Boyles Law / Burnt Latkes Fuel Metering Calculation

[Ratguy]

This is more of an informational post for people who are starting out and are not familiar with Boyles law. I decided to post this because I was noticing inconsistencies with BurntLatkes Fuel Tool.

Boyles law states that pressure and volume have an inverse relationship with each other. (with no temperature change) If the volume doubles, the pressure halves and vice versa. The formula for this is Volume1*Pressure1 = Volume2*Pressure2. IMPORTANT: V1 and V2 are always absolute pressures (psia) not relative. In BurntLatkes explanation, he uses 90psi for the tank pressure and plugs that into the equation. In reality, that number should have been 104.7psi. Unless you obtain a pressure gauge that is vacuum calibrated, a normal gauge will read 0 psi at atmospheric (~14.7psi)

Example calculations for my cannon are below. In my case, I have an adjustable regulator, so I am matching my pressure to a predetermined meter volume. You could apply the same mathematics if you have a predetermined pressure and are looking to get the appropriate meter volume:
Chamber - 160 cu. in.
Meter - 2.15 cu. in.

Volume 1 is the meter volume 2.15. Pressure 1 is what we are trying to find. Volume 2 is the volume of propane that we want in the chamber -- 160 *0.042(4.2%)=6.72. Pressure 2 is the pressure that the propane will be at once it enters the chamber, which will be atmospheric pressure -- we'll use 14.7 as the average (Atmospheric pressure varies based on altitude, weather, etc.)

Solving for P1:
V1*P1 = V2*P2

2.15*P1=6.72*14.7

2.15*P1=98.78

(2.15*P1)/2.15=98.78/2.15

P1=45.94 psia - atm(14.7)

P1=31.24 psig