This article is designed to demystify some of the peculiarities of propane cylinders.
This information is currently designed for DOT propane gas cylinders and does not cover liquid propane cylinders such as those designed for forklifts. We'll cover that at a different time.
Unlike gasoline or kerosene, propane cylinders are sold by weight instead of volume. The standard barbecue propane tank
Few things seem as confusing as the weight of propane. Take the 20# (20 lb.) propane cylinder normally associated with barbecues. That tank is called a 20# tank. But when it is filled, the tank weighs more than 20#. This makes no sense.
The reason is that many years ago, propane was sold by exclusively by weight, not by volume. The reason is that propane as a gas expands quite easily to 270 times its volume (270 to 1). It will boil at any temperature above -44°F (-42°C), so the most reliable way to see how much propane you have in a cylinder is to weigh it. Some dealers still sell it by putting the tank on a scale, but most now sell it by the gallon, but the tanks themselves are still sold by the weight they can carry which can add to some confusion.
Propane tanks are designed to be filled with 80% liquid and 20% gas to give the liquid an area to expand into, since propane is generally used in liquid form or in gaseous form and it is highly inadvisable to mix the use. If your effect is designed to work with propane as a gas, it is a very bad idea to attempt to run it as a liquid. This means that a 20# propane tank can hold more than 4 gallons of propane, but the safest amount is to fill them with about 4 gallons.
20# 4 gallon
30# 7 gallon
40# 10 gallon
60# 15 gallon
100# 25 gallon