Unlike other sensors that provide instantaneous measurements, any rain gauge needs to be powered and run continuously. In the case of analog rain gauges (with a reed switch, i.e. a "pulse" output) also the logging ESP needs to be powered and wait for signal continuously.
Any cheap rain gauge contains a tipping bucket mechanism. Every time one of the two buckets gets full, it tips over and is emptied. The process of tipping over triggers the signal ("pulse").
Rain gauges measure precipitation accumulated over a certain period of time (e.g. mm/h). One tip of the bucket can correspond to e.g. 0.1 mm of precipitation: with such a rain gauge three tips (pulses) within an hour would correspond to 0.3 mm/h.
Mounting requirements
In order to provide a more reliable measurement, the rain gauge should be installed at a representative location. This means that it should not be placed directly under the trees or around any other obstructions (such as walls), but also somewhere where it is relatively protected from strong winds. A forest clearing would be optimal. It also needs a sturdy base and has to be level, otherwise the tipping bucket mechanism inside can be triggered at incorrect times.
Analog rain gauges
If we replace the Sentec rain gauge with an analog one, we will need an ESP that's constantly running and triggers on a rising edge of a signal, i.e. we need to implement a pulse counter.
We currently do NOT plan on having an ESP running constantly at the central mast: this would be a large disadvantage of an analog (pulse) rain gauge over a digital one. A digital one needs to be powered constantly but does not require a powered ESP.
A possible (pulse) alternative if we decide that the Sentec digital rain gauge is not suitable: Davis Aerocone with a CMOS for pulse counting.