Tuning a Regulated PCP

[rsterne]

bstaley.... an interesting thought.... Whether O-rings are the answer, or something firmer, I don't know, but it might be worth fiddling with to find out.... The problem is, that to extend the level part of the velocity curve past the high point (ie 1300ish) you then need the buffer to get softer again.... It is typical for the plateau of the curve (above the setpoint) to increase slightly as the pressure downstream of the reg. creeps downwards a bit as the tank pressure drops, when you are using a downslope tune.... That doesn't leave much room for the velocity to increase before your ES is too great.... and of course a low ES is the reason for a regulator in the first place.... The "bump" in the velocity curve occurs when you are no longer under regulation, but below the setpoint, in a downslope tune....

Bob

[Tonycalves]

Hi Bob, and thanks for all knowledge you share.

So I fell off the fence and put a regulator in my Prod. Was set low 600fps FTTs for 30-40 shots from 2k fill down to 1k. 10lb HS, poppet cut down from 7mm to 4mm, exhaust opened .140, .140 TP. WAR NBH, TimH depinger, and 3 orings

Yesterday after installing the Milo74 regulator  (I got second hand) in place of the gauge block, and putting back the original Hammer Spring no preload, took out the depinger and 3 orings, kept same hammer through(2t cw?), I filled to 1750, reg came set to 1300 the gun got 14,15 shots in the 590s before fps  started falling.

Filled to 2k and set it down for a while. Pick it up later on and lost a few psi and shot 550FPS.  So I filled to 3k overnight

This morning in had 2700psi and took 16, 650FPS shoots taking it down to 2475. Filled back up to 3k and see if it leaks again.

What can cause the fluctuation in velocity?

[rsterne]

Velocity fluctuations with a regulator are usually either a sticking hammer or a problem with the reg. itself.... It is possible for a regulator to "creep", have a slightly higher output pressure when the tank is full than near the setpoint.... That can result in either a higher or lower velocity when full, depending on the tune.... They can also "leak" slowly above the setpoint, until the pressure is high enough to seal the HP seat.... That results in a lower velocity when shooting every few seconds, and a higher velocity if you leave the gun sit for a while....

Bob

[Motorhead]

In the case of a USED regulator and not knowing if previous owner jacked with adjustments or worse ... bottomed out adjuster screw crushing seat to seal face.
Would suspect "Set Point Creep" from a worn or damaged seat within regulator.

generally a dead give away to set point creep is that after sitting awhile ( If gauge is present ) a slightly higher pressure will read on gauge.  And due to this higher pressure within plenum/valve, upon poppet being struck by hammer it become more difficult to crack the valve open and velocity of first shot will be lower.

Taking relatively quick sequential shots that shoot faster until you set gun down and wait a bit finding again the 1st shot fires slow.

Yup ... regulator seat weeping / creeping   

** Here a copy paste made over on the Marauder forum

Re: Regulated my Prod
As asked on the GTA ... Reg set point creep may be suspect ... Or

Also building quite a few regulated P-rod / 1701 & 1720 regulated pistol conversions, when you take guns operating pressure within valve to @ 1300-1500# the required hammer strike really falls away !!
Factory hammers become too heavy ! and in the case of Travis's NB version, not sure where weight is relative to OEM ?
* Also if the No Bounce is setting to quickly it could be hanging valve open slightly or slowing down the closing speed screwing with valve dwell which will screw with velocity consistency.

What i would do is install OEM hammer and see if velocity instability still exists ... eliminate one part of the working system at a time until you isolate the offending part/reason.

Scott

[Tonycalves]

Cool I'll put the oem hammer back in. Should i remove some weight from the oem hammer? The NBH weights just about the same as oem. The reg came set to 1300 and wasn't messed with from what I've been told.

I shot a 3k string and got a 48 fps spread. Started at 605 648 600 last 4,5 shots fell to 578. The JB tank gauge showed it shot down to 1000psi 64 shots.

