Flame KingYSN03 3lb Steel Propane Tank Cylinder with Gauge and OPD Valves for Grills and BBQs, Camping, Fishing, & Outdoor Activities, White

Q: Accurate information, please! the cylinder itself says 7lb/3 kg. the description says 3lb, so which is it?

The TW (tare weight) and T (tare, same thing) at the top of the bottle collar refer to the weight of the empty bottle and valve assembly. The WC (water capacity) at the collar bottom tells you the volume of incompressible liquid (eg. water) the bottle can hold, and how much that water would weigh all by itself. The mix of avoirdupois, imperial, and metric systems is never confusing, as anyone who worked on the original Hubble telescope project could certainly tell you. Propane is filled into the bottle as a liquid under pressure at a standard temperature (ideally). Because temperatures and pressures will vary, and propane volume will vary with them and with use, the only constant measure available to determine how much propane is in the bottle is weight, which, for you physicists out there, is a proxy for mass. This bottle will hold 3.25 liters of water, which is sufficient volume to hold a bit over 3.0 pounds of propane, with, since people bring it up, enough room left over for safe transition to the gaseous phase. The bottle, with 3.0 pounds of propane in it, will therefore weigh a total of 10.0 pounds. These things are pretty well regulated and common, so if you're buying one by a reputable manufacturer from a reputable seller, you'll get what's advertised. A 3lb bottle will hold three pounds of propane, a 5lb bottle five, etc. Tare weights may be different between brands and on the basis of what is permanently attached, like the valve and the welded-on collar and base, but the bottles will hold what they say, and you will determine how much is inside by comparing the measured weight with the tare weight. Pressure also works, but is not as reliable because it is so dependent on temperature. Note that bottles are not weighed and stamped individually, so there will be some variability within whatever the manufacturer's quality control parameters are. The designs are engineered to account for that. Cheers!