Size of plenum and time to recover preassure

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Emu

Emu

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Jan 15, 2020
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Queretaro, Mexico
 

Hi,

I would like to know if in a regulated rifle the size of plenum is related with the time of recovery of preassure after each shot. That would mean that with a bigger plenum, the wait between shots should be longer to get the same preassure within the plenum and therefore the same fps.

I will appreciate your advice.

Thanks!
 
Yes, there is a direct correlation between recovery time and plenum size. It also depends on the design of the regulator itself. Some pass HP Air faster than others. For guns with larger plenums (like the Impact or new Maverick) you can hear the plenum refill after each shot. On my Impact its only a second or two. Perhaps if you were shooting an event like Speed Silo where a good shooter can knock down 20 targets in under 25 seconds, you might be able to "get ahead" of a gun with a large plenum and slow regulator...
 
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A bigger plenum means that the pressure after the shot is not reduced as much, as in a smaller plenum, between shots so it does not need as much air to replenish it. So time to full pressure is not increased with a larger plenum but reduced. Still it is a small difference in time we are speaking of here and most guns are ready to shoot again before you are.
 
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Only a portion of the air in the plenum gets depleted each time you pull the trigger. The plenum never gets totally emptied, else efficiency would be atrocious. Thus recovery time has more to do with the volume of air used per shot than it does the size of the plenum per se. And because the volume of air used per shot correlates reasonably well to FPE, that’s probably a more useful way to think of it. The higher the FPE, the more air used, the longer it takes to replenish.

And as Bob said, some regulators recover faster than others. A regulator having a large orifice will recover quicker but is simultaneously more susceptible to both creep and nonlinearity (plenum pressure moves up and down along with the fill pressure). The optimum balance of these tradeoffs depends a lot on how it’s being used.

A hard, relatively incompressible valve seat helps as well provided it has suitably high quality surface finish so as not to creep.
 
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When Matt Dubber talks about setting up the FX maverick, he says a large plenum, and a gun setup for shooting low power, the large plenum can in some cases make the reg not to notice the pressure drop, and a smaller plenum can be better is some cases https://www.youtube.com/watch?v=2lgsFum2JJw. The maverick have the option of both big, and small plenum, in the same gun

I personally have an FX impact with power plenum, and it shoots .177 JSB heavy accurate, with litle speed variation. I can barely hear the air filling the plenum, as it fills quiet slow. But I do not shoot those pellets at very low speed, so it is not an issue getting the gun to shoot them accurate.
 
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FWIW, if a shot uses so little air that the pressure hardly changes...so little that the regulator doesn't notice, neither shall the next pellet.
Click to expand...

Well I do not know, but I think I read somewhere that some of the UK sub 12 ft-lb guys had greater experiance with the impacts shooting with the smaller previous generation plenums. But that would also mean a smaller valve. I know when my power plenum impact was setup at 75 bar, which is as low as I can go on the reg, I had to use slack on hammer wheel, and valve adjuster turned all the way in, until it stopped, and the slowest speed it would shoot a JSB heavy was around 810 fps. But I do not plan shooting that low, I just did go down there while tuning, to check for accuracy. But I guess it would limit the options of a guy trying to tune below 12 ft-lb, as the gun seems to be to powerfull for that purpose. 





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As I understand what what Matt Dubber talks about on the maverick video, is that the reg can actually get "stuck", as the tiny pressure drop is not enough to get it open. And then it might start filling after several shots, where the pressure has droped more. So according to him it will give more stable ouput pressure if the plenum is smaller, in a low power scenario. But the maverick has a very large plenum, and is probably not an issue on most guns.





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Yeah, I’m not denying the behavior exists. So for example, the spongier the valve seat, the poorer the regulator's response to a small drop in its output pressure. However if the gun has anything approaching a decent state of tune, it is of no practical consequence.

If interested, one can observe the extent to which a regulator responds (or doesn't respond) to a small change in pressure by simply taking it from room temperature out into the cold. For example going from 70F to 32F would drop a 120 bar plenum down to 110 bar if the regulator's valve seat is stuck, so to speak, and a 10 bar (~145psi) delta is big enough to easily see on a typical gauge. It's probably worth noting the outcome of this experiment will be influenced by the extent to which the regulator creeps since the temperature change will be gradual but it can give some insight.

Another way this experiment could be set up is to back off the hammer spring tension to the point where the hammer strike is barely enough to blip the valve. Thus each trigger pull is releasing but a miniscule amount of air. Keep firing until you can discern a drop in the plenum pressure, and keep going until the regulator opens and replenishes the plenum to its normal setpoint.
 
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