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 Post subject: Tuning a Regulated PCP
PostPosted: Sun Sep 14, 2014 3:03 pm 
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Location: Coalmont BC
There have been a lot of shooters who are disappointed in the initial results when first fitting a regulator to their PCP.... There are two primary reasons for this, one is that often there is not a large enough plenum (chamber) between the regulator and the valve, and the pressure at the valve seat drops during the shot to a much greater extent than in the unregulated version at the same pressure, leading to lower velocity than expected.... Assuming that is not the case, then the most common thing the shooter notices is that the shot count is much lower than he expected.... This almost always happens if the hammer strike is not reduced when the regulator is installed, and is a result of the gun having been previously tuned to peak at, say, 2200-2400 psi and now it is running on a much lower pressure, making it an air hog....

Fortunately, the solution is relatively simple, and in most cases all you need to do is reduce the preload on the hammer spring to retune the gun to the now lower pressure.... If you measure and plot the velocity while you back off the preload, and assuming the gun was previously tuned to peak at a higher pressure than what your setpoint is now, you will get a graph that looks something like this....

Image

You will notice that as the hammer spring preload is reduced, at first there is virtually no change in the velocity (the plateau), but the efficiency increases.... Between 4-5 turns out on the adjuster (in this case) the velocity starts to drop (the knee), and then past 5 turns out it declines nearly linearly with further decreases in hammer strike (the downslope).... What you are doing, of course, is you are changing what would be the peak of the bell-curve if the gun was still unregulated.... This second graph of what the velocity does in those three regions may help you understand what is happening.... Note this second graph is using a 1500 psi setpoint and does not correlate to the graph above, other than in concept....

Image

First of all you will notice that when the gun is tuned on the "plateau", which it likely would be with the original, unregulated, setup, there is a slight slope to the velocity as the tank pressure drops due to the output pressure of the regulator creeping.... This is common in most regulators, more severe in some designs than others, but is usually present, and the output may be 4-8% higher when the tank is full than when it is at the setpoint pressure.... Since the gun was tuned to peak at 2200-2400 psi, it is well down on the downslope of it's unregulated bell-curve, so small drops in pressure will (usually) cause the velocity to drop slightly.... Then when the tank pressure reaches the setpoint (in this case 1500 psi) the velocity starts to drop rapidly with decreasing tank pressure.... Remember, that since the efficiency is also low when the regulated gun is working up on the plateau, the shot count will suffer.... If the hammer strike is very high for the pressure, the chance of air-wasting hammer bounce is increased, and the gun becomes a real air-hog....

If the hammer strike is reduced to retune the gun to the "knee", what you have done is effectively tuned the gun as if it was unregulated with the peak of the velocity curve at, or very near to, the setpoint.... This means that from a full tank to about 100 psi below the setpoint, the gun is operating right in the "sweet spot", and the chance of hammer bounce is much reduced.... and the velocity variation is virtually nil (in fact usually due only to pellet variations).... You have given up only a few fps and may have doubled your shot count.... This is the way I tune ALL my regulated guns....

If you further reduce the hammer strike, you are now operating on the "downslope" (eg. at 6 turns out on the previous graph).... The velocity is significantly lower, and as the tank pressure drops, the velocity usually shows a slight rise until you hit the setpoint (due to regulator creep), at which point it then increases significantly before dropping off.... The peak of that bump in the velocity is where you have now tuned the gun to if it were unregulated, in this case 1200-1300 psi.... While the gun is shooting above the setpoint pressure, it will be VERY efficient, with virtually no chance of hammer bounce occurring, and this is a good tune for target shooting, or something like FT, with one exception.... that jump in velocity below the setpoint.... If you are competing in a class where the FPE is limited and have the gun tuned for, say, 19 FPE when it is above the setpoint and they check your velocity at the end of the course and the pressure is below the setpoint you could be over the allowable FPE level.... It could also cause you to start missing targets due the increase in velocity you weren't expecting....

I personally ALWAYS tune my regulated PCPs to the knee of the curve, ie I back off the hammer spring preload until the velocity just starts to drop.... Generally that extends my shot string 100-200 psi below the setpoint, giving me additional shots on top of the already efficient setup.... If the gun is shooting harder than I want, then I reduce the regulator setpoint a bit, reduce the hammer preload to get back to the (now lower) knee of the curve.... and end up with even more shots.... Yes, I could just back off the preload, but then I have to worry about that bump in the velocity curve below the setpoint, so instead I drop the regulator setpoint and retune to the new knee....

Bob

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PostPosted: Sun Sep 14, 2014 4:02 pm 
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Location: Southern canada
Bob, you know more then I could ever aborb! Great read, and it gets me thinking about those little things I just deal with without thinking about.

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PostPosted: Sat Apr 28, 2018 11:51 am 
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With time comes increased clarity and understanding (ie I'm always learning).... so I thought I would revisit this subject to add a chart that might help you understand what happens as you tune a regulated PCP with progressively less velocity than the plateau.... Here is an updated version of the chart in the first post of this thread....

Image

The 3%, 5% and 10% below Plateau velocities are approximate only, but will give you an idea of what happens as you reduce the hammer strike, and hence the velocity, on a regulated PCP.... It will show you exactly why I like to tune 3-5% below that plateau.... because (at about 5% down) you get a few more usable shots below the setpoint before the velocity drops off.... or at the very least (at about 3% down) you save a lot of air compared to tuning up on the plateau, with very little loss of power.... The chart also shows what happens if you tune about 10% (or more) below the plateau.... Note that below the setpoint the velocity increases significantly.... and while operating in that realm gives you extremely good efficiency, for some applications (eg. Field Target, or in some countries where there is an FPE or velocity limit) that bump could put you over the limit....

Every gun is a bit different, and yours might produce a curve like the "5%" one above only 3% below the plateau, or it might be 7% below.... but to date those would be about the outside limits I have found to produce a tune that has a usable shot string (ie within a ~1% ES) extending slightly below the setpoint, which is the tune I prefer.... I hope this chart allows you to visualize what I have tried to confer in this thread....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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