Thanks Guys

[rsterne]

That kind of string is what you get with an unregulated PCP, it almost looks like the regulator isn't working....

Bob

[lloyd-ss]

Bob, Honestly, you do have a gift for technical writing. And all the hours and hours of designing and building and testing and analysis that you do beforehand is an incredible gift to the airgun community.  Thank you, sir.

I am reviving this thread because of the guidance it is giving myself and Miguel in tuning the Marauder pistol with a Huma regulator.
http://www.gatewaytoairguns.org/GTA/index.php?topic=100748.0
When I first saw the Huma claim of 31 shots at 18.8fpe from a P-Rod pistol with a standard 66cc reservoir, well, I have to say I was skeptical. But with the info from Mr Andriessen from Huma, and others, and this thread, well.... I am becoming a believer.  Trying to eek out every last shot from such a small reservoir, I can see where every variable must be addressed, and that for best efficiency, the gun must be tuned for one pellet weight at one velocity.
So here is the question, plenum volume? Given that the normal reservoir is only 66cc, it would be best to keep the plenum volume to a minimum (without jeopardizing fpe) so that the HP volume can be kept to the max. With the current setup, the plenum volume, including the volume inside the valve, is approximately 8 cc.  Seeing that this is well below 1cc/fpe, it seems that trial and error might be the only definitive way to determine how small the plenum can be.  I would assume that a higher set point would require a smallish plenum, but maybe because the practical set points will all be within a 500psi range,  one plenum volume can be decided upon that will work within that range.  So, to further refine the question, I guess I should first achieve my "optimum" tune, and then try whittling away at the plenum volume until the power drops, realizing that a hammer spring tweak might be needed?  At some point, it will be splitting gnat's hairs, because exchanging one plenum cc for one HP cc won't yield any further improvement in shot count.  Again, I can see that this is only significant in guns with tiny reservoirs.
Bob, any thoughts would be appreciated.
Thanks,
Lloyd

[rsterne]

Lloyd, we did a quickie analysis of this not long ago.... If you have virtually unlimited reservoir (ie a large bottle feed a plenum through a regulator), then the 1 cc per FPE that I have been recommending for a while is, IMO, still the best compromise.... Going smaller will cost you in FPE and efficiency at any given pressure, requiring you to increase the setpoint to get the FPE back.... while going larger doesn't really pay much dividends.... However, once you are using an "in-tube" regulator, and trading HP volume for plenum, things change.... It seems that around 1/2 cc per FPE is a better choice, and of course even that is flexible.... The higher the power you are trying to achieve (relative to unregulated at the same pressure), the more plenum you need.... If you are going for low power and large shot count, then you can shrink the plenum more.... Plenum volume, BTW, includes valve volume (all the air that can affect the pressure at the valve seat during the shot cycle)....

You have it exactly right, that trading HP volume for plenum will eventually find an optimum for any given FPE level.... As you increase the plenum size, you can use a lower setpoint, so even though you have less HP volume, the "overhead" (the pressure range from fill pressure to setpoint) is greater, giving you almost the same amount of air (pressure range times volume).... When you decrease the plenum size, the opposite occurs, you need to increase the setpoint, losing overhead, but gaining on HP volume.... Within small changes in plenum and setpoint, there will be several combinations that COULD produce about the same shot count at your chosen FPE.... EXCEPT....

When you tune to the correct point on the "knee" of the curve (I'm talking here about a plot of velocity vs preload).... as I started this thread with.... you end up being able to "cheat" out a few extra shots.... THAT is the combination that will prove to be the BEST for your chosen FPE level.... If your pressure is high, and you are on the downslope, your velocity below the setpoint will climb.... If your pressure is too low, and you are on the plateau, you get a sudden drop in velocity below setpoint.... However, when you find the OPTIMUM point on the knee of the curve, you get a nearly flat velocity while above setpoint, and then a VERY slight increase (a percent or so) just below setpoint, and then a gradual initial drop in velocity.... This can easily give you 10-20% more shots as the pressure drops just below the setpoint, without even noticing the ES.... IMO, that is THE best way to tune a regulated PCP.... The setpoint is JUST before where the peak of the velocity would be, unregulated....

