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

[AirGunEric]

I seem to recall someone was experimenting with mixing HPA and Co2 in a rifle tank/reservoir and getting some very good results. Anyone remember who this was?

[FRANK]

It was me.

I was told not to mix but I did to test it out. and my shout count was hight then HPA but the speeds where a bit slower but not enuff to make me stop, but ever since I bought a HPA Pump well
I don't mix.

it gave me no problems but was told not too do it.

[Voltar1]

When exactly was that Frank?

It was me.

I was told not to mix but I did to test it out. and my shout count was hight then HPA but the speeds where a bit slower but not enuff to make me stop, but ever since I bought a HPA Pump well
I don't mix.

it gave me no problems but was told not too do it.

[rsterne]

I don't have a CO2 tank, and I'm not interested enough to spend the time and money to drive 4 hours to purchase one and get it filled, just to do the experiment.... I was just suggesting something that could possibly settle the debate....

at these pressures both the air and the co2 are actually supercritcial fluids

If I read Steve's argument correctly, the air in the tank isn't affecting the CO2 in any way, it is only affected by the temperature.... Therefore the CO2 follows the accepted chart (below) completely.... That would mean that at 70*F (for a tank between full and 1/3rd full), the CO2 was in the green (liquid/vapour) area of the chart, not the yellow (supercritical) region.... It would also mean that for a tank filled to 100%, if you increased the temperature to 120*F, the pressure of the CO2 would increase to 1900 psi, and then you ADD the air pressure onto that to get your total.... That was his safety concern.... If you started at 70*F (where the CO2 was only 850 psi), then added air to bring the pressure to 2000 total (ie 1150 psi of air).... then warmed the tank to 120*F, the total pressure would be 1900+1150 = 3050 psi....



Personally, I don't see how either can be the case.... I would argue that if you follow the 70*F line to the right, and then upwards to where the actual total pressure inside the container is, that is what the state of the CO2 is.... eg. at 70*F and 1500 psi it is in the middle of the blue (all liquid) area.... My argument was (and as I think about it, still is) that the air reduces the space available for the CO2, just as if it was in a smaller (flexible) container.... In other words, the HPA makes the CO2 think it's in a smaller container than it really is (because of the higher pressure) and it condenses to all liquid.... Find where the temperature line crosses the total pressure, and you will know what the CO2 is (liquid, vapour/superfluid, or both).... I'm pretty sure Walter agrees with me, but I'm not going to put words in his mouth.... My theory is that, starting with the above example, the air would be used up first, the pressure dropping, and the CO2 state (liquid) "sliding" down along the temperature line until it reaches the border between the liquid (blue) and liquid/vapour (green) phase interface.... At that point (in this example 853 psi), the CO2 would begin to boil, and then you would start using both air and CO2 at constant pressure (the air in ever decreasing percentage, as the CO2 vapour is being replaced as the liquid boils) until both are exhausted, or you refill the tank.... That theory, and $1.50, will buy you a coffee if you don't go to Starbucks....

Someday, somebody will care enough to find out....

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

[Voltar1]

AT44 has a gauge so could repeat this test at some point.
Fill the AT44 reservoir on its side using the bleeder to vent excess liquid would get you about 45% by volume. Good enough for the test.
Walter.....

[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

[Gippeto]

It was Walter IIRC.....

Found it....

http://www.network54.com/Forum/79537.../CO2+%2B+air--

[Voltar1]

Here are my real results in a 2260 PCP with a mix of CO2 and Air May 28 2008, 2:35 PM


I have my 2260 setup for the Discovery games and so limit it to the 2,000psi that the Disco gets.
Made a drop tube for CO2 filling and set it up for 55% by volume in my 19.5 x 0.745" reservoir.
Filled with CO2 to that volume and then topped it up with Air to the 2,000psi limit.

So went outside in the shade to shoot..... first ten would not read on my Chrony......
So setup in the bright sun and got these numbers: using Crosman 14.3gr HP
32 shots ave 611fps
next 32 shots ave 612
next 32 shots ave 627
next 32 shots 634
next 32 shots 653
next 32 shots 674
next 32 shots 684
shot 10 more until the fps fell to 595 and the ave was 624fps
Discounting the first 10 this little dual-fueler got 234 shots from a 'fill'

Will test some more after lunch to determine CO2 only and Air only results.

Walter....

edit: 3,062.7fpe from that 140cc reservoir...................

[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