Water boils at 212 deg F at sea level, of course, but the
boiling temperature of pure water is a pretty strong function of the ambient pressure. This figure shows the boiling temperature of
water at different pressures.
Atmospheric pressure at sea level is around 14.7 psi, and that point is
marked on the figure with a red circle. The standard operating pressure for pressure cookers is 15
lbs which means 15 psi above atmospheric pressure. That point is marked with a green triangle in
the figure. You can see that the boiling
temperature for water is around 250 deg F inside a pressure cooker operating at
sea level.
Atmospheric pressure changes from day-to-day, but those
fluctuations are pretty small. This
figure shows the average atmospheric pressure at different elevations above sea
level. At sea level, the air pressure is
14.7 psi, but close to the top of Mt. Everest, it would be around one-third of
that.
We could use the second figure to determine that at 10,000
ft, the atmospheric pressure would be about 10 psi, then use that number in the
first figure to find that the boiling temperature of water at that elevation
would be about 193 deg F.
Instead of
doing all that work one point at a time, we could just combine the two figures
to get this figure which shows the boiling temperature of water, both inside of
a pressure cooker and on an open stove, at various elevations. It is interesting to note that the effect of
altitude isn’t as great inside the pressure cooker. On an open stove, the boiling temperature
drops almost 20 deg F when going from sea level to 10,000 ft, while inside the
pressure cooker, it only drops about 10 deg F.
I guess high altitude mountaineers should lug a pressure
cooker along with them if they want to do gourmet cooking during an expedition.
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Here are the same three figures in SI units:
So does that mean that gas powered stoves that backpackers use would require less fuel at high altitudes to bring water to a boil than at low altitudes? Or do the stoves operate less efficiently at higher elevations, somewhat negating the lower boiling temperature?
ReplyDeleteI suppose that there might be a slight drop in combustion efficiency with the lower partial pressure of oxygen, but since it is an open diffusion flame, I doubt that would be very significant. So yes, you'd require less fuel to bring the water to a boil at elevation, but your boiling water wouldn't be as hot as at low altitude, either.
ReplyDeleteThanks for your clear and concise presentation here.
ReplyDeleteI'm going to head off to do some extended trekking in the high Himalayas. Both altitude (I'll be up to 20,000ft) and locally available food sources (rice and beans - dhal) conspire against attempts at fuel efficiency. And no wonder there's been so much deforestation in the populated high valleys.
I'd like to take a pressure cooker with me, but I've found that light packable pressure cookers just aren't available (they're all over a kilo in weight and the lighter models have long protruding handles).
I know most trekkers may have porters carrying in bags of dehydrated foods, but IMHO that's a bit of a cop-out. Outdoor gear manufacturers could help matters by anticipating this latent* need and selling something that fits these simple needs. (*No pun intended.)
Rant over :)
Sounds like a market opportunity is waiting. Best wishes on your trip...
ReplyDelete