Hi,
I've been doing a number of tests which have involved removing the battery and can't say that I've seen any "phantom" rain readings either. I normally remove the rain cable before the batteries (and replace after), but not every time.
Orion wrote:perhaps I ought to remove the receiver batteries first as it will loose contact with the transmitter anyway ?
I doubt if that will help. The data is only transmitted once every 48 seconds, so the transmitter has to accumulate the number of tips of the sensor and transmit a total every 48 seconds. To prevent data being incorrect if a few transmissions are not received (it seems to need about 5 missing messages before lost contact is actually registered), there is probably some sort of "running total" which eventiually overflows back to zero. I don't know if anybody has worked out the exact data format, but I've just tried a few experiments:
The transmitter unit seems able to accumulate at least 160 tips (in 48 seconds) so the next transmission increments the total by over 50mm. However, manually rocking the see-saw even faster for most of the 48 seconds results in NO additional recorded rain, but a short period of very fast rocking does produce an increase. So it looks as if the counter has little high frequency filtering (I haven't yet tried a faster electronic oscillator*) but hits some sort of overflow limit (at least 200 tips) which it then treats as an error (and ignores the measurement).
Maybe when the battery in the transmitter is changed, the "accumulated rain data" takes a random (or zero) value which the receiver can recognise as incorrect (i.e. a battery change) and resets accordingly. But (due to a fault or bad luck) your unit generated a "plausible" value which the receiver processed as phantom rainfall.
*EDIT: I've now tried connecting an electronic function generator and am a little surprised that the transmitter appears to accurately log short bursts of 50 pulses per second and even responds to 300 pps (but I cannot be sure if all these are counted). So any "intermittent" contact whilst unplugging/replugging the cable
might produce a phantom rain measurement. But this is unlikely to happen as the reed switch should be open circuit unless the seesaw is actually tipping. More likely, if the transmitter is being handled in a "mains environment", is that "mains hum" is being introduced onto the live rain sensor pin (the input impedance is 120k ohms).
Cheers, Alan.