What Does the Air Weigh?

[Geronimo]

a question that will probably muddy the waters and make me understand even less of what's going on but here it goes. what effect does the cooling of the rapidly expanding air have on the effieciency of the air used when you fire a shot? does this contribute to higher pressure, smaller sips being more efficient overall? some really cool stuff to learn here, thanks to those that understand and have been explaining it.

[rsterne]

When you fire a PCP, the air cools to some extent, but the expansion is nowhere near Adiabatic, as some believe.... If pure Adiabatic expansion was taking place, the exhaust plume would be so cold as to freeze the moisture in the air to snow.... I use a model where the expansion while the valve is open in Isothermal, and after it close is Adiabatic, but even that is probably wrong (although it models well).... The expansion in both cases is probably only partially Adiabatic....

To answer your question, the cooling of the air, to whatever extent that happens during the shot, reduces the energy available to accelerate the pellet.... That is quite a different thing than why higher pressures tend to be more efficient.... With a higher pressure, more acceleration takes place early in the barrel, so you can close the valve sooner.... The air trapped between the valve and the pellet then has further to expand, extracting more energy, and producing a lower residual pressure at the muzzle.... That is why in an unregulated PCP, the report is less at the beginning of the shot string than at the end.... The small sips of higher pressure at the beginning of the shot string is more efficient than the larger gulps of lower pressure towards the end of the string....

HTHs....

Bob

[ackuric]

When you fire a PCP, the air cools to some extent, but the expansion is nowhere near Adiabatic, as some believe.... If pure Adiabatic expansion was taking place, the exhaust plume would be so cold as to freeze the moisture in the air to snow.... I use a model where the expansion while the valve is open in Isothermal, and after it close is Adiabatic, but even that is probably wrong (although it models well).... The expansion in both cases is probably only partially Adiabatic....

To answer your question, the cooling of the air, to whatever extent that happens during the shot, reduces the energy available to accelerate the pellet.... That is quite a different thing than why higher pressures tend to be more efficient.... With a higher pressure, more acceleration takes place early in the barrel, so you can close the valve sooner.... The air trapped between the valve and the pellet then has further to expand, extracting more energy, and producing a lower residual pressure at the muzzle.... That is why in an unregulated PCP, the report is less at the beginning of the shot string than at the end.... The small sips of higher pressure at the beginning of the shot string is more efficient than the larger gulps of lower pressure towards the end of the string....

HTHs....

Bob

I certainly observe a lot more shot noise as you shoot at lower psi's, I can even tell the shot report noise difference in just about 15~ fps...quite a difference!

[Geronimo]

you drew a very clear picture with that explanation bob, thank you. now I think I'm getting it

[kbstingwing]

Bob is the Einstein of Airguns...... Quantum Pneumatics......... E=MRod2........

[Geronimo]

HAHAHAHA! that's hilarious. I'll never be able to quote Einstein again without saying MROD