Yeah when in use, keep the cap off. As Mark said, you can get some storage fluid.

If you are taking it out to clean the tank, just keep it submergerd in some tank water till you finish your maintenance.

As for your different readings, other equipment can cause interference with the reader.... It just depends on what is close by to it (other electrical equipment, lights, or ballasts).

I kinda wonder if line ac noise from devices on shared outlets would cause problems too (but doubt it since you would think they would test for that when designing the controllers)