Calibrating Tipping Bucket Raingauges

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GT Woodin
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Joined: Sun Dec 13, 2015 7:53 am

Calibrating Tipping Bucket Raingauges

Post by GT Woodin » Thu Dec 08, 2016 6:53 pm

If, like me, you wish your tipping-bucket gauge to give daily/monthly totals that agree closely with a manually-read check gauge, you might be interested in my experiences.

Manufacturers often calibrate their TBR's to read correct at high rainfall rates, typically 2" per hour, such gauges will over-read at low rainfall rates, i.e: the vast majority of rainfall events in lowland Britain.

You need to statically calibrate each bucket to tip at the amount specified by the manufacturer, or if not stated, a simple equation related to the funnels diameter obtains the value. A 10ml measuring pipette can be bought from Jaytek.

Repeatability of this method on cheaper gauges is difficult. One manufacturer, who claims to be the largest raingauge manufacturer in the world, has a gauge with a design fault. Water runs off and underneath the plastic buckets when they tip, and a variable-sized blob of water sits on the bucket-stop screws. This acts like glue on the empty bucket, and can delay the tip of the next filling bucket, so that the gauge under-reads.

More expensive gauges have better pivots, (lower friction), and do not have design faults like this.

Of course, if you wish your TBR to be more accurate at high rainfall rates, the buckets should be adjusted to tip with a lower volume of water than this static calibration. This is to compensate for rain lost pouring into the bucket that is tipping during heavy rain. If doing this method, it is best to buy a rain gauge calibration kit, such as sold by Novalynx, and run a known volume of water through at, say, 2" per hour, and get the stated number of tips, 119 for 946ml dripped through, for 8" diameter TBR's with buckets set up for 0.01" tip. Full details of various diameter gauges comes with the Novalynx 260-2595 Precipitation Gauge Calibrator. Andrew Overton, (do internet search for "A guide to Siting") describes a home-made calibrator.

It might be an idea to have 2 TBR's, one statically calibrated to read correct at low rainfall rates, and the other dynamically calibrated to read correct at 2" per hour. This latter gauge would capture fine detail of a heavy rainfall event, but would consistently give higher totals than the statically-calibrated TBR.

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