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Author Topic: Tuning a Regulated PCP  (Read 3340 times)

rsterne

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Tuning a Regulated PCP
« on: November 16, 2016, 08:10:58 AM »
Two years ago I wrote this thread on the GTA.... I keep getting so many people asking for information covered in it I thought I would add it here as a sticky.... I have copy pasted all the initial posts with the data and graphs, and then added a few (edited) comments where I answered a few important questions at the bottom.... I hope you find it useful....

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....



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....



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....

I am locking the thread for now until I get it copied here....

Bob
« Last Edit: November 16, 2016, 08:26:26 AM by rsterne »


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rsterne

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Re: Tuning a Regulated PCP
« Reply #1 on: November 16, 2016, 08:12:33 AM »
The second graph was not taken from actual data, I drew it to show the trends ONLY.... However, the first graph, on the QB, was from data, and I ended up running that gun at about 4.5 turns out, at about 845 fps.... so only down about 15 fps from the plateau, ie about 2%.... Other guns will be a bit lower to find an efficient point on the knee, it all becomes a matter of balancing shot count vs. velocity.... Here is a similar graph for my 2560....



I tuned that gun for 950 fps, achieving this shot string at 1.01 FPE/CI.... about 4% below the plateau velocity....



That represents an approximate range below the plateau for the knee.... 3% might be an average drop, certainly well worth it from a shot count and efficiency point of view.... With really powerful guns (which tend to use a lot of air) you may have to drop 5% below the plateau to get the shot count you want.... and the plenum size relative to the FPE may have to do with how far you have to drop as well.... Some guns won't provide a perfectly flat plateau, it will still have a slight slope to it, and some actually lose a bit of velocity with big increases in hammer spring preload, but finding the point where the plateau and the downslope transition, ie the knee, is the important part anyway..... Your ideal tune will be somewhere on that curved portion, exactly where is a matter of personal choice, balancing power vs. shot count....

Bob
« Last Edit: November 16, 2016, 01:29:56 PM by rsterne »
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rsterne

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Re: Tuning a Regulated PCP
« Reply #2 on: November 16, 2016, 08:13:09 AM »
Here is some actual data from a .177 cal Int'l FT rifle of 12 FPE that I built a couple of years ago, showing the velocity rise below the 1500 psi setpoint....



The velocity crept up from 800 fps at 3000 psi to 820 fps at 1500, at which point the air usage increased slightly, the velocity increased to peak at 843 fps at 1200 psi, and then dropped, getting back down to the 800 fps where it started at only 800 psi....  This rifle is a classic example of tuning a regulated PCP on the downslope....

Yes, the gun really did get 250 shots from 3000 psi down to 800 on a 13CI tank.... Each data point on the above graph is the average of 10 shots.... I ended up reducing the setpoint pressure and hammer spring preload to flatten the curve....

Bob
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rsterne

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Re: Tuning a Regulated PCP
« Reply #3 on: November 16, 2016, 08:13:59 AM »
Here is some more data which you may find useful.... It shows what happens when you change pellet weight in a regulated PCP....



As you can see, the position of the knee shifts to more preload as you increase pellet weight.... With 12 gr. Hobbies, it is at about 4 turns out, with 18 gr. Heavies it is at about 2 turns out, and with EunJins the knee cannot be reached even with the hammer spring at coil bind.... Doing testing such as this and graphing the results gives you a complete picture of exactly what is happening inside your regulated PCP.... It is worth doing just for the learning experience alone, at least once....

Bob
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rsterne

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Re: Tuning a Regulated PCP
« Reply #4 on: November 16, 2016, 08:16:13 AM »
There is a relationship between regulator setpoint and the optimum hammer strike.... Hammer strike is a combination of hammer weight, travel, and average spring force.... Hammer travel and spring force affect both hammer energy (lift) and momentum (dwell).... Hammer weight only affects the momentum (dwell).... Generally, if you are regulating a PCP that originally operated at, say, 3000 psi.... and using a setpoint of, say 1500 psi.... you have to drastically reduce the total hammer strike or you will be wasting air.... That is the primary message of this thread.... You can reduce hammer strike by reducing any or all of hammer travel, weight, and spring force.... Motorhead has found that for many PCPs, when you fit a regulator, they work much better with a lighter hammer.... The downside is that if you make the hammer too light, you will have to then increase the travel, or spring force, to compensate.... This can become an issue in high-powered PCPs, as if may make the gun difficult to cock....

