The Helium Effect Explained

[DU-BB]

And not by me, this is a quote I came upon by accident, but it needed to be shared here:

"The limiting factor on the speed of an airgun, firearm, or light-gas gun is the speed of sound in the working fluid—the air, burning gunpowder, or a light gas. This is essentially because the projectile is accelerated by the pressure difference between its ends, and such a pressure wave cannot propagate any faster than the speed of sound in the medium. The speed of sound in helium is about three times that in air, and in hydrogen 3.8 times that in air.

The speed of sound also increases with the temperature of the fluid (but is independent of the pressure), so the heat formed by the compression of the working fluid serves to increase the maximum possible speed. Spring piston airguns increase the temperature of the air in the chamber by adiabatic heating; this raises the local speed of sound enough to overcome frictional and other efficiency losses and propel the projectile at more than the speed of sound in the ambient conditions."

[Parellax]

When I was playing around with the stuff I came to the conclusion that the bb's were just behind the sound wave. If you remember according to my Chrony that's right where all of my fastest shots were.

The sound is totally different than shooting HPA though. There is a definite cracking which lead me to believe the helium escaping after the bb was breaking the sound barrier.

If I had a DrozdMax smooth bore I would be really tempted to give it another shot. Now that I have a compressor I can pump helium from a large tank at lower pressure into a 90 4500 using a trash bag as a diaphragm.

I don't think I ever mentioned it, but when I was in the mountains with the helium air mix I also had a bottle of helium. I shot my Bumblebee with a stock barrel on it extremely consistent just above 800 fps. I will have to go back, and watch to see what pressure I was using for straight helium. I have two 90 4500's. One is at 1000 psi, and the other is 1600 so it was not the pressure I had the mix in.

[DU-BB]

Now that I have a compressor I can pump helium from a large tank at lower pressure into a 90 4500 using a trash bag as a diaphragm.

There has to be some way to connect the inlet on the compressor directly to the helium tank. Some kind of adapter. I haven't messed with helium, yet, so I don't know what kind of outlet it has. But in case that a high pressure feed is too much for the compressor, a pressure regulator for a propane tank, or even better, a scuba tank would do the trick. Something better than the "bag over her head" approach, which I admit, was brilliant for a field expedient solution, but if you are going to pass gas, you might was well do it right, you know?

[Parellax]

Makes a lot of sense. The closer you get to the SB it gets exponentially more difficult. It's like you can knock on that door all you want using air the honest way. My fastest shot has been 1060 something.