Originally Posted by DavidsonOverlander
I thought that one of them still had some propane in it because it's 8 pounds heavier than the other
Grant, look at the markings around the collar of the tanks. One of the markings should have the letters TW followed by some numbers. TW stands for Tare Weight, which refers to the weight of the tank without propane. If you have an empty tank, it should weight at or near the TW.
If both tanks are the standard 7-gallon size, their TWs should be fairly close, though not necessarily identical. Our two tanks were bought side-by-side, same store, same manufacturer. They have different TWs.