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Thermodynamics of filling a PCP, ever notice this?

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Birdo

Member
Jan 27, 2017
242
4
Kentucky
I filled my PCP with air from 100 Bar to 200 Bar. The tank is a large 480 cc carbon fiber tank. The next day I noticed the pressure had dropped to 190 Bar. I thought maybe I had a slow leak, but after several days, no more pressure drop was found.

I suspect I haven't noticed this in my other rifles that have smaller steel or aluminum tube tanks. The aluminum or steel acts as a heat sink during the filling process. The light weight CF tanks and the large volume of air means the heat can't absorb fast enough, and then cools after filling resulting in a pressure drop.

Haven't taken thermodynamics in over 30 years,but I'm pretty sure this is whats happening.
 
When filling my Mrod I notice a slight drop (about 100psi) after a minute or less and I add a little more to bring it back up and it seems to stay there, although I usually shoot pretty soon after filling so hard to tell. But when filling my CF Great White from empty, there is a large drop in a few hours as the air cools - from 4500 down to 4000psi. It's new to me but I bet the next time i fill when I top off the tank I'm guessing the drop won't be as much due to putting in less hot air - we'll see.
 
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It doesn't take much heat to expand and much cooling to contract gas. I work in the gas industry...... Even the sun on a stagnate pipe will expand gas enough to lift a relief valve. Of course different gasses have different rates. Compressing air will raise your psig, and as it cools it contracts. You are right, metal will dissipate the heat quicker than a carbon covered bottle.
 
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"Birdo"I filled my PCP with air from 100 Bar to 200 Bar. The tank is a large 480 cc carbon fiber tank. The next day I noticed the pressure had dropped to 190 Bar. I thought maybe I had a slow leak, but after several days, no more pressure drop was found.

I suspect I haven't noticed this in my other rifles that have smaller steel or aluminum tube tanks. The aluminum or steel acts as a heat sink during the filling process. The light weight CF tanks and the large volume of air means the heat can't absorb fast enough, and then cools after filling resulting in a pressure drop.

Haven't taken thermodynamics in over 30 years,but I'm pretty sure this is whats happening.

Same here; I took Thermo in 1979. But you are correct, the change in pressure is due to compressing a gas adiabatically. Over time as the gas cools, you noted the appropriate pressure drop wrt temperature. 

Try filling your air rifle tank more slowly, then after the tank cools to ambient temperature, top the tank off. 

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"Menttaliist"It depends on the filling tool you use. When using a handpump, the bottle/cilinder does not fill that quickly like when using a tank and there is not a noticeable drop on the pressure. I'm using a handpump to fill a 480cc cf bottle up to 230bar and as long as you let the pump cool down properly the pressure drop is unnoticeable
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You sir, have my respect. That is a LOT of work.
 
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Smaug
"Menttaliist"It depends on the filling tool you use. When using a handpump, the bottle/cilinder does not fill that quickly like when using a tank and there is not a noticeable drop on the pressure. I'm using a handpump to fill a 480cc cf bottle up to 230bar and as long as you let the pump cool down properly the pressure drop is unnoticeable
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You sir, have my respect. That is a LOT of work.
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Hahaha thanks :)
The combination of a Fx 4 stage pump and have 19yrs makes the work muuch easier, b.t.w it's a nice way to be in a good physical condion ;)
 
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It also depends on whether you're compressing air into your gun or -subtle but important difference here- releasing compressed air into your gun. Theoretically, filling from a separate bottle should actually cool the air as it leaves the source. In this case, there should be a small (perhaps immeasurable, depending upon a number of variables such as the size of the source bottle and amount of air transferred) RISE in pressure after the air in the just-filled tube adjusts to ambient temp.

Haven't looked for this effect yet, but I will next time I refill my Wildcat from my 100 cu ft Omega bottle. Probably won't see any effect since the amount of released/xferred gas is very small proportionally. 