To answer your base question, 1/2 cc per FPE is likely in the ballpark for plenum volume when using an in-tube regulator.... You will need to use a higher setpoint than what gave you your desired FPE before you put in the regulator.... For example, if you were getting 20 FPE at the bottom end of the unregulated string at 1600 psi, you will need at least 10% more pressure than that (I pulled that number out of the air).... let's say try 1800 psi for a setpoint.... Try a couple of shots at each preload setting, and plot the graph of velocity vs. preload.... DON'T get lazy, do the graph, you will learn a ton.... If you really want to understand how important the preload is to efficiency, shoot an entire string at each setting and calculate the FPE/CI.... You will find that being on the plateau is NOT where you want to be.... Pay attention to what the velocity does as you cross the setpoint, and you will find how far down the downslope you can go before the velocity rise gets too great to be usable.... Once you really UNDERSTAND what your gun is doing at 1800 psi with the size plenum you have, and what you can get for the optimum combination of FPE and shotcount at that pressure.... THEN you can decide if you need to change the setpoint or plenum size....

If after finding the optimum tune, where the velocity just seems to hang on for 200-300 psi below the setpoint, you decide you need more FPE, then you have two choices.... a higher setpoint pressure or a larger plenum.... Obviously, changing the pressure will have a much greater effect, unless your plenum is really small.... If the plenum is too small for the FPE you are trying to get, you will need to increase the setpoint more than seems reasonable.... I haven't played much with in-tube regulators, but logic tells me that if the gun is insensitive to relatively large changes in setpoint pressure, the plenum is too small (for your FPE level).... That would be because although you are changing the pressure at the BEGINNING of the shot cycle, the plenum is just too small compared to the barrel volume (and the amount of air you are using per shot) to maintain reasonable pressure throughout the shot.... Conversely, if the plenum is up around 1 cc per FPE, setpoint pressure changes will have a large effect on FPE when tuned optimally....

Bob

[lloyd-ss]

Bob, Thanks for the detailed reply.  It is very helpful.  I have already been graphing various regulated strings with various hammer preloads, and most of them are of the shape of your downslope velocity in your first post in this thread.  So it seems like i need to go back to an unregulated setup and graph the velocity vs preload at several different pressures to find the knee at the desired fpe, whatever that particular pressure turns out to be.  I agree 100% that graphing the real data reveals a lot of hidden information that isn't obvious when just looking at a page of numbers.  And I do understand about tuning to get those extra shots as the pressure falls off the regulator.

So here we are back at the plenum question again.  I am going to go with the suggestion of about 1/2cc per desired fpe.  I assume that when I am doing the initial unregulated testing to find the knee at a specific psi (vel vs hamm preload) that it would be desirable to mock up the "stingy" plenum volume within the reservoir, just as it will be when the regulator is installed.  That should help to eliminate any surprises when the regulator is installed and set at the pre-determined pressure.   BTW, I have a fairly accurate gage with fine enough resolution so that I can determine the actual psi used per shot during the initial unregulated testing, and thus the efficiency of each shot. The determination of an "optimum" setup can be as involved as you want it to be, I guess.

 Bob, also, just as a point of reference, this is .22cal, 12" barrel, the barrel port is .125 (31% of bore area), the T-port and ex port are both .138 (39% of bore area), and the valve throat effective area is 51% of bore.  I am assuming that with the short barrel, and low fpe requirement, those smallish numbers are ok.

Lloyd

[rsterne]

I'm not sure how you can find the "knee" in an unregulated PCP.... it requires a constant pressure to plot velocity vs. hammer preload and get a plateau, knee, and downslope.... If you are feeding the plenum in your gun from an external regulator, at constant pressure, (ie tethering the gun) then you certainly can do that.... If you have an accurate gauge, and plug the plenum to the desired volume, you should be able to determine your optimum setpoint and preload without ever installing the in-tube regulator until your testing is all done.... Adjust the setpoint, install it with the same size plenum, and you should get the same results....

