Helium in PCPs - Why?

This topic has been mentioned a few times in the past, and although it is pretty well known that the use of He instead of Air in a PCP can improve performance, not much has been written about why that is the case.... There are a few things different about Helium, and some of them make a big difference to how it performs in a PCP....

What is Helium?.... It is an inert gas, and the second lightest element, next to Hydrogen.... Since it is inert, it is non-flammable and non-corrosive, and so relatively safe to use, although you can't breathe it for prolonged periods.... That is not because it is poisonous, it simply doesn't contain Oxygen, so it can't support life.... It exists as single atoms, with an atomic weight of 4, which compared to Nitrogen (78% of air) which exists as N2 (molecular weight of 28) and Oxygen (21% of air) which exists as O2 (molecular weight of 32) is very light.... Air has an average molecular weight of 29, so Helium, at a given pressure, weighs only 4/29ths as much, in other words air is 7.25 times heavier than Helium.... This gives it quite different physical properties, some of the important ones for us are below:

Speed of sound in air = 1126 fps (at NTP).... in He it is 3040 fps....
Average molecular velocity in air = 1646 fps (at NTP).... in He, it is 4435 fps....
The viscosity at NTP (20*C and 1 atm.) is similar, however....

When you fire a PCP, not only do you have to accelerate the pellet down the bore, but also the gas propelling it.... You might think that the weight of gas is insignificant, but at high pressure it definitely is not.... As an example, a stock Disco at 2000 psi, shooting a 14.3 gr pellet, might shoot 850 fps (23 FPE).... If you took away the mass of the air, somehow applying the same force to the pellet, the velocity would be about 1013 fps (32.6 FPE).... that's a 16% loss in velocity.... and a 30% loss in power, just because of the weight of the air being accelerated down the barrel.... If we used Helium instead of air, still at 2000 psi, we would expect 984 fps (30.8 FPE).... That's only a 3% loss in velocity and a 6% loss in energy.... or looking at it compared to using air, it's 16% more velocity and 34% more energy.... That doesn't take into account any other factors, JUST the difference in the lower mass of Helium.... and that in a gun where the pellet is travelling well below the speed of sound inside the barrel....

When we look at the other factors, and start looking at a PCP that is really pushing the limits of what is possible with air, then it is possible that the higher speed of sound in Helium, and the higher velocity of the atoms, can give us even more performance.... The possibility of sonic choking in the restricted parts of the porting will almost vanish, for example.... The theoretical limit of 1646 fps for a PCP running on air will likewise increase to 4435 fps.... I don't of course have any hard data to quantify what those gains might be.... but just the difference in performance due to the lower mass of Helium shows about a third more power available in a modest gun like a Disco....

For those of you concerned about the practicality of using Helium, all I can say is that it has the ability to move through a standard Buna Nitrile O-ring very easily.... While you might be able to do some testing, the gun would lose a lot of its pressure overnight, right through the O-rings.... I know there are other materials that can prevent this, but I have no experience with them to make any recommendations.... Plus, of course, Helium is rare and expensive....

Bob