Simply put:

<table><tr><td><pre>

IF chamber does not displace air as propane is injected

THEN

P<sub>m</sub> = 0.6174V<sub>c</sub>

-----

V<sub>m</sub>

IF chamber does displace air as propane is injected

THEN

P<sub>m</sub> = 0.59241V<sub>c</sub>

-----

V<sub>m</sub>

</pre></td></tr></table>

Where:

P<sub>m</sub> = pressure of propane meter in gauge PSI

V<sub>c</sub> = volume of combustion chamber

V<sub>m</sub> = volume of propane meter, (in same units as volume of combustion chamber).

EDIT: that's the second time I've done that. For some reason, if you put P<sub>m</sub> as being in absolute terms... it doesn't work. It really annoys me too because one side of the equation is in PSIA and one side is in PSIG. I think it has something to do with propane staying inside of the meter after injection, so here's my hypothesis.

You have a combustion chamber and a meter pipe. The combustion chamber is 100 cubic inches and the meter pipe is one cubic inch in volume. Atmospheric pressure is in terms of PSIA, and final pressure of the meter is in terms of PSIA.

If you inject using the above formulas and use the output in terms of PSIA and not PSIG, then when injecting, one atmosphere at one cubic inch will remain inside of the meter pipe, unless you allow enough time for the meter to completely diffuse it's propane into the chamber.

Since most spudders just open and then close their ball valve, (which is more efficient because you don't have to bleed the meter before injecting every time), the absolute output of the equation won't cut it. You will be one cubic inch short because that volume of propane stays inside of the meter.

Adding another 14.7 PSIA to your meter pressure will bring that cubic inch back into the chamber, and leave another inside the meter. So, the output would more correctly be written in terms of absolute PSI as:

<div align="center">

</div>

<div align="center">Where P<sub>m</sub> is in PSIA</div>

Of course, since the average spudder rarely wants an answer in PSIA, we can simple use one of the above equations, and say that P<sub>m</sub> is in PSIG.