I fill propane tanks here at the shop and seen various kinds of these, however after time or a slight "dropping" of tanks they seem to read inaccurate. I can only agree with Phoenix, the tare weight is stamped on the tank (TW) and depending on your tank size, being a 30# or 40# add the (TW) to the tank size and you will have the full limit of your tank. You should not have to write anything down, unless the filler is not giving you your moneys worth. A bathroom scale will work for this just fine. A 30# tank when full should be about 54 lbs. Aluminum of course is lighter and the TW would show this.