there is a sensor in the tank and you can use it as part of an auto shutoff that most recovery pumps have, but that is more often needed for certain higher pressure refrigerants, the butane isn't at as much risk of overpressure at most ambient temperatures and unlike in refrigerant recovery, from the get go you arent gonna overfill the tank, but that is a concern when using the system for its refrigerant recovery purpose. You need a tank that has a large dead space at the top to safely accommodate the fact that, unlike water, butane liquid expands and contracts very much with temperature changes, so you need lots of spare room. Id use the 50 if I built a 2.5 gallon one, my 30 is about half full when cool and the 1 gallon pots use almost all of it for a run. but you don't have to fill the pots up to the top, so there is room to experiment. let me know if you make one, I have some additional tips on running it. good luck, be careful.