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rsterne · Moderator

I was having a look through some of my earlier data and ran across a graph that is very interesting.... It was taken by shooting shot strings at various hammer spring tensions on an unregulated HPA conversion....

There are a couple of interesting things to note.... One is that if you look at the top of all the curves, they pretty much line up.... with the falling parts of the curve forming almost a straight line.... This basically means that less pressure produces less velocity in a valve that is opening "fully".... Yeah, a no-brainer I guess, but pretty neat that four curves combine to form that pattern so precisely....

The second thing is that for each hammer spring setting there is a pressure at which the velocity peaks.... and that as you reduce the hammer spring preload, that peak moves to a lower pressure and velocity.... Again, no surprise, it is that feature of a valve becoming self-regulating (and eventually partially "locking") that produces the typical bell-curve in an unregulated PCP....

What was interesting to me was that this graph also shows what I have been talking about in some of my recent posts.... that there is an optimum regulator pressure for a given velocity desired in any given setup.... Normally, in a regulated PCP, once the pressure drops below the setpoint, the velocity drops.... However, if the regulator pressure is higher than needed for the desired velocity (ie you have dialed the RVA out to get less velocity and more shots).... when the pressure drops below the setpoint the velocity will actually INCREASE before dropping off.... This translates to fewer usuable shots than if you had a lower regulator pressure.... Shocked

If you look at the graph, you can actually figure out what is the optimum regulator pressure for a given velocity IN THIS GUN.... To reach ~920 fps, this gun required 1800 psi.... However, the peak of the 3T out curve is at ~890 fps, and that occurred at ~1650 psi.... The 4T out curve peaks at ~ 865 fps at ~1500 psi.... At 5T out, you get ~830 fps with only ~1350 psi.... If, for instance, you had a 1700 psi regulator and set the RVA to give you 800 fps (5 T out) once the regulator reached its setpoint, the velocity would then follow the 5T out curve, climbing over the next 35-40 shots to ~830 fps before falling back to ~800 over 60 shots below the 1700 psi setpoint.... Obviously, if you wanted 800 fps, you would be better off to have the regulator set for ~1100 psi in this case.... The "best" regulator pressure for a give velocity would fall along the "top" of all the curves.... along that "straight line" formed by where they all flow into each other....

Interesting how looking back over something you did a while back produces new information you missed the first time.... It's also VERY interesting how much velocity you can get at relatively low regulator pressures.... Cool

Bob

packrat · Senior Member

That certainly explains how it's possible for me to keep right on shooting down past the 1600psi setpoint..in fact with pellets 18gr and below, I can actually shoot down to just under 1000psi with no discernable loss of power..

rsterne · Moderator

I always try and set up my guns so that the velocity below the setpoint is right near the top of the bell-curve.... If you get it just right, you can extend the shot string quite a bit....

My .22 Bench Rest rifle has a regulator that starts at 1600 psi (at 3000 tank pressure) and gradually drops to 1550 (which is the true setpoint).... During that part of the shot string, the velocity actually increases about 10 fps.... Over the next 3-4 shots, the velocity increases about another 5-10 fps, rolls over the top of the bell-curve, and then begins a gradual drop.... I get about 1/4 of my shots below the regulator setpoint.... and I can shoot down to 1300 psi and still have all the shots within 4-5% of the peak velocity.... I can get another 4-5 shots within a 10% velocity window and end up shooting down to about 1100 psi....

If I crank in the RVA just a turn or so to get about 20 more fps.... My usuable shot string drops from about 50 shots to about 35.... Shocked

Bob

packrat · Senior Member

Well, thanks for all that info...looks like I've got mine set-up pretty close to as good as it can get, going by those numbers......with 14.5 ultra mags I get 48-50 shots..if I remember correctly, I get about 10-12 shots from 1600 (indicated) down to just under 1000...say ~950. One thing is for sure...Of all my guns (5) and all the cool ones I see posted on various websites,,,there are few that offer as much fun and such high preformance as my little 2250 it's just amazing what they can be made to do Mr. Green