Calculating vapor pressure of propane during test firing

I will be relying on the vapor pressure of a propane tank to maintain a certain feed pressure for my rocket igniter (which I will measure with a pressure gauge). But will the boiling/evaporating of the propane while it's being used lower the temperature enough that there's a significant reduction in feed pressure?

Using figures mostly from here:
http://en.wikipedia.org/wiki/Propane_(data_page)

propane heat of vaporization:
356kJ/kg (or 356J/gram)

propane liquid molar heat capacity:
98.36J/(mol*K)

propane molar mass:
44.1 gram/mol

propane liquid heat capacity:
98.36J/(mol*K)/(44.1 gram/mol)
=2.23J/(gram*Kelvin)

Thus a 4 second firing consuming .5 grams of propane a second for a total of 2 grams will cool the liquid propane in the tank by:
2grams*356J/gram=712Joules
which will change the temperature of half a liter (roughly 500 grams) of propane by:

712J/(500grams*2.23J/(gram*Kelvin))
=
.639 degree (in K or C)
that's roughly 1 degree F (or Rankine)


It's quite possible that the whole tank won't equalize in temperature, but even so, it does mean there's not a big enough drop in total temperature to lower the overall vapor pressure by a considerable amount over the length of the burn.


Here's a chart of the vapor pressure of propane (and other substances) as it changes with temperature:

Wolframalpha.com says that propane at 77 F has a vapor pressure of about 138.1 psi, and at 76 F, it's 136.1 psi. So, if we're operating the tank at 77 F, we'll get roughly a 2 psi change in feed pressure over the length of the burn, assuming 500 grams of propane in the tank, a feed rate of .5 grams of propane per second and a 4 second burn length. That's low enough that we don't care.

BTW, since we'll be wanting to operate at closer to 80psi for the propane tank, that means we need it to be at 41 Fahrenheit (5 C), which means an ice bath or something like that.
http://www.wolframalpha.com/input/?i=propane+vapor+pressure+at+5+Celsius
Still need to figure this out, but I will be doing some experiments with flow-rates and orifice sizes (I'll be drilling them my own).