@NickBuilder I now have looked at the code and (of course) you are right, the interrupt is used for the pulse count which I think is the best way of programming this function. My sensor is based on an old version of this code but modified for use with Openhab (which does not give any start value back, this function has to be own programmed). Another modification is that same unit monitors a number of temperature sensors. The "thing" has been in operation since my first post above and works well but I guess I should make an effort to modernize the code. Maybe a task for the long and dull autumn/winter days here.

@ericvdb Yes, I know about the 5V. But the adapter brings it down to 3.3V for the radio, that is the reason for using it and the adapter is equipped with the AMS 1117 3.3V chip. Seems to work OK but I really do not have any hard facts to prove it. One thing I may test is to wire the 5V also directly to this adapter instead of taking it out from the Arduino 5V pin. This is to find out if this Arduino Nano clone might have too small capacity for feeding the radio at certain occasions.