I don't think you will have any problems with those port sizes.... My Grouse Gun has a 1/8" transfer port and a 14.5" barrel, and gets 20 FPE (18 gr. @ 700) unregulated from 2000 psi down to 1200....

Bob

[lloyd-ss]

Bob, yes, I will be using a regulated supply to determine the the knee.  That seems to be a bit easier than draining the gun and removing the reg to re-set it, then reassembling and refilling for each different pressure setting. So it does sound like doing the testing at the "final" plenum volume is essential.  I will do that.
Thanks,
Lloyd

[rsterne]

Yes, I would definitely use the final plenum volume.... What changes is the pressure at the end of the shot, and that affects the average pressure while the valve is open, of course, and hence the velocity.... The smaller the plenum, relative to the barrel volume (well actually the volume of air used in the shot), the greater the pressure drop, and the lower the resulting velocity... It is easy to calculate, and in fact your spreadsheet can do the math quite well, just use the plenum volume for the reservoir volume.... You will quickly see what happens as you change the plenum.... and how much you need to compensate for a smaller plenum by increasing the setpoint (starting pressure).... to get back to your desired FPE....

Bob

[lloyd-ss]

Here is a little bit of data related to small plenums.  This is a P-rod with12" barrel,  a Huma regulator, 48cc HPA, 10cc regulated plenum.
Set point was about 1740 psi, fill pressure was a bit over 3k, final pressure was abut 1600. 

24 shots 18.1gn, avg vel 682fps, avg 18.7fpe, ES 10fps (1.5%), avg air efficiency approx 1.23fpe/cuin

25 shots 14.5 gn, avg vel 747fps, avg 18.0fpe, ES 10fps (1.3%), avg air efficiency approx 1.27fpe/cuin

This info is from this thread:
http://www.gatewaytoairguns.org/GTA/index.php?topic=100748.20

It seems like a small plenum can work in this short-barrel situation.
Llloyd

[rsterne]

Glad that 10 cc plenum worked out OK for you Lloyd.... what is that, about 0.55 cc per FPE (plus minus) ?.... That is the same ratio as what I have on my QB79, however when I change the tube only to a QB78 tube, with a larger plenum, I gain FPE for the same volume of air used per shot (same number of shots), so also gain efficiency.... The difference is that I don't lose HP reservoir volume when I do that because it's a bottle gun....

Bob

[shorty]

This is interesting between the small plenum size and 12" barrel vs a Bottled QB with a barrel over 20".

If just calculating a 12" barrel without the extra behind it, it's just under 8cc's. A longer barrel with larger plenum would make sense for using the air more efficient to produce the same power.

Should optimized plenum size be based on desired fpe and barrel  volume to be the most efficient?

[lloyd-ss]

Yes, this is getting down into the splitting hairs region.  It seems like it might be getting near the low limit of plenum volume for producing that power. But I think if you only wanted 12fpe from this same gun, you could probably go smaller than .5 cc/fpe.

[Bill G]

This is interesting between the small plenum size and 12" barrel vs a Bottled QB with a barrel over 20".

If just calculating a 12" barrel without the extra behind it, it's just under 8cc's. A longer barrel with larger plenum would make sense for using the air more efficient to produce the same power.

Should optimized plenum size be based on desired fpe and barrel  volume to be the most efficient?

