I installed a new rain gauge at the end of September 2009 as the old Rain-O-Matic was starting to fall apart after 10 years in the sun.

I've checked the calibration of this gauge and despite being stated at 0.01 inch or 0.254mm per tip, I found using a 25ml burette that it actually tips at 0.5mm when I was trickling water through the funnel using the calculation below. The funnel diameter is 111mm, smaller then the 6 inch funnel I had on the previous gauge.

As there are no adjustments on the tipping bucket, I have to accept the 0.5mm resolution and have adjusted my weather station calculations to suit. Overall, it shouldn't make much difference to the totals on a wet day but may miss the odd shower.

The tape on the bottom of the gauge is to prevent the funnel from detaching itself with wind vibration.

The slots in the gauge are small, hopefully small enough to keep spiders out!

 

 

Previous Rain Gauge - Feb 2000 to September 2009


I was using a Rain-o-Matic rain gauge for my project. I have checked the calibration and the resolution is 0.2mm.

The WX175 kit counts the raingauge tips and these are in turn, read by the weather PC. This enables the rainfall to be displayed every 10 minutes.

Calibration
In order to check the calibration, I used some information posted on the uk.sci.weather newsgroup.
Posted by Rob Bale 19 Feb 2000

"Measure diameter [of the funnel] in cm. For a 6.5 inch gauge it should be 16.51cm. Square this number and multiply answer by 1.964. This will give you the vol of water for a 25mm rainfall in cubic cm. or ml. Check that the tipping bucket has no residual water in it. Measure out volume of water required (535.3ml. for a 6.5inch gauge) If you can get hold of accurate equipment from a local college or friendly pharmacy so much the better. Pour water in VERY slowly down side of gauge."

I now have 25ml burette to check the calibration occasionally.