I have 2 one hundred pound tanks that are used to heat my garage and also are used for the grill. The last year or so I am using more propane then I have used in past years. The propane company came out and put a gauge on the lines to check for leaks and none were found. When one tank is empty it switches to the other tank automatically. I think there is a problem with the valve switching to the other tank before the first tank is empty. When they replace the tank I have no idea how much propane is left in the tank that is taken away. Any ideas? Thanks...