Canister stoves are easy, lightweight, and convenient to use in the backcountry. Simply attach your stove to a canister of fuel, turn the knob, light it, and you're done. But have you ever wondered what exactly is in those canisters? And why they start to burn with less intensity as their contents dwindle?
Backpacking fuel canisters hold a mix of propane and butane under pressure, which keeps the fuel in liquid form. (You can hear it sloshing around inside if you shake the canister.) Gaseous vapors rise from the liquid, emerging from the top and into the burner when a stove is attached. For this reason, canisters must always be used in an upright position. (If you tip the canister over, liquid fuel reaches the burner and causes a major flare-up.)
As you cook, liquid fuel is continuously transformed into vapor to replace what's burned off. This process requires a small amount of heat. This is why canisters also cool down while in use—you'll often notice condensation forming on the metal.
In many ways, propane is a more desirable fuel than butane because it contains more heat for its weight. However, it is also much more volatile and must be held at higher pressure. This is why pure propane canisters are made of much thicker, heavier metal. (Think of the classic green Coleman propane canisters.) To get around this problem, manufacturers blend in butane, which can be held at a lower pressure. The resulting mixture can be safely contained in lighter weight canisters more suitable for backpacking.
As you start to use the fuel mixture, the propane burns off first, providing high-heat cooking power. But as the propane content is depleted, the remaining butane burns less intensely. Add to this a decrease in internal pressure as the fuel dwindles and you have a much fainter flame once the canister reaches its last 10 to 20 percent.
Cold temperatures also affect fuel performance. As the mercury drops, so does the volatility of the pressurized liquid fuel, reducing the amount of gaseous vapors needed to power your stove. Mixes with regular butane stop working reliably right around the freezing point. Most mixes, however, use isobutane, a variation of butane that will continue to work down into the 20s. They are often marketed as "four-season" mixtures.
If you're stuck with a cold non-functioning canister, you can always warm it up by holding it close to your body. This will provide you with a potential cooking window even in sub-optimal temperatures.
“Equipped” is an AMC Outdoors blog, written by Matt Heid.