Some simple things to check first.
Is the pro gauge partially blocked by debris ?
Is the Pro gauge horizontal ? If not the tipping buckets may not tip properly.
Some people have reported on other forums that the tipping buckets were sometimes sticking in one position, so worth checking they move freely.
More complicated but worthwhile checking sometime.
Have you checked the Pro gauge is recording correctly ? It's possible to check calibration by slowly dripping a known accurate volume of weighed water into the rain gauge this will give an accurate measurement of the output.
You will need the following formula where:
v = volume
pi = 3.14
r = radius of the funnel
h = height of the funnel (measured depth of the funnel)
h=v divided by pi r squared
This example is from the Weather Observers Handbook, and advises using 500 of water and uses metric measurements centimeters throughout. It's best to carry this calibration/measurement two or three times and taking an average of all readings.
a Pro2 tipping bucket gauge has a funnel diameter of 165 mm so has a radius of 82.5 mm (8.25 cm)
If 500 mm of water is dripped slowly into the gauge, it should give a result of 2.34 cm (23.4 mm of rainfall)
e.g. h = 500 /3.14 * 8.25 * 8.25
Can I also just quote Dr John Dann of Prodata weather systems here in the UK.
Can I add just a small word of caution here. First, I wouldn't recommend anyone altering the screw settings on their Davis rain gauge
unless they're really convinced that it is misreading significantly and have completely ruled out other possible explanations. Often an
apparent miscalibration has another explanation, eg the gauge increment is not correctly set on console or software or the reference rainfall readings are not properly comparable, which can be for a variety of reasons.And in practice it's pretty difficult to return the
screw heights exactly to their factory settings once altered if you subsequently decide that the error wasn't as great as you initially
thought and you didn't keep an accurate record of any changes that were made.
JC