Bob
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rsterne

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Re: Tuning a Regulated PCP
« Reply #5 on: November 16, 2016, 08:17:17 AM »
If you are stuck on the plateau and can't get down to the knee, you have two choices....

1. If you want more power, increase the setpoint....

2. If you have the power you want, and wish to increase your shot count.... reduce the hammer strike by lightening the hammer, shortening the throw, or installing a weaker or shorter spring....

or, of course you can just leave it, and enjoy what you have....

Bob
« Last Edit: November 16, 2016, 08:46:03 AM by rsterne »
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rsterne

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Re: Tuning a Regulated PCP
« Reply #6 on: November 16, 2016, 08:18:32 AM »
As you approach and fall through the setpoint:

On the plateau (thigh) the velocity falls quickly, possibly starting it's decline slightly above the setpoint if you have way too much hammer strike....
On the downslope (shin) the velocity rises noticeably as you pass the setpoint before falling after the pressure is some distance below the setpoint....
On the knee the velocity holds nearly constant, perhaps rising slighty, as you pass through the setpoint, before falling gently, then more rapidly....

When you are on the knee, the closer you are to the plateau the less chance of the velocity rising, and it will start to fall just below the setpoint.... If you are closer to the downslope, you will get a larger velocity rise, extending the usable shot count well below the setpoint, before the velocity drops too far to be usable.... When the rise is about 1% ES, that results in the highest shot usable shot count for that setpoint for most conditions....

Bob
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rsterne

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Re: Tuning a Regulated PCP
« Reply #7 on: November 16, 2016, 08:24:08 AM »
Unregulated PCPs were very often tuned for extended strings for FT work by leaning on the hammer spring and choking up the transfer port.... You can certainly reduce the power of a regulated PCP very simply by installing a smaller transfer port, and if the gun is operating efficiently before, it will generally get even higher efficiency (in FPE/CI) when the power is reduced.... and it does get quieter in proportion to the FPE reduction.... It can be done externally with a adjusting screw, if that is incorporated into the design, such as my retracting bolt that can be restricted, or the velocity screw in an MROd valve port....

You can also reduce the power, and increase the efficiency dramatically, by reducing the hammer strike (preload) to use smaller sips of air, but if you go that route, you MUST stay above the setpoint, or you will experience a drastic velocity increase once the pressure drops below the regulated output.... Look back at the first post in this thread to see what I mean.... The great thing is this is instantly adjustable and reversible without any disassembly....

The velocity can also be reduced by dropping the setpoint pressure and readjusting to the knee of the now lower curve.... This gives more shots from the greater pressure range, and increased efficiency as well, although is more complex to do, and not as easily reversible as simply dialing back the hammer preload....

Generally, the very highest efficiency is obtained by using tiny sips of high pressure.... That means the valve is closing very early, and the air is still expanding as the pellet leaves the muzzle.... The shot will be very quiet as well.... The only drawback, as I said, is the velocity increase below the setpoint.... This likely makes using reduced preload the best method to detune temporarily, just watch the pressure gauge....

Bob

PS, I will unlock this topic so that you can ask questions and I will try and answer them to the best of my ability.... PLEASE STAY ON TOPIC or I will lock it to prevent clutter....
« Last Edit: November 16, 2016, 08:42:08 AM by rsterne »
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Motorhead

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Re: Tuning a Regulated PCP
« Reply #8 on: November 16, 2016, 11:06:14 AM »
Indeed all this data helps wrap ones mind around all the ways to get to the same place taking each part of system and marry it to another.  Think whats so frustrating for tuners is that there are so many ways to manipulate the working parts.  No real definitive Right or Wrong but like baking there is some talent required to get it right.
Certainly some ways work better than others, but sadly it is still an applied science to use what best fits the PCP your working with. * Which leaves us short of having a "Do This" to get there simple response for many HP to LP regulation conversions.

Great data Bob ... I'm sure it will help many for years to come who wish to take a bottomless trip down the rabbit hole of PCP tuning possibilities.  8)

Scott
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Re: Tuning a Regulated PCP
« Reply #9 on: November 16, 2016, 02:38:47 PM »
Bob,
Awesome work and post on "how to tune a regulated gun". As soon as the reg is in "it is what it is" "unless you pull it apart and start changing things again".

