There's a better and more absolute method to calibrate any tipping-
bucket gauge, as follows. This should be performed annually as the
adjustment does drift over time. If you haven't checked your TBR
calibration - undertake this simple check, it'll take just a few
minutes ...

This extracted from Andy Overton's very useful guide to settying up
and calibrating AWS, which can be found on the Royal Meteorological
Society's website:
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"To calibrate using the method of measured amount introduced to the
raingauge, the area of the raingauge collector must first be
measured. For a square or rectangular collector multiply the lengths
in millimetres of the two sides together; for a circular collector
measure the diameter in millimetres, square it, divide it by 4 and
multiply the result by . Finally, divide 500 000 by this result to
give the amount of rainfall in millimetres which 500 ml of water
should register with your raingauge.

Next, it is necessary to measure 500 ml of water in a good quality
measuring jug, ensuring that the comparison of amount is taken from
the bottom of the meniscus of fluid. Measuring jugs are not perfectly
accurate but will suffice; if a more accurate measure is available,
use it. Pour the water extremely slowly into the raingauge. It
cannot be stressed too highly how slow this must be as errors in
registering become very significant with high precipitation rates.
The flow should be no more than a slight dribble, giving time for the
buckets to tip before introducing a fresh trickle. If a syringe is
available, use this to better control the flow. This will be time
consuming but the large amount of water goes a considerable way
towards equalling out the inevitable errors caused by uneven tipping.

Once all the water has been run through the raingauge the registered
precipitation should be noted and compared with the result to be
expected from the calculation above. It is good practice to repeat
the exercise to ensure accuracy and then average the results. If an
adjustment is necessary this must be done by manually calibrating the
raingauge assembly, an offset cannot be applied to the measurements as
the error is incremental and the introduction of the total offset for
500 ml of water in all circumstances would make a mockery of lesser
readings. The manufacturer's manual should be consulted for details
of how to make adjustments. For tipping bucket raingauges this is
typically by adjustment of the screws upon which the buckets rest in
the empty position. Lowering the screws will require more water in
the buckets to make them tip and so compensate for a raingauge which
is over-reading. Raising the screws compensates for a raingauge which
is under-reading. It is vitally important that the screws are
adjusted by equal amounts - it may be helpful to mark the screw heads
in some way to assist with equal positioning. If the raingauge is out
of calibration by as much a 10% or more do not be surprised if
considerable alteration in screw height is needed. Once adjustment
has been made it will be necessary to repeat the calibration process
to check what amount of water is now registered and make further
adjustments as necessary. A correctly adjusted raingauge should be
accurate to around 2%."


Stephen Burt
Stratfield Mortimer, Berkshire