by Michael Hodgson

Are propane/butane blend cartridges (the kind used in small backpacking stoves or compact heaters) really as good as they say they are? Well, yes and no. Touted by many as the answer to butane-only inefficiency, the 20% propane 80% butane blends boldly claim to be hotter burning and offer superior performance in colder temperatures.

That claim, taken at face value, is rather deceptive. According to product designers at both Coleman and Mountain Safety Research (MSR), butane, isobutane and propane deliver nearly the same BTU's (registered heat output) thus putting a shadow over the claim of hotter burning. In other words, you are not going to boil a quart of water any faster with a canister of propane or slower with butane.

While everyone does agree that propane will outperform butane in colder temperatures, the consistent superiority of the blended fuel (propane/butane) is in question.

In order to understand the issue, one must understand that each of these liquid fuels has a different boiling point or point at which gas is produced. Butane (N-Butane or normal butane) has a boiling point, of 31 degrees F. Isobutane (think of this as the premium gasoline of the butane line) has a boiling point of 11 degrees F. Propane by itself has a boiling point of -43 degrees F.

Simply put, at below 31 degrees F, normal butane has difficulty vaporizing which is why the performance of a butane stove drops considerably. The cartridge must be kept warm to keep burning. In theory, the addition of propane to the canister adds a kick to the fuel, keeping it delivering premium gas at lower temperatures. But, since propane has a considerably lower boiling point and doesn't actually chemically bond, won't the propane burn off first, leaving the butane behind?

Tests performed at MSR labs indicated that fuel efficiency did drop off dramatically during the final 1/3 of a canister hinting that the propane had burned off and only N-butane remained. Since MSR distributes isobutane, however, those results were interesting, but could not be taken as gospel-after all MSR has a lot to gain by proving propane/butane blends are not what they say they are.

So, we went to Randy May, product designer at Coleman. Intrigued by the question, May conducted his own battery of tests that substantiated the MSR assertion.

"The canisters of fuel we tested began with a gauge pressure of 73 psi at a room temperature of 72 degrees F. After 2/3 of a bottle burned at room temperature and was allowed to rewarm, it showed a pressure of 46 psi. Another bottle, chilled in ice water and burned 2/3's empty showed a pressure of 38 psi. This appears to verify that indeed the propane goes off first," says May.

Suunto, distributors of the Gaz butane/propane blend cartridge, confirm the drop in performance, but add that "the remaining gas still has traces of propane and will continue to outperform normal butane cartridges."

What does all this mean to you, the consumer? In sub-zero temperatures, be warned that while the first 2/3 of the propane/butane blend canister will deliver as expected, the canister may need to be warmed to keep the flame from dying-it's either that or elect pack out partially full canisters. At temperatures above 15, isobutane offers a more consistent delivery of fuel, without the need to keep the canister warm during the final 1/3 of its canister life. Above 40 degrees F and the issue is a moot one.

© 1999 Michael Hodgson; All Rights Reserved