 
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"Mousefart"It also depends on whether you're compressing air into your gun or -subtle but important difference here- releasing compressed air into your gun. Theoretically, filling from a separate bottle should actually cool the air as it leaves the source. In this case, there should be a small (perhaps immeasurable, depending upon a number of variables such as the size of the source bottle and amount of air transferred) RISE in pressure after the air in the just-filled tube adjusts to ambient temp.
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When I refill my Air Wolf CF bottle from 100-230 Bar, using a 6000 PSI Nitrogen tank, the CF bottle gets really hot. I agree that gut instinct suggests that since the Nitrogen is going 'down hill', from 6000 to 3000 PSI, the CF tank should cool, but that's not what happens. Same thing will happen if you go to a dive shop with a tank farm: most of the air you are getting is coming from the tank cascade, not from the compressor, and your bottle still heats up, which is why its worth leaving overnight for a top up if you can

Thermodynamics is amazing and not intuitive. What happens is that the gas molecules in the CF bottle are being forced closer together as the pressure rises, so it heats up just like it would if you were filling using a compressor. The 6000 PSI source tank will cool, but because this has such a massive heat capacity due to the thick steel walls, its hard to feel the temperature drop.
 
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"Regal_US"
"Mousefart"It also depends on whether you're compressing air into your gun or -subtle but important difference here- releasing compressed air into your gun. Theoretically, filling from a separate bottle should actually cool the air as it leaves the source. In this case, there should be a small (perhaps immeasurable, depending upon a number of variables such as the size of the source bottle and amount of air transferred) RISE in pressure after the air in the just-filled tube adjusts to ambient temp.
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When I refill my Air Wolf CF bottle from 100-230 Bar, using a 6000 PSI Nitrogen tank, the CF bottle gets really hot. I agree that gut instinct suggests that since the Nitrogen is going 'down hill', from 6000 to 3000 PSI, the CF tank should cool, but that's not what happens. Same thing will happen if you go to a dive shop with a tank farm: most of the air you are getting is coming from the tank cascade, not from the compressor, and your bottle still heats up, which is why its worth leaving overnight for a top up if you can

Thermodynamics is amazing and not intuitive. What happens is that the gas molecules in the CF bottle are being forced closer together as the pressure rises, so it heats up just like it would if you were filling using a compressor. The 6000 PSI source tank will cool, but because this has such a massive heat capacity due to the thick steel walls, its hard to feel the temperature drop.
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You are right! The only way to get a cooling effect is expansion of gas.
 
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It is true that there is a cooling and heating effect, but it is only significant on the smaller vessel of air. The 480cc tank is being pressurized so there are more molecules of gas bouncing off the sides of the tank, thus heating it up. Once the average kinetic energy begins to lower then the pressure will lower too. If you could measure precisely enough then you would find that the big tank (75cuft) would in fact cool down. In order for this to be substantial you would need to be filling of another tank of at least 20cuft to be able to feel the temp lower on the main tank. 
 
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"Regal_US"
"Mousefart"It also depends on whether you're compressing air into your gun or -subtle but important difference here- releasing compressed air into your gun. Theoretically, filling from a separate bottle should actually cool the air as it leaves the source. In this case, there should be a small (perhaps immeasurable, depending upon a number of variables such as the size of the source bottle and amount of air transferred) RISE in pressure after the air in the just-filled tube adjusts to ambient temp.
Click to expand...
When I refill my Air Wolf CF bottle from 100-230 Bar, using a 6000 PSI Nitrogen tank, the CF bottle gets really hot. I agree that gut instinct suggests that since the Nitrogen is going 'down hill', from 6000 to 3000 PSI, the CF tank should cool, but that's not what happens. Same thing will happen if you go to a dive shop with a tank farm: most of the air you are getting is coming from the tank cascade, not from the compressor, and your bottle still heats up, which is why its worth leaving overnight for a top up if you can

Thermodynamics is amazing and not intuitive. What happens is that the gas molecules in the CF bottle are being forced closer together as the pressure rises, so it heats up just like it would if you were filling using a compressor. The 6000 PSI source tank will cool, but because this has such a massive heat capacity due to the thick steel walls, its hard to feel the temperature drop.
Click to expand...
Interesting. Thanks!
 
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