HPA and Co2 mix- someone around here somewhere has done this...

[Hammer Sandwich]

WOW!.... I chatted with Walter a bit about this .... It wouldn't take a very expensive gauge to find out who is right, or if the answer lies between....

Bob

Whoa whoa whoa....let's not cloud conjecture with facts!

That's not what the interwebs are about....

[SeanMP]

Your experiment is going to be a little tricky Bob

I think what it is going to show is how many moles of each gas exist in the gaseous state. Using Daltons Law of partial pressures.....( This is where the confusion exists I believe) Daltons Law of partial pressures states that the total pressure will be the sum of the pressures exerted by each gas

Reading that it would seem that steve is correct,

however it is a misinterpretation because the calculation is based on moles of gas and at these pressures both the air and the co2 are actually supercritcial fluids.

[SeanMP]

Even if the container were filled with co2 once it hits the critical point the liquid and the gas cease to exist and the whole mass becomes supercritical fluid allowing whatever other supercritical fluid that is present to mix completely into the whole. But at that ratio the proportion of air would be so small it would have little effect on the physical properties of the co2.

However heading in the other direction I agree there might be some interesting possibilities
The reason why HPA is so ideal is that for our purposes we are using it 100% of the time as a supercritical fluid. Because it's not crossing back and forth across phase lines it has a predictably stable pressure within the range that we are using it at.
Soooo yes. If you mixed enough supercritical air into supercritical co2 to change the CP to say 70-75deg. The air would inherit fluid density from co2 and the co2 would inherit phase stability from the air.
At that point I agree you would have a gas that would not suffer from the usual co2 issues. It would behave as air within the normal temperature range but with an increased density and thereby an increased shot count.

Bloody hell we need to recruit a PhD to do some phase diagrams Bob

[Voltar1]

Interesting thread fellas :)
Signed up so I could reply here.

The 'test' was as written up on the Yellow. Results were posted as discovered. Might could do it again with an AT44 or sum such......

Cheers

[Voltar1]

Shots on just air May 28 2008, 4:57 PM


filled to 2000psi

10 went 510-578
10 went 568-596
10 went 597-623
10 went 623-648
10 went 648-682
10 went 681-717
10 went 717-756
10 went 756-789
10 went 789, peaked 791 down to 712
next 7 went from 711 down to 594
stop

Made no changes to gun setup for the test on air.
So blead off the little reamining air and filled with CO2

muzzle down for a minute and then one shot at 654fps

letting gun stabilize in shade.

Walter....

[rsterne]

So far, Frank, this is all just math and a couple of guys trying to understand the physics involved.... Walter used 55% CO2 in a Disco and then topped up with air to 2000 psi and got 234 shots totalling 3000 FPE.... I'm not comfortable telling anyone it's safe until some experimenting is done with different percentage fills and chilling and warming the reservoir while watching the pressure gauge.... but it looks promising....

Bob

[rsterne]

HI Walter, and welcome.... I know you published the velocities, they are in that thread on the Yellow, linked below.... I think a simple pressure test, using three widely spaced temperatures, would settle what is going on.... Is my summary of Steve's and your positions below accurate?....

Bob

[Voltar1]

Shots on CO2 May 28 2008, 5:31 PM


30 ave 680
30 ave 680
30 ave 678
10 ave 672
10 from 672 down to 622 ave 646
next five 622,619,615,612,609
stop.

weighed gun full = 5.08 pounds
weighed after shots = 4.96 pounds
CO2 consumed = 0.12 pounds = 1.92 ounces

Walter....

[AirGunEric]

I think someone should put a gauge on things and give it a try. Of course, that would be me trying to figure out how to get 55% Co2 in a bottle which I'm bound to ne 'approximating' more than anything. Will see if I can find some time soon for this...

[SeanMP]

What I'm suggesting is that because they are both supercritical fluids they are going to permanently mix and this will alter drastically the phase shift of the co2

CP for air(nitrogen) 34Bar/-147degc
CP for CO2 72 Bar/26degc

So both are above the CP and therefore the standard phase shift for CO2 will not apply

All supercritical fluids are completely miscible with each other so for a mixture a single phase can be guaranteed if the critical point of the mixture is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
T_c(mix) = (mole fraction A) x T_cA + (mole fraction B) x T_cB.