What some people may want to know is (and this is a question) "what" to expect if they add a regulator to their existing gun and what pressure,plenum sizing, or porting they should use to achieve that shot count or power level. So, they get it right the first time with what they are looking for in regards to power and shot count with their existing gun.

I know this may be a big explanation and starting another thread but, I think it may compliment the "how to tune" an existing regulated gun.

I know every gun is different along with barrel lengths which can change the efficiency but, do you have a method that others can follow to understand what they are going to get if they do X and Y to the gun before even adding the reg ?

Like Scott says, take the frustration out of regulating your gun.

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rsterne

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Re: Tuning a Regulated PCP
« Reply #10 on: November 16, 2016, 04:36:05 PM »
It would be wonderful if I could write a simple thread on the "best" setup to use when you install a regulator, but as you say, not really part of this thread.... This thread is pertinent to how to get the most efficient operation from a PCP that already has one.... Some PCPs come regulated from the factory (and some of them are quite poorly tuned).... Others are created by installing a regulated bottle on a CO2 gun, or to convert an unregulated PCP, and you have more flexibility when creating the plenum, as it is a separate entity from the bottle, and changing the size of it doesn't affect the reservoir volume.... Still others are converted by installing an "in-tube" regulator in an existing reservoir, so you have an additional compromise to make, because increasing the plenum reduces the primary reservoir, creating yet another variable / balancing act....

The one constant is that for any given combination of barrel, ports, and projectile, the velocity should primarily be set by the regulator setpoint.... If you try and get too much velocity for the pressure, you will end up with an air hog.... If you dial the velocity way down, you will have to deal with a "bump-up" in velocity below the setpoint.... Only by tuning in the vicinity of the "knee" of the curve will you get the best combination of velocity, efficiency, and shot count.... THAT is what this thread is all about....

Bob
« Last Edit: November 16, 2016, 04:38:04 PM by rsterne »
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charmin99

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Re: Tuning a Regulated PCP
« Reply #11 on: November 19, 2017, 10:05:53 AM »
Hi Bob,

I read this whole post with great interest and learnt a lot.

I'm in the UK and as you prob know have max 12ft/lb by law. I have just fitted an Altros regulator to my Air Arms S510 Sportet Ultimate, as you say the ft/sec has dropped slightly from 786 to 775 but to have 40 level shots is great, there is about 4ft/sec variation before it hits the downward knee, the velocity starts to drop what seems pretty linear with the drop in chamber pressure.
It does seem to be a bit of an air hog, reg set at 135bar, 200bar fill, I used to get about 35 shots across the power curve that were useable for FT, so as you said I was hoping for a few more regulated.
Is it worth reducing the hammer spring down when just being able to use 12ft/lb, I have no idea how to find a suitable spring and there is no adjustment apart from removing the seating collar on the end of the spring, about 2mm, and fit a very thin washer instead or find a weaker spring somehow.
The transfer port adjustment is optimised for max power which is the 775, it's a .177 as well.
Any thoughts....
Many thanks

Malc
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rsterne

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Re: Tuning a Regulated PCP
« Reply #12 on: November 19, 2017, 10:26:35 AM »
Without the ability to try different hammer spring preload settings, it is pretty tough to know where you are on the power curve.... If you are up on the plateau, then reducing preload will decrease air use without a loss in velocity.... If you are down on the downslope, then any reduction in preload will lose velocity....

Bob
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bnowlin

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Re: Tuning a Regulated PCP
« Reply #13 on: November 19, 2017, 12:39:24 PM »
Ok let's say you need a lighter hammer from Travis' MDS hammer do you mill out some of the center brass if it is thick as min is and make sure there is enough room to have plenty room to screw the cocking screw in without the end of the crew not touching the spring. or use a weaker spring?
Bobn

charmin99

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Re: Tuning a Regulated PCP
« Reply #14 on: November 19, 2017, 01:53:01 PM »
Hi,

OK, understood, so is there any other way to find where I am on the plateau or down the knee apart from playing with various spring preload, what if I was to re-set the regulator set-point higher gradually and if needed, lower gradually, measure the output power,
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