Intervalometer (Slight Return)

An intervalometer is an extremely simple measurement device that measures the intervals between the impact of raindrops on a surface. In our case, we used a piezo-electric element put in a small 3D-printed holder. When a raindrop hits the surface, a voltage is generated that is detected by a simple CPU, such as that of an Arduino. A softwarr-based signal filter is applied to filter out frequencies that are too far removed from the Eigenfrequency of the element. Once an impact is detected, a transistor shorts out the the piezo, after which it is ready for the next drop. It takes about 20 ms to register a drop. The design follows the open hardware philosophy and all codes, including those for 3D printing, can be found at https://github.com/nvandegiesen/Intervalometer/wiki/Intervalometer.

The intervalometer has been tested under different climatic conditions. The main focus here is on a campaign in Tanzania. It turned out that the intervalometer gives surprisingly consistent results and with some calibration, rainfall rates can be determined. Because the arrival rates are known in great detail, one can check to see if the Poisson assumption, underlying many rainfall models, is valid. For tropical Tanzania, it turned out that only under rare drizzle-like conditions does Poisson hold.