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

[rsterne]

CP for CO2 72 Bar/26degc
So both are above the CP and therefore the standard phase shift for CO2 will not apply

I found a slightly different value for the critical pouint of CO2, namely 7.38 MPa at 31.1 °C (two different sources).... In SI units (as per the graph), that is 1070 psi at 88*F.... That is consistent with the point on the chart where all three colours come together, which is my understanding of the term.... The yellow area can further be divided by a horizontal line at 1070 psi (ie at the top of the green area).... In the lower left portion (ie where the pressure is less than 1070 psi) the CO2 is a gas.... In the remainder of the yellow area, the upper portion (ie where the pressure is greater than 1070 psi and AND the temperature is greater than 88*F) it is a supercritical fluid.... Therefore, you are correct, in the yellow area of the chart (above 1070 psi), the CO2 is a supercritical fluid.... However, in the blue area, where the temperature is BELOW 88*F, it is 100% liquid.... even if the pressure is above 1070 psi....

Now we get to the second part of your post:

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,
Tc(mix) = (mole fraction A) x TcA + (mole fraction B) x TcB.

One mole of air (as a mix) weighs ~29 grams.... At STP it will have a volume of 22.4 litres.... Therefore, at 850 psi (58.6 bar), the volume of 1 mole of air is 22,400/58.6 = 382cc....
One mole of CO2 weighs ~44 grams (1.55 oz.).... A 9 oz. CO2 tank would contain 9/1.55 = 5.8 moles of CO2.... Using the "100% fill = 7/18 oz./CI" from the chart, the volume of a 9 oz. CO2 tank would be 18*9/7 = 23.1 CI = 380 cc.... However, the liquid CO2 in the tank has a density of 770 g/L, so that 9 oz. of CO2 occupies 9x28.35/770 = 331 cc out of that 380 (ie the tank is about 87% full of liquid).... That leaves only 49cc for the air (for the moment I'm assuming all the CO2 is a liquid).... Since 1 mole of air requires 382cc of space at 850 psi, we have 49/382 = 0.13 moles of air if the CO2 tank was pressurized to 1700 psi (850 psi of air plus 850 psi of CO2).... The total number of moles is 5.8+0.13 = 5.93 moles, so the mole fractions are 5.8/5.93 = 0.978 for the CO2, and 0.13/5.93 = 0.022 for the air....

Working in degrees Kelvin (absolute), the critical temperature for air is 132.4*K and for CO2 is 304.2*K.... That means....

Tc(mix) = (mole fraction A) x TcA + (mole fraction B) x TcB
Tc(mix) = 0.978 x 304.2 + 0.022 x 132.4 = 297.5 + 2.9 = 300.4*K = 27.3*C = 81*F

For the critical pressure, I assume the same formula applies.... The critical pressure for air is 37.7 bar and for CO2 is it 73.8 bar.... That means....

Pc(mix) = (mole fraction A) x PcA + (mole fraction B) x PcB
Pc(mix) = 0.978 x 73.8 + 0.022 x 37.7 = 72.2 + 0.8 = 73.0 bar = 1059 psi....

So if my calculations are correct, a 9 oz. CO2 tank filled with 9 oz. of CO2 (at 70*F) and then pressurized to 1700 psi total by adding 850 psi of air should have a critical point of 81*F and 1059 bar.... While that isn't in perfect agreement with the chart, it's only 7*F and 11 psi lower.... My point is, that the CO2 is by far the dominant factor in terms of the amount of material inside the tank.... I still think my "theory" about how the CO2 acts at 70*F (ie 100% liquid, the air used first, then at 850 psi the CO2 starts to boil) is pretty close, for this example.... The fact that there is so little air and so much CO2, however, would make one ask if there was any advantage to doing this.... and yet Walter's results speak for themselves....

To answer that, we need to look at what Walter did.... If I understand correctly, he only filled the Disco 55% with liquid CO2, and then added HPA until the pressure was 2000 psi.... In addition, IIRC, he was working at only 55*F.... He would have had a smaller percentage of CO2, occupying a smaller volume, leaving more room for the air.... This would have lowered the critical point further....

140cc reservoir, 55% full of liquid CO2 = 77cc CO2 = 770x77/1000=59.3 gr. CO2 / 44 = 1.3 moles CO2....
140cc, 45% remaining = 63cc available for air.... 2000 psi (total) - 700 psi (CO2 @ 55*F)) = 1300 psi = 1350 / 14.5 = 93 bar x 63 cc = 5859cc / 22400 = 0.26 moles air....
Total moles = 1.30 + 0.26 = 1.56.... mole fractions, 1.30/1.56 = 0.83 CO2.... 0.26/1.56 = 0.17 air....
Tc(mix) = 0.83 x 304.2 + 0.17 x 132.4 = 252.5 + 22.5 = 275*K = 2*C = 36*F....
Pc(mix) = 0.83 X 73.8 + 0.17 x 37.7 = 61.3 + 6.4 = 67.7 bar = 982 psi....

