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Thursday, November 24, 2011

Gas Blends and Cold Weather Performance (Why not just use propane?)

I got a good question recently:
Hikin Jim, I see you refill [your backpacking canisters] with butane. What is the advantage over straight propane if propane works in lower temps? For that matter, why are these cartridges blended at all? I have wondered about this for a while.

An excellent (and important) question. It has to do with vapor pressures. Take a look at the following boiling points table.
 Boiling point
n-butane    -0.5C    31F
isobutane    -12C    11F
propane      -42C   -44F
 
As you can see, propane will vaporize (boil) at extremely low temperatures whereas n-butane ("regular" butane) has a vaporization point some seventy five Fahrenheit degrees higher. Quite a difference!

Let's say you have liquid propane in a container and the ambient temperature is a nice, comfortable 75F (24C). 75F is some one hundred nineteen degrees (Fahrenheit) above the boiling point of propane. Propane at that temperature desperately wants to boil and exerts tremendous vapor pressure against the walls of the tank, vapor pressure so strong that you need a fairly heavy steel container to hold it safely, such as those found on the big green 16.4oz (~460g) propane canisters from Coleman. The little lightweight canisters that backpackers carry could literally turn into a hand grenade at those pressures.

On the other hand, n-butane is relatively benign at those temperatures. Take a look at a clear plastic lighter some time. That clear liquid inside is n-butane. Yep, all that's needed to contain n-butane is some flimsy plastic. Contrast that with a 16.2 ounce Coleman canister! Of course the problem with n-butane is that if the fuel temperature falls below about 40F/5C, the vaporization isn't strong enough to run a typical gas stove.

Well, if propane is great but would catastrophically burst a backpacking canister and n-butane is easily contained but useless in cold weather, what to do? Well, you blend the two. The liquid blend takes on properties that lie somewhere between low pressure n-butane and high pressure propane. Better still, you get a chemist to rearrange the internal structure of the n-butane molecules a little and create isobutane. Isobutane is what is known as an isomer of butane. It's the same stuff, but the internal components have been put into a different arrangement, an arrangement that in this case works in our favor in that the boiling point drops by twenty degrees Fahrenheit! Nice, eh? It's like re-packing the trunk of your car. It's the same stuff, but in a different order. In the case of your car's trunk, things fit better. In the case of butane, the properties change a bit, and we've got a better cold weather fuel. The best cold weather blend for upright canister stoves is therefore isobutane mixed with propane. Note that I stressed the word "upright." More on that in a minute (see the second of the two below links).

The problem with this blending arrangement is that the higher pressure propane tends to boil off a bit faster than than the n-butane or isobutane. Toward the end of the life of the canister, all you have left is the "lesser" (in terms of cold weather performance) of your fuels. This is one reason why in cold weather you can get the dreaded "canister fade" where your flame slowly fades out toward the end of the life of your canister, and you can't operate your stove even though, when you shake the canister, you can still hear fuel sloshing around in there.

There are two things you can do to get good performance and avoid canister fade in cold weather.  One, you can take action to warm the canister or two you can switch to a liquid feed gas stove.

With respect to liquid feed gas stoves:  If gas is kept under sufficient pressure, it liquefies, i.e. the form of the fuel changes from a vapor into a liquid.  If you feed the fuel in liquid form to your stove, you no longer have to worry about which fuel in your blend has a tendency to boil off faster into a gas.  With liquid feed, the blend you start with is the blend your finish with.  Since the blend doesn't change with liquid feed, it's not quite so critical that the non-propane component of your fuel be isobutane.  Again, since the blend doesn't change, the cold weather performance of your fuel is just as good at the end of your canister as it was at the beginning, and you don't get such pronounced "canister fade" toward the end of the canister.

There are a lot of advantages to using a stove in liquid feed mode in cold weather, but be aware that not all gas stoves can be used in liquid feed mode.   Be sure to read the above linked article to find out what type of stove may be used safely.  You also need to buy the right brand of gas in order to get good cold weather performance.  Not all brands are equal.  For more information, please see my post on What's the Best Brand of Gas for Cold Weather?

As for refilling backpacking canisters, butane is cheap, readily available, and of lower pressure than the original contents (and therefore reasonably safe). Although reasonably safe, the one problem with refilling backpacking canisters with 100% butane is that they're no good if the fuel temperature gets much below about 50°F/10°C.  So, I just use my refilled canisters on fair weather trips.  I tend to prefer fair weather trips anyway, so refilling backpacking canisters works very well for me.  YMMV.  :)  I wish I had a source of cheap isobutane, but alas I do not. Refilling with 100% isobutane would also be reasonably safe if the canister originally contained a propane-isobutane blend. Refilling a backpacking canister with 100% propane could be a very deadly enterprise and should be avoided. But this post is really about why we blend gas and why we don't use 100% propane in backpacking canisters.  At some future point, I'll dedicate an entire post to the subject of refilling canisters.

Hope that clears things up a bit.

HJ


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7 comments:

  1. This was a pretty clear explanation. It is not safe to store propane in a flimsy butane canister. At room temperature, the vapor pressure of propane could cause a butane canister to burst.

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  2. Thank you for this. I've read about the difference in isobutane blends for winter use before, and your's is a nice, succint explaination.

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  3. Thanks, Bill. I hope it's helpful.

    HJ

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  5. so if i want to go winter car camping, using a propane stove or heater (in the green cylinder) is no problem then right (until -29C according to the coleman website)? I'm looking at a camping stove for winter camping (not backpacking) and have been trying to choose between the coleman single burner dual fuel stove or an inverted canister stove, both roughly 100 dollars (canadian), but using a propane stove could save me a significant amount of money. Thanks in advance for the clarification.

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    1. Oh, heck yeah. If you're doing car camping, just get propane. The only reason not to use propane is the weight of the big heavy steel tanks. But for car camping? Just use propane. You'll get better cold weather performance and the fuel is typically cheaper, particularly if you buy in bulk.

      HOWEVER, be really careful with catalytic or other types of heaters regardless of fuel type. Maybe you already know this, but in an enclosed space, any kind of heater or stove could emit carbon monoxide. One slumbers but never awakes in the presence of carbon monoxide in sufficient quantities. Sorry if I'm telling you something you already know.

      HJ

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