High Sierra Topix

Interesting thread this...
HJ -
Your chart seems to show that increasing altitude affecting the lowering of the temperature window for "decent" canister gas ignition, (you called this slack?). IE - the higher you go, the less pressure, "depressing the vaporization temperature of the fuel." - gas canisters should then "burn" easier at somewhat lower temps at altitude.

Concerning this altitude/ lowering of pressure/vaporization hypothesis...As the isopropane/butane mix is in a liquid state and under pressure inside of a metal canister at constant temperature, (remote from the vaporization area), why would any (slight) external pressure change outside (at the stovehead) matter? The gas is still a liquid while inside the can?

Temperature of the canister and/ or if your stove has a generator?...just wondering?

HJ,
On some of the images I could see you were writing something on the canisters. I assume you were measuring and recording how many grams of fuel you were using with each test? It boiled 500cc of water in just over 2 minutes, how many grams of fuel did it use on that test?

markskor wrote:Interesting thread this...
HJ -
Your chart seems to show that increasing altitude affecting the lowering of the temperature window for "decent" canister gas ignition, (you called this slack?). IE - the higher you go, the less pressure, "depressing the vaporization temperature of the fuel." - gas canisters should then "burn" easier at somewhat lower temps at altitude.

Concerning this altitude/ lowering of pressure/vaporization hypothesis...As the isopropane/butane mix is in a liquid state and under pressure inside of a metal canister at constant temperature, (remote from the vaporization area), why would any (slight) external pressure change outside (at the stovehead) matter? The gas is still a liquid while inside the can?

Temperature of the canister and/ or if your stove has a generator?...just wondering?

Great question.

OK, so why is the "gas" inside a canister in liquid form? Because it's under very high pressure, pressure so high that the gas changes state into a liquid.

What does the valve on a stove do? It in essence releases pressure. As the pressure is released, the fuel changes state from a liquid to a gas through a process commonly called boiling. Now boiling water is quite hot, 212 F at sea level, but something can boil at a much colder temperature. Butane for example boils at 31F. Break open one of those clear plastic butane lighters some time on a cold day (below 40F), and you can literally watch the butane boil, but put your finger in it and it will be cold.

So the issue with an upright canister stove in cold weather is to keep the gas boiling (turning into a vapor). Just as with water, the boiling point of the fuel in your canister drops as the atmospheric pressure drops, therefore you can operate a canister gas stove in colder temperatures at higher elevations. The Lindal valve in your canister is really nothing more than a sophisticated hole. Whether you break open a little plastic hand held butane lighter (unsophisticated) or open the Lindal valve (sophisticated), the principle is the same: You're releasing the internal pressure. That internal pressure will continue to be released so long as the hole remains open.

It is the difference between internal canister pressure and the outside atmospheric pressure that causes gas to flow out. Reduce the outside pressure, and more gas flows.

This matters less with a stove with a generator/pre-heat loop, but you still need enough pressure inside the canister to drive fuel into the generator.

HJ

hikin_jim wrote:In other words, if you can operate a particular stove with a propane/isobutane fuel mix at 20 degrees Fahrenheit at sea level, you can operate that same set up at 0 degrees Fahrenheit at 10,000 feet....Now, is the graphic, precise? No. But it communicates the idea very effectively.

It communicates the idea, but imprecisely. At 10,000 feet your hypothetical fuel mix would only work down to 5°F on average, not zero. And since the barometric pressure changes not only with elevation but also with time of day, season, latitude and weather, you really have to speak about a range.

hikin_jim wrote:Actually only about 3 psig is required for decent pressure on an upright canister stove.

How did you determine that? I don't believe it's true, at least not in general.

hikin_jim wrote:Wow, that's warm. I've camped in the teens in lower elevations than that. If you're not getting below 20F, then bringing an inverted canister stove isn't absolutely necessary, but God help you if the temperature drops unexpectedly low. Of course at 13,000', you've got a lot of "slack" because of the low pressure which depresses the vaporization temperature of your fuel. And you'll do better with a fresh canister with an upright canister stove since the propane content is still fairly high when the canister is full.

Often there is an inversion and it's colder at lower elevations. I don't know what you mean by "slack". Except for when there are inversions the typical winter lapse rate exceeds the rate that elevation lowers the boiling point.

