I playing with testing the rain sensor on the Watson 8686 PWS Clone of HP2550
The diameter of the rain funnel is 11cm (110mm)
so the capture area (area of a circle) is pi*R*R (PiR2)
3.141 * 55 * 55 = 9503mm2
so using this online calculator
http://www.calctool.org/CALC/other/default/rainfall0.1L (0.100732L) of water would be about 10.6mm of rain
hopefully worked it all out OK
Its a single bucket , so its never going to be accurate with high rainfall and also if there is less than 0.1 so the bucket does not tip, then that will be an error anyway the accuracy is +/- 10% anyway
hopefully on the right track here
welcome thoughts