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This type of raingauge works on the principle of counting drops of a known constant volume as they pass an optical sensor. These drops of constant volume are formed from a precision tube under zero-head conditions. Rain from the main collector is funnelled into a small reservoir chamber. This reservoir is kept constantly primed, maintaining the critical water level within the system, allowing a quick response to the onset of rain. A pipe links from the top of this reservoir to the precision dropper tube, as the water level in the system increases during rain, the excess water in the reservoir flows out causing drops to form from the precision tube.

The benefit of this technique is the rapid response to rainfall. A single drop is equivalent to 0.0033mm accumulation of precipitation (based on the standard collector), compared to the standard tipping bucket gauge, which triggers with 0.2mm accumulation of precipitation.

Three sizes of collector can be used with this raingauge, the standard size is 150cm2, a larger, 324cm2 (8”), collector enabling better resolution at lower rainfall rates. A small 75cm2 collector has been successfully used in tropical areas.

A detailed description of the drop counting raingauge, together with examples of typical measured data, is given in ‘A rapid-response rain gauge’ by J R Norbury and W J White, Journal of Physics E: Scientific Instruments, 1971, Vol. 4 pp 601-602.