air displacement

[cooter472]

I need to know what would a good estimate for volume of air displaced during a shot cycle on a hot .25 S-Rod

[rsterne]

No idea what you mean by the volume of air "displaced".... but if you mean the amount of air used to create the energy of the shot you can get a rough estimate by assuming an efficiency of 1 FPE/CI, which is typical for a reasonably tuned PCP.... That means if you are producing 50 FPE, the gun will use 50 CI of air at one atmosphere (bar) of pressure.... If the gun is filled to 200 bar (2900 psi) that means it will use about 1/4 CI of air from the reservoir (50 / 200) or about 4 cc.... As the pressure drops, you will use a bit more HPA for each shot to get that 50 CI to produce the 50 FPE.... If your gun is less efficient, you will use more air, and vice versa....

Bob

[cooter472]

I'm trying to figure out the volume of air spent out the muzzle after projectile leaves barrel. I've been on hold for a LDC type of device and decided to do a little R&D to see if this is something I could pull off. From my research I've found you have to capture the same amount of volume of pressure that is spent to project the projectile. I have found that the Factory stripper is very inefficient and there is a lot of room for improvement.

[PakProtector]

I'm trying to figure out the volume of air spent out the muzzle after projectile leaves barrel. I've been on hold for a LDC type of device and decided to do a little R&D to see if this is something I could pull off. From my research I've found you have to capture the same amount of volume of pressure that is spent to project the projectile. I have found that the Factory stripper is very inefficient and there is a lot of room for improvement.

Picture a 1 L pop bottle to meet your spec. That being 61 ci. Volume rules in this game, even with significant numbers of right guesses  to shrink it.
cheers,
Douglas

[rsterne]

That 50 CI is what you are looking for in that case.... so like Douglas said, almost the volume of a 1 litre pop bottle....

Bob

[cooter472]

thanks guys

[Pellgunfun]

So to help improve my knowledge (Lord know I need a lot of help), the air volume needed to propel my tiny .22 cal 14.35 gr pellet to 900 fps is approximately the same size as the air volume of an empty 1 liter plastic bottle??  But that air volume is "squeezed" (compressed) down to fit in a holding area of about 4cc behind my pellet just before it is fired?  I know there is a bit more to it, but did I say that right?  :-)

So to capture all of that would mean that some device would have to be very fat or very long, or some of both?

[rsterne]

A 14.3 gr. pellet at 900 fps is about 26 FPE, so assuming the "typical" efficiency of 1 FPE/CI, the air to drive it, once expanded to atmospheric pressure, would be about 26 CI (426 cc, a bit bigger than a can of pop).... If your gun is using 2900 psi (200 bar) then that air is only taking up 426 / 200 = 2.13 cc of room in the reservoir before you fire.... At 100 bar (1450 psi) it would occupy 4.26 cc....

So, to "capture" the air would take a container larger than a pop can.... However, to contain the "lead dust" the volume can be much smaller.... The reason some guns are louder than others is that that the residual air pressure at the muzzle varies, and more pressure means more noise.... In fact, most PCPs are a lot louder at the end of a shot string (when the pressure is lower) because the valve stays open longer, letting out a larger volume of air, and the residual muzzle pressure is higher (and the efficiency lower)....

Bob

[Pellgunfun]

Believe it or not!  That made since to me:-)

Concerning your second paragraph.
I am intending on adding a regulator to my .22 cal B-50.  Would that have a positive effect on lowering the report, since it will be more efficient and consistent?  If so, would the difference be very noticeable or not much at all?

Sorry Cooter.  Didn't mean to jack your thread :-)

[rsterne]

Actually, adding a regulator will likely make the gun louder.... as ALL the shots will be towards the lower end of the pressure range where the gun is less efficient, the valve is closing later, and the residual pressure at the muzzle is greater.... Consider this graph for a stock Disco....



Note how as the pressure drops, the dwell increases, and the muzzle pressure also increases.... That is why a Disco (or any unregulated PCP) gets louder through the shot string.... When you install a regulator, you will always be running the gun like the 1200 psi graph (blue line), in the loudest, least efficient mode.... but more consistent for velocity....

Bob

[Pellgunfun]

Oh....



Shucks :-)  Mine will end up in the 1800 to 1900 range, but I see your point.

I was thinking that because the reg and plenum having a pre-measured dose of air, that it would be less likely to continue discharging air as the pellet exited the muzzle.  Only then would the air space between the reg and valve opening begin to refill.  I guess it doesn't work quite that way.  Hehe.

So how could someone set the dwell time on the valve to stay consistent?  I suppose you can't cause that would mean a trade off in some other area huh?

[rsterne]

If the plenum limits the amount of air, then the pressure in the plenum drops during the shot.... It might start at 1800, but if it's really small, might end up at half that, and work like an unregulated gun at 1400.... So, you are exactly right, the plenum only tops up after the valve closes, but because of the small plenum the gun has less power....

If the pressure is constant, like in a regulated PCP, the dwell is also constant.... In an unregulated PCP, if you had a valve with constant dwell the first shot would be the fastest and every shot after that would be slower as the pressure dropped....

Bob