Total FPE = avg. FPE per shot times shot count (or if you have it, the sum of all the FPEs of all the shots)....

Total air used (std. CI) = pressure drop (bar) x reservoir volume in CI .... One standard CI is one in^3 of air at 1 bar pressure....

Pressure drop (bar) = pressure drop (psi) / 14.5 (psi/bar)

You also use the HP reservoir volume in a regulated PCP, providing you don't shoot below the setpoint pressure.... because the plenum pressure doesn't change....

Reservoir volume (CI) = reservoir volume (cc) / 16.4 (CI/cc)

**Efficiency in FPE/CI = Total FPE / Total CI**

If you shoot below the regulator setpoint, you need to correct for the pressure drop in the plenum.... You are starting with TWO volumes (HP reservoir and plenum), at different pressures (fill pressure for reservoir, and setpoint pressure for plenum).... and ending up with the total volume (reservoir plus plenum) at the ending pressure....

The density of CO2 gas at room temp (70*F), is 1.83 g / L.... Therefore a 12 g. cartridge contains ( 12 / 1.83) litres x 61 (CI/litre) = 400 std. CI once expanded (and a 90 g. contains 3000 CI), so you can use that for the CI to calculate the efficiency.... If you know the weight in of CO2 in a bulk fill gun or tank, you can use this formula....

Expanded volume of CO2 at STP = 33.3 CI per gram = 945 CI per oz.... Do not assume a 9 oz. tank has 9 oz. of CO2 in it, you must use the actual weight of CO2....

There are two ways to calculate the efficiency of a pumper.... You can do it just like a PCP, using the valve volume and the pressure inside it before the shot (which requires a pressure gauge), which tells you the efficiency of the SHOT itself.... Alternately, you can do it using the amount of air pumped into the valve (swept volume times number of pumps), which takes into account the pumping efficiency of course.... If the pumper dumps ALL the air in a single shot, the calculations are straight forward.... If it retains air, then to use the PCP method, you need to know not only the volume of the valve, but also the pressure at the end of the shot as well as at the beginning of it.... If you are calculating the efficiency based on the swept volume and number of pumps, then for a retained air pumper you would use the number of pumps used to refill the valve between shots....

Lastly, you can use a similar method for a Springer.... Since they are typically one pump (cocking motion) per shot, you can use the swept volume of the compression chamber (area of ID times piston stroke) as the Total CI.... Note, however, that you will get MUCH HIGHER values for the FPE/CI efficiency than for a PCP, because you get Adiabatic compression occurring, which heats the air being delivered to the pellet.... This increases its energy more, so you will get far higher values than for a PCP, where the expanding air actually cools and loses energy that is then not available to provide FPE to the pellet....

HTHs....

Bob