Personally I've never cared at all how much liquid propane may be left in the tanks as I've marked them CURB SIDE / STREET SIDE and always run them sequentially - i.e. with the second turned off I run the first until it empties then switch to the second, offering me plenty of time later to get the first filled. Sure, there's always the possibility of running the first tank dry in the middle of the night and having to go outside to switch over to the second but at least I CAN ... if I run both tanks together and run out in the middle of the night then I'm really pooched. :M