Helium in PCPs - Why?

[rsterne]

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

[Bwalton]

Bob I do not know if you saw this but there was some that have tried it......


October 12, 2010 12:04PM       Registered: 5 years ago
Posts: 48
Good morning gentlemen...here are some numbers with Helium
Gun used is what Brent and myself made....40 cal. shooting a 200 grain slug sized to .408, bullets were purchased from Hunters Supply...Lead does have brinell hardness, not dead soft...
Charged to 3500 psi
1st shot...1056 fps
2nd 1037 fps
3rd 978 fps
Charged to 4000 psi
1st shot 1050 fps
2nd 1020 fps
3rd 1050 fps
4th 890 fps

215 grain slug @ 4000 psi
1st shot 980 fps
2nd 982 fps
3rd 956 fps

Notes to ponder...Using nitrogen the gun can be charged to 2800 psi, any higher and valve lock occurs...using Helium and valve lock occurs at about 4600 psi...fps difference between nitrogen and helium is about 200 fps...bullet grouping did not change with increase speed...all clover leaf @ 50 yds...Chronograph was set at 30 ft from muzzle...I wanted to test more but Gods waiting room got busy (Wish I could see his daily planner)

[Habanero69er]

Interesting reading.

[Bwalton]

here are some results:

http://www.airgunrendezvous.com/rendezvous/read.php?2,9096,9096

[rsterne]

Did you see the data which gave the fastest shot with Helium at 1050 and with Nitrogen at 900.... That's a 16.7% gain, pretty close to the 15.8% I predicted.... Fancy that!.... Even the 1080 vs. 880 numbers are only a gain of 22.7% velocity.... What I find more interesting is that the Helium seems to allow a higher fill pressure for the same hammer strike.... That, to me, means that the valve is staying open longer with Helium, which I didn't allow for in my simplified calculations.... I used the same dwell for each example I gave....

The force required to knock the poppet off the seat would be proportional to the pressure.... Much of the closing force comes from the pressure in the valve throat acting on the area of the valve stem, so again that should be the same.... The remainder of the closing force, however, is caused by the drag of the air mass going past the head of the poppet, and causing what amounts to a pressure differential across it.... I can only guess that maybe the Helium passes over the head of the poppet easier (maybe each atom, being lighter, hits it with less momentum?) and therefore the closing force is lower with Helium, creating more valve dwell, and hence more potential power.... The writer also said that the gun used a LOT more Helium than air, which we might expect if it is producing 50% more power, but if the valve is staying open longer, then the Helium use would increase even more.... Without changing the tune, he was talking about a 50% increase in power, but with twice the amount of Helium used as air (2 shots instead of 4).... That loss of efficiency would be consistent with more valve dwell as well....

So, perhaps, based on just that one guys data, we might say that you can expect AT LEAST a 33% increase in FPE with Helium (due to it's lower mass).... and perhaps as much as 50%.... As I said, those other factors I mentioned are unquantified at present....

Bob

[Tommy]

This is gonna sound stupid but could you mix 50/50 air and helium or would one degrade the other?
Tommy

[Bicycleman]

Now you got me thinking.  The party stores in town will sell you a no-return, five gallon Helium tank for about $30.  What I am wondering about is the pressure in the little tanks. 

[StevenG]

The "helium" in many party stores is in fact a blend.
It is generally between 250 and 300 psi.

[Bicycleman]

Thanks, Steven.  I was thinking that I could start with the 250 - 300 PSI and then add air.  But that won't work if it is already "cut".

[DOKF]

The "helium" in many party stores is in fact a blend.
It is generally between 250 and 300 psi.

the party tanks are usually only about 10% He.  Just enough to give the balloons some lift.

[StevenG]

It depends on the source and type of tank. I am talking about the cheap tanks that look like propane tanks.
Party city has pure helium in even those, 95%+.

Like the ever wise rsterne said, if you do that and let the gun sit overnight all you will have is air in the tube. Since helium is a noble gas it does not react with air, so no risk of "degradation".

