hikin_jim wrote:It's been 30+ years since I went through physics, calculus, chemistry, etc. I understand in a general sense what's going on, but the calculations are beyond me.

You're probably selling yourself short. The math isn't that hard.

1. Isobutane saturated vapor pressure = 10^(A + (B/(temperature+C))

2. ISA: elevation = D*(1-(pressure/E)^F)

A,B,C,D,E,F are constants

Put equations 1 and 2 together and you can make that chart.

The saturated vapor pressure is the equilibrium gas pressure for a given temperature. You can generate tables for different fuels here:

http://webbook.nist.gov/chemistry/fluid/" onclick="window.open(this.href);return false;

Antoine equation parameters (the formula I wrote above for isobutane) are in the "phase change data" link for each chemical species, like this one:

http://webbook.nist.gov/cgi/cbook.cgi?I ... ermo-Phase" onclick="window.open(this.href);return false;

If you want to mix fuels it's easy:

total pressure = x1*P1 + x2*P2 + x3*P3 + ...

where P1,P2,... are the sat. vapor pressures of the fuels and x1,x2,... are the mole fractions

Example: Say you have propane and isobutane, 20/80 by weight at 20°F.

The molecular weights: propane (44 g/mol) and isobutane (58 g/mol).

So the mole fraction of propane is: .20/44/(.20/44 + .80/58) = 0.248, call it 25%.

Thus, the mixture is 25/75 in terms of the number of molecules.

At 20°F the sat. vap. pressures from tables: isobutane = 17.8 psia; propane = 55.8 psia.

(The Antoine values are a little off: 17.9 and 57.5 psia)

So the total pressure = .25*55.8 + .75*17.8 = 27.3 psia.

At sea level: 27.3 - 14.7 = 12.6 psig; at 10,000 feet: 27.3 - 10.1 = 17.2 psig.

With a spreadsheet you can generate endless charts like this.

A monkey could be taught to do it.

hikin_jim wrote:
I think I know what the answer is going to be, but if you ever wanted to make a short video showing (via voice, subtitles, or just time into the video correlated to specific psig) the flame at various psig, say 3, 5, and 10 since those are what we've previously discussed, I'd love to post it on my blog. I think it would be helpful to people to understand very clearly the impact of low pressure and why pressure needs to be boosted by warming canister before use and keeping the canister warm during use.

A video sounds like too much work.

Here's another way to think about it though.

At full power my stove is rated at 10,000 BTUs. These fuels contain 46 kJ/g so to generate that rate of heat output requires 3.8 grams of fuel per minute. At 5 psig the fuel flowed through my stove at a rate of 1.5 g/min which means the heat output was 4000 BTUs.

In terms of time, suppose you wanted to melt enough snow to produce 2 liters of water. If the stove were transferring heat at 50% efficiency it would take about 8 minutes at full power to melt the snow. So at 5 psig it would take around 20 minutes. You want it boiled? Add at least another 20 minutes.