dc42:
That's a neat idea, using oil to separate the wet air from the dry air! I guess expansion of the two air pockets may make the reading temperature-sensitive, but probably not enough to matter.
The sensor is temperature compensated. I'm not sure if the oil added any inaccuracy to the reading given it will expand and contract at different rates to air.
I have a water pump that pumps up the system until a set pressure point. Open a tap etc and the pressure drops turning the pump back on. Same as a caravan water system. Of course if the tank should empty, pressure will never be reached and the pump would run continuously dry, something it's not designed for. What I have ensures the pump won't run dry. Having said that, it's not been setup long and in the UK it's done nothing but rain!