Air Gun Home :: View topic - Observations on Backpressure Affecting Lift in a PCP Valve

rsterne · Moderator

I made some lift measurements on my .30 cal Disco Double this morning, at 3000 psi, with the preload adjusted for a 4% ES shot string starting from that pressure....

Normal shot with pellet: lift 0.07"
Open bore, no pellet: lift 0.06"
Bore blocked ~95%: lift 0.08"

The bolt on this gun doesn't have a probe, you push it forward to feed the pellet to it's proper position, and then withdraw the full diameter end of the bolt back, clear of the transfer port (the slot for the bolt handle is "J" shaped).... The last shot was with no pellet and the bolt left forward, blocking the barrel, other than the ~0.004" clearance around the bolt in the bore.... There was very little report, and very little pressure drop during the shot.... so very little air escaped....

It is difficult to get accurate measurements with such a small gap between the O-ring and RVA screw, so I have to use feeler gauges and try not to disturb the O-ring.... That is why I only gave the measurement to 2 decimals.... It would seem reasonable to argue that when there is no pellet the air easily escapes from the bore, so the pressure in the throat/exhaust port of the valve is lower, causing less closing force on the stem area, but there is a larger pressure difference across the head of the poppet.... The opposite happens when the barrel is blocked, it acts like a VERY heavy pellet.... causing high pressure on the exhaust side of the valve, a greater closing force on the stem area, but a lower pressure difference across the head of the poppet....

It would appear that in this case (at high pressure, small lift) the pressure difference across the poppet has more affect on lift than the closing force from the air pressure acting on the stem area....

I then repeated the experiment at 1900 psi with the following results:

Normal shot with pellet: lift 0.12"
Open bore, no pellet: lift 0.12"
Bore blocked ~95%: lift 0.13"

This shows the same trend, but in this case there was no measureable difference between pellet and no pellet.... In both these cases, there is lots of flow through the valve, and with the much larger lift, I'm guessing that the pressure difference across the head of the poppet is just balancing that acting on the stem area.... With the port blocked and no significant pressure difference across the head of the poppet, the lift increses, even though virtually the full reservoir pressure is available to act on the stem area.... Once again, it seems that the pressure difference across the head of the poppet has more (or at least as much) effect on the lift than the pressure acting on the stem area....

Bob