Choked flow in an air gun

The reason for the higher efficiency of a Springer is that is uses Adiabatic Compression to heat the air and further increase the pressure.... while a PCP expands the air, and a high percentage of that expansion is Adiabatic, so the temperature drops.... You can feel this cooling effect on the barrel and even the reservoir.... and if you have a digitial pressure gauge on the reservoir you can even see the pressure recover somewhat after the shot as the heat from the reservoir soaks back into the air.... proof positive that the air cools when you shoot....

I don't think the term "thin orifice plate" could be applied to any of our guns, be it Springer or Hatsan.... The total area of the six holes on the outlet of a Hatsan valve is much larger than the transfer port, so I don't see how that would choke the flow.... It is true, Tim, that the transfer port volume in a Springer is very small compared to a PCP, and also true that if you seat the pellet deeper the velocity drops.... because the pressure in the transfer port drops due to the larger volume available for expansion before the pellet moves.... This has nothing to do with choked flow, because the air velocity, like the pellet velocity, is very low until the pellet moves a long distance from the breech....

It is a common misconception that the airflow through a valve is always occurring at high speed.... Initially, before the pellet starts to move, all that is happening is that the air in the reservoir is increasing the pressure in the exhaust and transfer ports and barrel chamber, at the random vibration speed of the air molecules, which is 1650 fps.... For a 1" long transfer port system, it takes about 5 uSec (0.000005 sec.) for the air to get to the pellet, and maybe only twice that for the pressure behind the pellet to reach nearly full valve (reservoir or plenum) pressure.... At that instant the airflow through the valve is essentially zero, because the pellet velocity is only a few fps.... Most of the air in the valve and ports is just "rattling around", waiting for the pellet to start to more.... Since the valve in a PCP is open for at least 100 times that long, for our purposes we can consider the pressure rise at the pellet to be basically "complete and instantaneous", but the larger the wasted volume in the porting system the lower that pressure will be.... For example, in a PCP with a 20 cc plenum at 2000 psi, if there is 1 cc of wasted volume in the porting system (between valve seat and pellet) the pellet can never see more than 1900 psi....

Bob

Or the pellet breaks free and starts heading down the barrel before the pressure being delivered to it has peaked .... the continued release of air from valve into the exponentially expanding volume behind the moving pellet buys you little in greater speed.

Above is somewhat sarcastic being those who are schooled in PCP operation already know this ...
So for you it is simply to further your understanding of another item you may not be aware of.

Scott

Hi Bob, the valve stem only projects .095 from the back of the valve, and the valve return spring is soft for about half that distance. So I think you nailed it. The gun is .22 cal. with a .171 transfer port, the valve throat is .227, the barrel is 8"  Thanks, James