First thing in the morning the manometer is down 6". Quite disappointing. Pump it back up to the reference line and go about other tasks. Doing a layup later, so put the heater on. When I come back the shed has gone from10C to 16C, and the manometer is UP 4". It is only then I realise how big a factor in this the local atmospheric pressure and temperature is. When I leave the shed with the temperature 17C I have let the pressure out to get down to the reference line twice.