I've had a pretty easy time down to about 10°F. Below that it takes more time to get the stove primed. Bootstrapping a near empty canister on a cold, windy morning is the worst case. But there are ways to deal with it that don't require any heroics.

hikin_jim wrote:Actually only about 3 psig is required for decent pressure on an upright canister stove.

Try this:

Take your stove and open the valve. Press the canister side to your lips and blow as hard as you can. How much air got through? A human can blow with a pressure of about 2-3 psi.

At sea level, the boiling point of water is 212 Fahrenheit. At 10,000 feet elevation, the boiling point of water is about 193 Fahrenheit, a decrease of 19 degrees. The boiling points of propane, isobutane, and butane (or a mixture thereof), fall by the same amount. Therefore, if a stove can be reasonably operated at temperature "X" at sea level, the same stove can be operated at about the same level of effectiveness at 10,000' elevation at a temperature of "X" minus (roughly) 20.

The rule of thumb, and it is just that, a rule of thumb, is that one can operate a stove approximately 2 Fahrenheit degrees colder for every 1,000 feet of elevation gain. Note the use of the word "approximately" and the use of the phrase "rule of thumb."

I'm stepping away from a PSI based argument here. Pick whatever PSIG one feels is appropriate at sea level, and one should have that same PSIG at 10,000' in temperatures that are 20 Fahrenheit degrees colder. If one wants a PSIG of 5, very well, choose a fuel mix and go out in an appropriate temperature for that fuel mix for 5 PSIG at sea level. With that same fuel mix, one should be able to still get 5 PSIG at 10,000' in temperatures 20 degrees colder than at sea level.

Likewise for 10 PSIG, 15 PSIG, etc. Whatever PSIG one desires can be had if one goes out in temperatures appropriate for one's fuel mix.

Naturally, there are variations in air pressure due to factors other than elevation alone. There also variations in canister content. These are not "reagent grade" fuels. The percentages of propane and isobutane may vary. Other gasses such as propylene (if memory here serves) may be present. Even in canisters labeled as only containing isobutane, some n-butane will typically be present. Therefore, the closer one operates to the lower operating limit of a stove's temperature range, the more one should be prepared to take steps to mitigate the effects of cold on canisters.
1. Select a fuel that is labeled isobutane/propane only (contains no n-butane)
2. Start with a warm canister (sleep with it, put it under your shirt, etc.)
3. Warm the canister (put it in a bowl of water, use a chemical hand warmer, etc.)
4. Shield the canister from cold winds (place it in a sheltered spot, use a wind block, etc.)
5. Insulate the canister from the ground

HJ

markskor wrote:Your chart seems to show that increasing altitude affecting the lowering of the temperature window for "decent" canister gas ignition, (you called this slack?).

The use of the word "slack" was perhaps not the best choice of words. What I means is that one typically has more pressure in a canister than just that derived from isobutane alone. Isobutane boils at 11F, but there's also propane in the canister which boils at -44F. Now, the propane does boil off at a faster rate than the isobutane, leaving one with only isobutane at the end of the canister. But for much of the life of the canister, one has propane, therefore a higher pressure, and therefore one can operate in colder temperatures.

Also, one typically uses a canister stove above sea level. Those camping in Death Valley take note!

So, given the propane content and the fact that one is usually higher than sea level, one typically has more pressure in the canister than would be implied by my 20F lower operating limit advice, a lower operating limit based on isobutane alone. The difference between the pressure of isobutane alone at 20F and the actual fuel + the effects of elevation is what I meant by "slack". Perhaps "margin for error" or "pressure over and above the lower operating limit of a stove" would be better terms.

However, canisters do empty and propane does burn off faster, so I typically say "20 F" as the lower operating limit knowing that this is a fairly conservative number.

HJ

hikin_jim wrote:At sea level, the boiling point of water is 212 Fahrenheit. At 10,000 feet elevation, the boiling point of water is about 193 Fahrenheit, a decrease of 19 degrees. The boiling points of propane, isobutane, and butane (or a mixture thereof), fall by the same amount....