To produce the same power and achieve more efficiency, Probably be better suited with the longer barrel smaller plenum and higher output psi..... for the same power.  The longer barrel is gonna cause the efficiency to increase by causing the velocity to increase, which is gonna require you to drop the plenum size and/or lower the psi in order to keep the fps the same.  higher psi with smaller plenum seems to always increase efficiency.  So in my thinking, there should be some optimal condition based on barrel volume that renders the best power to efficiency.  Bob has been all over it for some time now with the 1cc/fpe and at this point I see that as law.  That's not to say that it can't be done with less, but is more of a very reasonable goal when setting out to design or build a gun.  Bob has also pointed out that guns that are producing higher efficiency are closing the valve around the same time that the projectile has reached ~30% of the barrel length.  This would suggest that you could propel the same projectile at the same speed from a dump gun using a dump plenum of 1/3 the barrel volume at the same psi.  That's not likely reality.  If that were a 20", .25cal barrel, 1/3 vol would be ~7cc.  The question is, can you achieve say 950fps from 7cc at 2000psi, with a 25.4gr pellet?  If so, then we are flirting with reality.  There are cross over characteristic between standard PCP's, regulated PCP's and Dump valve guns.  I contend that when we are able to recognize and isolate the characteristics, we can then really unlock the power and efficiency of a charged gun.  A regulated PCP is basically a hybrid between a pcp and a dump valve gun.  Just the dump valve get refilled really quickly and that leaves us some grey area of under standing. So we are left to ponder the valve timing in a more isolated and controlled condition.  I've spoken of this before and probably fail to convey or articulate the concept very well but just imagine a valve design that has a plenum that is designated with a volume, has regulated psi, and when fired, the plenum is allowed to go to zero before it is refilled.  When refilled, it is refilled to a set psi, under a set volume to be cycled again.  That would be as close to a square sine wave as I could imagine.  The valve is open until it is empty then shuts to establish the exact same condition. Valve timing is no longer an issue and the plenum is sized according to desired performance. Tuning would be changing the regulated psi. Bigger plenums equal less efficiency but higher power potential.  The question is, can we get 950fps with 7cc of air at 2000psi from a 20" barrel, shooting a 25gr pellet?  If so then we can do better with a longer barrel and higher psi and a heavier slug.             

[Bill G]

Yes, this is getting down into the splitting hairs region.  It seems like it might be getting near the low limit of plenum volume for producing that power. But I think if you only wanted 12fpe from this same gun, you could probably go smaller than .5 cc/fpe.

LOL. Lloyd that might be getting cocky right there.  But I do agree.  In fact the power output could be decided by simply choosing a working psi (regulated) and increasing or decreasing the plenum.  If you wanted a 50fpe pellet shooter, then you'd have a 7cc plenum at 2000psi and a 20" barrel.  But if we wanted a slug shooter that generated 120fpe, we'd probably need something more like a 30" barrel and a 23cc plenum (which is close to 100% barrel volume).  Even though, 23cc is a fairly small dose of air at 2000psi.  Of course, if the psi is raised, the barrel can be shortened or the plenum made smaller.  Also, if we had a slug shooter that generated 120fpe and wanted to shoot pellets, we could simply have all the features that are required to produce 120fpe.  the 23cc plenum the 30" barrel but can just lower the reg psi to say 1000-1100psi and get 50fpe pellet shooter. I imagine that the efficiency would increase in comparison but would it be optimal?  No not hardly.       

[Motorhead]

The question is, can we get 950fps with 7cc of air at 2000psi from a 20" barrel, shooting a 25gr pellet?  If so then we can do better with a longer barrel and higher psi and a heavier slug.             

Not going to happen .... 7cc's at 2000# is no longer at that pressure ever falling lower as it fills the space between valve poppet and pellet.  Greater the throat area & transfer size gets, greater is the loss in pressure filling the space.

[Bill G]

Correct.  But take all the space between the poppet and the pellet out of the equation.  A chamber that is 7cc charged to 2000 psi is allowed to expand to 23cc is an exit psi of about 600ish psi. That is exerting 29lbs of force at the base of the pellet when it breaks free of the bore. The force is sufficient to attain that fps as long as the friction is 10lbs or less.  I think it is for a .25 pellet. 
I'm thinking outside conventional wisdom and dismissing what we are familiar with.  No dead space behind the pellet and most likely an in-line flow dynamic with nozzle.  Nozzle converges to .1875 only to open up to .250. No turns and throat is as close to zero length as you can get (probably need a small flat to avoid erosion or something other than Aluminum).