Re: is snowpeak gas in a primus stove ok?
- From: "J. Curl" <jhcurl@xxxxxxxxx>
- Date: 4 Oct 2005 12:59:26 -0700
Bill Tuthill wrote:
> J. Curl <jhcurl@xxxxxxxxx> wrote:
> >
> > Even the few manufacturer websites that specify the mixture are suspect.
> > For example, MSR (IsoPro) and Jetboil (Jetpower) both claim identical
> > fuel compositions on their websites, but these fuels behave differently.
>
> That could be because Jetboil canisters have lower pressure, or something.
Higher, not lower. But the reason there are pressure differences (at
the same temperature) is because the mixtures aren't the same.
> > So a 20/80 propane/isobutane boils at .2*231K + .8*261K = 255K (-18C)?
> > That's not how you calculate the boiling point. If you assume an ideal
> > mixture with a 20/80 ratio by weight, it works out to about -23C. In
> > reality, higher volatility components in the mixture depress the initial
> > b.p. significantly. Initial b.p. isn't a very useful statistic anyways.
>
> Do you know the formula to calculate it?
I did it iteratively by using tables for saturated vapor pressure (you
can find them at http://webbook.nist.gov/chemistry/fluid/). If you
assume propane/isobutane forms an ideal mixture (a good assumption),
then you can use Raoult's Law to calculate the total vapor pressure:
VP = x*Pp + (1-x)*Pi, where x=propane mole fraction, Pp and Pi are the
sat. vapor pressures of propane and isobutane at a given temperature.
Pick a temperature, calculate the total vapor pressure. If it's equal
to the atmospheric pressure, you've found the boiling point; if not,
try another temperature.
If you're starting with weight fractions (is the 20/80 by weight or by
moles?), you need to convert first. Use the fact that propane is about
44 g/mol and butane (n- or iso-) is about 58 g/mol. A 20/80 pro/iso
mixture by weight is approximately 25/75 by moles.
> Perhaps the Century stove that screws into Coleman/Century propane bottles
> is less efficient than the Primus Yellowstone Classic I got to replace it.
> According to the REI website, the Yellowstone Classic burns 50:42 minutes
> on 215g of Primus fuel. Whereas according to the Campmor website the
> Century Scout burns 53 minutes on 465g of Coleman/Century propane fuel.
>
> Can you explain that?
It's confounding because you're not only comparing fuels, but also
stoves, and probably not under identical conditions. Also, I'd be
leery of specifications on a third party website. The propane in a
16.4 ounce cylinder releases about 22,000 BTU of energy when burned.
If it burns at the 8000 BTU/hr claimed, that would give it a lifetime
of 2 hrs 45 min, which is closer to what I've experienced with my
propane stove (also a Century). Does yours really die after less than
an hour? Maybe it has a leak.
Those specifications also claim a faster boiling time for the propane
stove, which sure isn't what you're reporting. There are just too many
factors that influence boiling time. Unless you control all the other
variables first, presuming that the difference is due to the fuel is
jumping to a conclusion.
.
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