...The rule of thumb, and it is just that, a rule of thumb, is that one can operate a stove approximately 2 Fahrenheit degrees colder for every 1,000 feet of elevation gain. Note the use of the word "approximately" and the use of the phrase "rule of thumb."

I'm stepping away from a PSI based argument here. Pick whatever PSIG one feels is appropriate at sea level, and one should have that same PSIG at 10,000' in temperatures that are 20 Fahrenheit degrees colder.

Your rule of thumb is inaccurate.

At 10,000 feet water boils 18.8°F lower than at sea level but isobutane boils 16.5° lower and propane only 14.7° lower. So your estimate of a 2°F drop per 1000 feet is too optimistic; it should be closer to 1.6°F.

It's kind of beside the point since the fuel isn't going to be at ambient temperature. Still air doesn't contain or conduct enough heat. The canister is cooled by fuel vaporization and heated by proximity to the flame, immersion in water and/or warming of the cooking environment (e.g. vestibule). How these factors balance is the key to success in the cold.

Yesterday, out of curiosity, I tested the flow rate through a Snowpeak canister stove. The MSR canister I used was on the low side, 80% empty. So the full throttle pressure was about half of what a new canister would have produced.

3 psig: 1.2 g/min
5 psig: 1.5 g/min
10 psig: 2.1 g/min
22 psig: 3.6 g/min (full throttle at 61°F)

At 3 and 5 psig it was kind of a weak flame. Given how much fuel is necessary to melt snow and bring it to a boil it would be a long weight at the lower flow rates.

longri wrote:At 10,000 feet water boils 18.8°F lower than at sea level but isobutane boils 16.5° lower and propane only 14.7° lower. So your estimate of a 2°F drop per 1000 feet is too optimistic; it should be closer to 1.6°F.

Over a 3,000 meter elevation gain from sea level, the boiling point of hydrocarbons like propane, isobutane, and n-butane is depressed by about 9 degrees Celsius. Converting to Fahrenheit comes out to about 1.6, so I'll buy your number, but the difference between using your number versus the one I used over a 10,000 foot elevation gain is 4 degrees Fahrenheit, an amount which isn't going to have a huge impact on the operation of a stove. In addition, I can work with 2 in my head, and it's easy to remember. It's a rule of thumb not a precise calculation.

longri wrote:It's kind of beside the point since the fuel isn't going to be at ambient temperature. Still air doesn't contain or conduct enough heat. The canister is cooled by fuel vaporization and heated by proximity to the flame, immersion in water and/or warming of the cooking environment (e.g. vestibule). How these factors balance is the key to success in the cold.

Agreed. But you have to understand enough about how canisters work in order to successfully deal with the cold.

I usually tell people that above 50F/10C, you don't really need to worry about mitigating the effects of evaporative cooling as the fuel vaporizes. I tell people that they shouldn't expect an upright canister stove to work well throughout the life of the canister at less than 20 F/-7 C -- and that's using isobutane. These are round numbers. Yes, there are any number of factors that can affect those numbers. The point is that with a few simple numbers, the average person can have some rough rules of thumb that they can remember and use in the field.

To get really precise numbers, you'd have to know the exact temperature, the exact mix of fuel (which changes over time as the propane burns off at a faster rate and to be fully understood requires the calculation of the molar fractions), the exact barometric pressure of the air, the amount of heat loss due to convection, the amount of heat loss due to conduction, and on and on. For the average person, good luck. And out in the field? It's just not practical.

For field usable, practical values:
-Above 50F/10C, don't get too worried about canister cooling for normal boiling of say a liter or less of water.
-Think of 20F/-7C as the lower operating limit of a stove, and then adjust that by about 2 degrees F colder per 1000 feet above sea level, and don't use fuel with n-butane in it. The performance will degrade as one approaches the lower operating limit. Cold weather canister techniques are important as one approaches the lower operating limit (don't use n-butane, warm the canister before use, supply mild heat to the canister during use, insulate from the ground, protect from wind).

If the average person stays within those parameters, they should be fine.

Obviously, one can use can use an upright canister stove as cold as they like so long as they have good cold weather canister techniques. Screw up when it's really cold, and you could be in for a world of hurt. For that reason I generally recommend 20F/-7C as a practical lower limit.

HJ