(Math, I know, ugh, boring.)
Density values in lb/ft^3
rho_He = 0.1107
rho_Air = 0.0765
35% change in density, 44% increase.
20 fpe to 27 fpe = 29% change, 35% increase.
Close nuff.
-Matt
You're correct of course Matt. I got lazy about adding a % fpe change column for each pellet since they vary a bit and the spreadsheet would have provided accurate numbers.
A power increase of about 1/3 is probably a reasonable rule-of-thumb on this particular power plant and settings.
My original Helium tests 35+ years ago were done on a multi-shot .25 Career 707 under lever carbine. It had a power wheel and opened up it topped out at about 65 fpe on air and 105fpe on Helium. At an increase of 38% it's still in the same general range as the regulated Notos.
Helium won't always work as well as air in a particular airgun depending on unique aspects of the design and whether a person can also adjust the hammer and valve spring tension when using Helium. For example, a .22 Shortened Huben GK1 produced somewhat higher power on Helium than on air at 3 revolutions of the power bolt, but the pressure dropped so fast with Helium at that setting that all gas leaked out after the 3rd shot - that was not a problem with air. The Huben uses a unique pressure valve though compared to most airguns, so Huben results there don't necessarily reflect on non-Huben designs.
I can't think of any good reason to use Helium in areas where variable-power compressed gas airguns are legal and available. In jurisdictions where rifles are mechanically restricted to 12fpe, using Helium could temporarily raise the shot power to about 16fpe. But they aren't supposed to do it and people already have access to information on the mechanical modifications of most rifles that would result in the same levels of increased power.
JP