09/17/2026
Shake a full tank. That sloshing you hear is the part almost nobody accounts for.
A 20 pound grill cylinder is not full of gas. It holds about 4.7 gallons of liquid propane, roughly 17 pounds of it, lying in the bottom of the shell under its own pressure. The top fifth of the tank stays empty on purpose, because propane expands about 17 times more than water for the same rise in temperature, so that empty space is the cushion that keeps a tank parked in the sun from venting through its relief valve. It is also why a full cylinder feels heavier than people expect, with gross weight landing near 35 pounds.
The service valve draws from the v***r space at the top. Liquid boils off the surface, the v***r feeds the hose, and the boiling pulls heat straight out of the steel, which is why a hard working tank frosts over on a warm afternoon, and that frost band marks the liquid level rather than a fault. Propane boils at 44 degrees below zero Fahrenheit, so it v***rizes in any weather you would grill in, it just cannot keep up when the tank gets cold. At 70 degrees Fahrenheit the v***r sits at about 110 psi, and dropping the tank to 0 degrees Fahrenheit takes it down to roughly 24 psi, which starves a high output burner.
Lay that tank on its side and the valve sits in liquid instead of v***r. Now raw liquid enters a hose and regulator built for v***r only, it flashes to gas past the regulator at 270 times the volume it occupied in the tank, and the burner receives a slug of fuel nothing metered. Flames that leap a foot above the grate come from exactly this. Carry cylinders upright and strapped, with the relief valve up in the v***r space where it belongs, and never in a closed trunk.
Two numbers are worth keeping. Those 4.7 gallons become roughly 170 cubic feet of v***r, enough to fill a small bedroom. And the collar of every cylinder is stamped TW followed by its tare weight, usually 17 or 18 pounds, so a bathroom scale tells you the fill in ten seconds: gross weight minus tare equals pounds of propane left. Worth knowing before you drive anywhere for a refill, because exchange cages typically hand back 15 pounds in the same size shell, not 20, a practice the Federal Trade Commission challenged in a 2014 complaint. [FMPTF]