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Updated : 16/10/2016

GHowSAW Weather Rain Calibration

Introduction

For a tipping bucket rain gauge the method of calibration depends on whether or not the count of bucket tips is processed before display or not. If there is no processing of the data then the calibration must adjust the mechanical setting of the device to measure a specific volume of water per bucket tip. With data processing all that is required is to determine the volume of water per tip and there is no need to adjust the mechanical settings beyond that required for normal operation. From here on it is assumed that the counts from the device will be processed using the transfer function.

Rainfall(mm) = k .counts + c

Calibration

This procedure is designed around the Rainwise Rain gauge

Cross-sectional area of the collector:- Area(mm2) = π (Diameter(mm) / 2)2

Volume of one unit(mm) of rainfall:- Volume(ml) = Area(mm2)/1000    Note 1(ml) = 1000 (mm3)

For the Rainwise gauge with a diameter of 203.2 (mm) the volume V per mm of rainfall is 32.43 (ml) .

Using a burrette or separating funnel (A gravy separator will also suffice) calibrate as follows:-

  1. Fill the funnel/burrette with a multiple n.V ml of water

  2. Run Vml of water through the gauge at a rate of at least 5 seconds per tip.

  3. Count the number of tips (C) produced by running the remaining (n-1).V ml through the gauge at the same rate.

  4. The transfer function constant K is then equal to C/(n-1).

The DS2423 counter is continuously powered by a battery to maintain a cumulative total of rainfail bucket tips. The rainfall over any period is then determined by the difference in counts thus obviating the need for the offset value 'c' in the transfer equation.

As a cross-check of the gauge set up and installation the discharge from each side of the bucket tip can be collected and measured. Equal volumes of discharge indicate that the gauge is set up correctly. If there is a large difference in the volumes then adjust the calibration or stop screws. Screw in the screw on the side with the lower discharge and screw out the screw on the other side by the same amount. Adjust the screws by a maximum of a quarter turn at a time. Run more water through the gauge and compare the discharges again. Repeat until the discharge volumes are equal. Then repeat the calibration process.

Calibration Jun 2013

Calibration of Rainwise Rain Gauge (diameter = 203.2 mm )
Test No Volume (ml) Rain (mm) Tip Counts Rain Factor (mm/count)
1 325 10.002 34 0.294
2 325 10.002 35 0.286
3 325 10.002 35 0.286
Note: The Rainwise, as delivered, is set to 0.01 inch per count. This being the case then my factor should have been 0.254 (mm/count). Since the unit works well I elected not to mess arround with the calibration adjustment screws and use my experimentally determined calibration factor.
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