RE: [John Muir Trail] Canister stove performance notes and trick
Date: Wed, 4 May 2011 20:59:55 +0000
Subject: [John Muir Trail] Canister stove performance notes and trick
One performance issue commonly encountered with canister stoves is that the stove putters out in cold temperatures at high altitude. Even with a relatively volatile fuel (such as the popular MSR 80/20 isobutane/propane blend), this becomes an incresing problem as amount of fuel in the canister diminishes upon use.
The reasons for this are several-fold:
(1) As the pressurized fuel vaporizes, the heat of vaporization (i.e., the energy requred to change the fuel from liquid to gas) comes out of the can and fuel remaining in it, thereby lowering the temperature of the can and remaining fuel. The less fuel in the can, the lower the thermal mass of the can and its contents, meaning that, during use, their temperature will more quickly drop the emptier the can becomes.
(2) with blends, the more volatile component tends to burn off fastest, leaving the fuel heavy component-rich the emptier the can becomes (the lower the temperature of previous uses, the more pronounced this effect). For example, the remaining fuel in an initial 80/20 blend of isobutane/propane becomes more isobutane rich as the can is emptied (isobutane has a standard pressure boiling point of -11 degrees C whereas propane has a standard pressure boiling point of -42 degrees C.). The more heavy component-rich a fuel blend becomes, the less volatile it is at a given temperature.
(3) while, from a volatility standpoint, the lower pressures at high altitude initially provide at least some offset to lower temperatures, upon use they also provide higher pressure differential (between can and atmosphere), causing the evaporative cooling of the remaining liquid in the can to occur more rapidly. This partially explains why, at high altitude, a canister will sometimes start really well but quickly putter out, especially as the can becomes emptier (due to its lower thermal mass, as discussed above).
For all these reasons, even if attempts are made to keep an emptier can warm at high altitude (such as by putting it with you in your sleeping bag) it can still quickly putter out on a cold morning.
If I sense this may happen, I find that the following steps ususally work (unless I'm in really bad wind conditions, the can is really low on fuel, or it's just really, really cold), at least when simply boiling water in a small, lightweight pot:
(1) empty a bit of the water out of the pot so that it is no more than about 2/3 full and try to heat the remaining water until it is at least lukewarm;
(2) turn off the stove, detach the fuel canister, and immerse it in the heated water in the pot for a few seconds;
(3) reattach and relight the stove - there should be a marked improvement in the stove's heat output (immersing the canister in the lukewarm water even for just a few seconds should transfer enough heat to the fuel to increase its temperature and hence, vapor pressure, above a critical level).