowly allow the water to empty into the gauge.

 

This is also a good way to measure snowfall in the raincup in winter. All you have to do is place a container on the scales, zero it, take off the rain cup and melt it with a hair dryer into the container. You can then work out the rainfall equivilent by the weight remembering the 5.44ml per 0.2mm tip above.

 

My Davis raincup has the metric adaptor, but still read 25% under.

Edited December 6, 2013 by reef