I believe most rain gages use a "tipping bucket" (bucket is very small). 
There is a funnel above, and a teeter totter sort of arm with a bucket 
(maybe a fluid oz. capacity) on each end of arm with the pivot or 
fulcrum at the center. When  a bucket which is on the higher elevation 
end fills to some point where the weight forces that bucket down, and 
the other bucket rises which also moves it in position to receive the 
funnel outlet flow. Count the number of times the teeter toter flipped, 
and you have the rain quantity. When the bucket is in the lower 
position, the water overflows the low side, emptying. The buckets are 
inverted wedge shaped, when in the high position hold a known quantity 
of water, and in the low position, one side of the wedge is slightly 
sloped down from level to empty completely.

The gauge must be self emptying, otherwise could require emptying at 
most inconvenient times. Also if your temperature can get below 
freezing, needs heat.

See: http://www.raingauge.net/
and google rain gauge. There are inexpensive units under $50, some 
wireless including LCD indicator. I think a little searching would find 
the sensor by itself. Hardly worth the effort to build, maybe buy a 
sensor and build the circuit to interface with PC..

Dario Greggio wrote:
> What would be the best method to measure quantity of fallen rain? (meteo 
> station)
>
> A cup/glass and two resistive bars? possibly using AC instead of DC to 
> prevent oxidation? Or any other suggestion?
> And, does it need to be emptied once in a while?
>
> TIA everybody.
>
>   
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