Propane tanks are heavy, and it's not something you can drag onto your scale to weigh every time you need to find out how much propane is left in it.

Truma's LevelChecker uses ultrasonic signals to determine how full or empty your tank is. You just hold the LevelChecker's tip to the side of the tank, press a button, and it'll emit and monitor an ultrasonic blast that will tell you if your tank is full or empty in the current location. After a few tests you'll know how much propane is left.

[Truma]