User Guide
9
3. Check that the balance arm is free to move. This can be done by slowly
pouring a measured quantity of water (say 25 cm
3
) through the gauge and
counting the tips. It is worthwhile carrying this out at regular weekly intervals
(for example, every Monday at 0900) while leaving the gauge connected to the
datalogger. Providing a significant volume of water is used, these weekly
checks can easily be identified in the logged measurements. This simple
procedure confirms that the gauge is functioning, detects any marked change
in the calibration and (if carried out punctually) introduces an independent
time check into the records.
6. Calibration
The sensitivity of the ARG100 is set at manufacture to approximately 0.2 mm/tip
(optionally 0.25 mm/tip) and each gauge is subsequently calibrated as described
in Section 6.1. The calibration factor is given on a certificate and is also recorded
on the inside of the raingauge. For precise measurements use this value in your
program instead of the nominal value of 0.2.
6.1 Static Adjustment and Calibration
The following procedure is carried out during manufacture and may be repeated if
the calibration appears to have shifted:
1. Install the gauge over a sink unit as illustrated in Figure 2, ensuring that it is
correctly levelled.
2. Adjust the two calibration screws under the tipping buckets until the balance
arm tips in response to the correct amount of water dripped from a burette or
pipette (10.13 cm
3
for 0.2 mm sensitivity, 12.67 cm
3
for 0.25 mm).
It is not possible to set the screws very precisely using this method, but it should
be done with as much care as possible. It is obviously very important to ensure
that both buckets tip in response to the same amount of water. Many manufac-
turers and users of tipping bucket gauges aim to adjust the bucket settings until
exactly the correct calibration is achieved. However, a dynamic test (see below) is
required to check this calibration precisely after each readjustment and the process
becomes very time-consuming. In any case, it is virtually impossible to get the
adjustments absolutely correct, and it is generally preferable to adjust the settings
as closely as is reasonably practical, and then to derive a calibration factor for
each gauge individually.
6.2 Dynamic Calibration
1. Set up the gauge as illustrated in Figure 2, carefully levelled and connected to
the datalogger.
2. Fill the water container with 810.4 cm
3
of water for a 0.2 mm calibration
(1013.4 cm
3
for a 0.25 mm setting). This is usually achieved most precisely
and consistently by weighing the water on a balance capable of measuring to
0.1 g (0.1 cm
3
). An alternative is to use a good quality graduated measuring
cylinder.
3. Allow the water to drip slowly into the gauge (allowing approximately 100
minutes for the container to empty). This is a rate equivalent to a rainfall of
10 mm/hour, as recommended in BS7843 Section 2.1 for calibration
purposes. At the end of this period approximately 80 tips will have occurred.
The exact number is obtained from the datalogger, together with a visual
inspection of what fraction of a ‘tip’ is left in whichever bucket is still filling
as the flow of water finishes.