In this case, test temperature (55*F) is above the critical point, and the pressure starts well above 982 psi, so initially there should be no liquid CO2 in the reservoir.... If I understand Sean correctly, and provided I have done the math right (a BIG if).... that means the mixture simply acts as a simple gas (composed of 83% CO2 and 17% air).... If that is the case, then it should have no more safety concerns than the same pressure of just air.... Did I get that right, Sean?....

So if we've got a gas consisting of 5 parts CO2 and 1 part air at 2000 psi, where are the extra shots coming from?.... If we filled the Disco completely with air, it would hold 140cc at 138 bar = 19.32 litres / 22.4 = 0.86 moles.... By filling it with a mix of CO2 and air, however, we have been able to cram 1.56 moles of the mixture into the same space, at the same pressure.... That is 81% more molecules in the same space, and yet the pressure hasn't increased.... I was OK right up until this point, now I'm completely confuzzled again.... How is that possible without having some of the CO2 remain a liquid?....

Bob

[FRANK]

March or there about i was working on my qb trying to make it work and make it work better, i have a 13ci tank and i filled it with liquid co2 and pump it up, not to 3000 psi i keep it lower.
I only use hpa not at the amount of shoots i get now it's not worth it messing with co2

[SeanMP]

Wow...first off I can't believe you did the math but yes I agree it's all correct.
Yes the mixture acts as a simple gas and that's why Steve's hypothesis is not correct in the application of Dalton's Law.

As for the last paragraph Bob...no flippin idea. Supercritical fluid mixtures are strange. In the superfluid state they have the density of liquid and the properties of gas. When you mix two they inherit properties of each. It possible that the big fat co2 gas inherits properties from the more vigorous nitrogen. It's possible that Walter stumbled on an ideal supergas for our purposes.

What I know for sure is people are writing doctoral thesis on the properties of supercritical mixtures all the time.....soooooo WAY outta my league

[Voltar1]

I do like the results as found :)

Wow...first off I can't believe you did the math but yes I agree it's all correct.
Yes the mixture acts as a simple gas and that's why Steve's hypothesis is not correct in the application of Dalton's Law.

As for the last paragraph Bob...no flippin idea. Supercritical fluid mixtures are strange. In the superfluid state they have the density of liquid and the properties of gas. When you mix two they inherit properties of each. It possible that the big fat co2 gas inherits properties from the more vigorous nitrogen. It's possible that Walter stumbled on an ideal supergas for our purposes.

What I know for sure is people are writing doctoral thesis on the properties of supercritical mixtures all the time.....soooooo WAY outta my league

[rsterne]

Thanks for checking the math, I really appreciate it.... When I do a complicated post like that, I just work my way through it a step at a time, typing as I go.... so I have no idea what the results will be until I get to the last equals sign.... *LOL*.... Then if I look at the results and it makes no sense, or is confusing, I end up with thoughts like in the last paragraph....

It would appear to me that if the reservoir is filled with CO2 by weight, there is little room left for air (1/8th), and in fact there would be liquid CO2 (at least I believe that to be so).... It would appear that with the reservoir about half full of CO2 (by volume) there is no liquid, but a mixture of supercritical fluids.... If that is the case, then there is very likely an optimum ratio of CO2 to air to maximize the total FPE.... I have no idea if that optimum ratio would change if you played with the hammer spring and/or porting to alter the bell-curve (assuming there is one) or the FPE per shot (and therefore the number of shots).... Once the mixture is in this realm of a mix of supercritical fluids, I would guess that the gasses would be used in the same proportions as their mole fractions.... In the previous example, 5 parts CO2 to 1 part air in every shot.... All that happens is that the pressure gradually drops as the mixture is used up, the percentages stay the same.... I would also think that rapid shooting would not alter the pressure (no "boiling" taking place), and that temperature change would no longer affect the velocity, at least not any more than with HPA.... Would you agree that has to be the case?....

Bob

[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

[FRANK]

OK but after all this is it safe to do it? it would make my shoot count go up in my Disco enuff to do a full FT day? insted of pumping every 20 to 25 shots.

[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

[FRANK]

that is wonderfull. I did try it a few times but after getting the my qb modded to shot 270 in one fill of hpa it was not worth it for the QB.
but on the other hand the Disco is a different story. I get up to 28 full power shots but mostly from 22 to 25 shots. so the co2 hpa mix would be great for this.
I can tell you this maybe it will help in ur calculation.

I fill my 13ci tank with co2 was about 900psi then I pump it up too 1500psi with hpa and i did that about 3 times had a great shot count but was too much work so I modded the hammer spring with some of the forum help.
so I know full of co2 and 1500psi was safe in a 3000psi tank regulated at 850psi