Some blends can be that low, some are much higher.

[bnowlin]

Dang Bob,
Done gave me another headache
Bob

[Finwall]

Just something else to add.  Our supply of helium is finite so prices will only go up. Just google helium shortage. 

[rsterne]

I can't imagine mixing Helium and air could create a problem, but as stated if the O-rings are permeable to Helium you will end up with just air in the gun.... I have been told that Viton O-rings will hold Helium much better....

Bob

[NJMevec]

Bob stop giving me project ideas! You make saving money very difficult lol. Gonna try and find a source of helium now because for something like my 2240 carbine keeping it compact and giving it good hunting power will be tough and squeezing every little drop possible from it will be fun and almost required. See if we can get up to that 800 fps mark and not have the gun go over 35 inches!!

[dyotat100]

I did some helium testing a couple of years ago in my .25 and .308. condor.

I use to keep a tank full to shoot in my .25 but I drained it a couple of months ago and sold the tank finally.

In .25 with 3000 psi fill I went from 110 fpe to 233 fpe.

The .308 went from around 230 fpe to 414 fpe.

I haven't tried it since I got the .308 up to 300 fpe on air. I'm sure it would be well over 500 fpe.

When I bought my 80 cf tank the fill was $42. Last time I checked it was $230 to fill it now.

I have a booster and can fill to any pressure. Right now I have 1000 psi left in my tank.

Maybe tomorrow I will screw a .257 valve into my 400 cc tank and pump it up to 3000 psi and see what it will do. Maybe 350-400 fpe.

[rsterne]

I assume those numbers showing double the FPE are with the same bullet at the same rifle settings?... ie ONLY changing to Helium from air, and both at 3000 psi?.... If so, it's most impressive, and there is obviously a lot more going on than just the mass difference.... I'd love to see the hard data (bullet weight, velocity, etc.).... What bullets were you using in the .25 at 233 FPE?....

Bob

[Zebra]

My understanding is that the volume of air going from the valve to the barrel has a large impact on power which is why big bores use more air. If helium is lighter then it will take more of it to fill a cylinder to the same 3000 psi pressure. Things that are lighter can also travel faster when that pressure is released. Smaller molecules means a greater volume can pass through the same space under identical pressure. In other words, when the hammer hits the poppet, a lot more gas is going through the barrel with 3000 psi of helium than it would with 3000 psi of air with all else being equal.

From what I have seen on the sites of the companies that make custom big bores, the performance increase with helium is huge. They state 300 ft lb with air and 800 ft lb with helium for their guns.

I don't think a different hammer or spring would be needed because it is the psi that makes it difficult to move the poppet. An lb of feathers weighs the same as an lb of bricks.

The big bore makers advise not to use a mix of helium and air. They say you have to choose one or the other. I don't know why but I also don't know if air and helium mix or if we are talking oil and water.

If one has a tank of helium, you could run a hose into a hand pump using the threads where the pressure guage  would normally screw into and use that to pump 3000 psi into a PCP cylinder (if you could get the valve to shut after you drain all the air). Or, you could just take your 4500 psi air tank to a gas supplier and ask them to fill it.

I can't see a point myself. The big bore makers use it because they are hunting huge game with air guns. They are killing 1000lb animals with slugs up to 72 cal. If I needed to send a 400g slug at 1000fps, helium would be the way to go but for the small game hunting I do, it would be overkill. If I needed that much power, I would buy a firearm which is the right tool for 1000lb animals imo. A single shot big bore for $1500 does nothing for me. But that is just taste.

If you shoot indoors, you would end up talking like Mickey Mouse after a while with helium too.

[rsterne]

If helium is lighter then it will take more of it to fill a cylinder to the same 3000 psi pressure.

Actually, if you fill a container with Helium at 3000 psi, the gas in the container will only weigh 4/29ths what it would weigh if you fill that same container with air.... If you expand both gasses back to 1 atmosphere pressure, they would both occupy (nearly) the same volume....

Bob