How does rainfall sensor work?

Years ago I got a weather station from somewhere, don't even remember where exactly anymore. It had all kinds of goodies like usual temperature and humidity displays, with some history (few days), but also wind and rainfall sensors. I never installed wind meter (due to lack of suitable location) or rain meter (due to snow), so they just sat on my shelf.

When doing cleaning I found the rainfall sensor again. I was just about to throw it away, but then reconsidered and checked it again. How does rainfall sensor work, after all?

Sensor itself is cylinder, with conical opening on top, with some small holes at the bottom. Hmm.

 

Opening the device is enlightening. Inside there is small swing-like device. When enough water drips on upper part of this swing, weigh eventually causes it to fall down to other side, causing accumulated water to spill out through  opening in bottom. Then other side starts collecting water. That small metal-colored cylinder is magnet; inside the box is small board with reed (magnetic) switch which is used to count "swings".

So, let's go back a bit. The opening on top is approximately 11.2cm in diameter (quick'n'dirty tape measurement so accuracy is so-and-so), so opening is approximately 100 square centimetres total. I don't feel like experimenting on water volume needed for tripping the swing, but I think it is also some nice round number, let's say X, eyeballing the size of things suggests possibly few millilitres.

I am not exactly current with details of rainfall math, but all this means that every time magnetic switch triggers, X amount water had rained to area of 100 sq cm. With little more math its fairly trivial to figure out more detailed rainfall figures.  

Also in the box is some kind of RF transmitter. I also don't feel like experimenting with it too much to figure out how it exactly transmits rain amounts to base station, since this very likely works on one-way mode (i.e. no confirmation from base of received data), unfortunately I don't have time for that. My guess is that it transmits raw number of trigger pulses received, leaving detailed math to main unit; or then accumulated rainfall figure over last hour or so. For cheap unit I pretty much expect the first.

And now I can happily toss this thing from taking up space on my shelf.