GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on October 01, 2015, 03:13:10 PM
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There are always many questions about the effect of Plenum size on the performance of PCPs, so I thought I would try and explain what happens internally, using Lloyd Sikes Internal Ballistics Spreadsheet.... It models what happens to the pressure, velocity, and energy inside the barrel, with a huge number of variables you can change to see what happens.... For this set of graphs, I used a .22 cal, 24" barrel, and a fixed dwell of 2 mSec for the 1800 psi graph and 1.8 mSec for the 2000 psi graphs.... I used a pellet breakaway force of 2 lbs. and a bore drag of 1 lb., a transfer port volume of 0.02 CI, an overall mechanical efficiency of 55% (typical for a gun like a Disco), and included the air mass in the calculations.... The pellet was 14.3 gr., and I varied the plenum volume ONLY on the first two graphs to show it's effect.... On the third graph, I increased the pressure for the smaller setpoints to keep the FPE output constant.... Here are the results, based on a 30 FPE design point....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Plenum%20Volume%201_zpsikpscy0e.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Plenum%20Volume%201_zpsikpscy0e.jpg.html)
A stock Disco plenum is 135 cc, which at 30 FPE is 4.5 cc per FPE.... The other two volumes are 1 cc per FPE and 1/2 cc per FPE, based on the 30 FPE design point.... You can see immediately what happens during the time the valve is open.... it closes when the pellet is about 10-11" down the barrel, and the pressure is dropping as the air in the plenum expands into the barrel.... The smaller the plenum, the less air available to expand, so the greater the drop in pressure by the time the 2 mSec has passed and the valve has closed.... While the gun shot ~30 FPE with the full 135 cc reservoir available, that dropped to ~28 FPE with a 30cc plenum, and down to ~ 26 FPE with a 15 cc plenum....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Plenum%20Volume%202_zps3mkrnshu.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Plenum%20Volume%202_zps3mkrnshu.jpg.html)
Since we are interested in what happens in a Regulated PCP, and it is not possible to use the entire 135 cc reservoir for the plenum, the second graph looks at plenums that are 1 cc per FPE and smaller (1/2 cc and 1/4 cc per FPE, based on the 30 FPE design point).... I increased the setpoint pressure to 2000 psi, and adjusted the dwell using the 30 cc plenum to get 30 FPE, and the graphs show what happens as you cut the plenum volume in half, and then in half again.... The power drops to ~ 28 FPE and 25 FPE respectively.... The next graph shows what happens if you increase the pressure to get back to 30 FPE using the smaller plenums....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Plenum%20Volume%203_zpsjxuxm5b5.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Plenum%20Volume%203_zpsjxuxm5b5.jpg.html)
The title of this graph gives the plenum size @ the pressure required to get back to 30 FPE.... Dropping from 30 cc down to 15 cc requires increasing the pressure from 2000 psi to 2190 psi to get back to 30 FPE.... If you cut the plenum in half again, to 7.5 cc, you have to boost the pressure to 2560 psi to get back to 30 FPE.... These figures are all calculated using a constant dwell of 1.8 mSec, which means the valve is closing when the pellet is about 10" down the barrel.... However, similar curves can be drawn using different parameters....
I hope that this series of three graphs helps you understand the importance of having a decent size plenum in an Regulated PCP.... There is no "one" correct answer, many variations can be made to work.... However, the fact remains that for a given designed FPE level, reducing the plenum volume reduces the pressure during the shot, and therefore the potential FPE at a given setpoint pressure.... To get that power back, you must either increase the dwell (lowering the efficiency), or increase the setpoint pressure.... A small decrease in plenum volume below the 1 cc per FPE level won't require much pressure increase.... but using too small a plenum for the desired FPE will mean you have to make a much larger increase in pressure....
Bob
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Good stuff tho a bit overwhelming for us to digest the data with many of the same color graphing lines per chart ???
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That is great info. Had to read it a few times but wow super indepth. Thanks for the info.
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I didn't think about the colours being confusing, I guess because I work with Lloyd's Spreadsheet so much.... The lines starting upper left, with the inflection in them when the valve closes, are pressure (psi), and the values are read on the right scale.... The upper set of curved lines that start at zero are energy (FPE) and the values are read on the left scale.... The lower set of curved lines that start at zero are velocity(fps) and the values are read on the right scale.... On the bottom chart, where all three versions are producing the same fps and FPE, the three lines are nearly superimposed, and end at exactly the same value at the 24" point (muzzle)....
The thing that for me is the most interesting is the pressure drop while the valve is open.... You can see the drastic effect that reducing the plenum has on that.... If you look at the middle graph, using 2000 psi, at the moment the valve closes (when the pellet has moved 9-9.5") the pressure is about 1660 psi for a 30cc plenum, about 1430 psi for a 15cc plenum, and only about 1120 psi for a 7.5cc plenum.... Just understanding that gives you a good understanding of why plenum volume is so important....
Bob
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Thanks Bob -> I'll re-read in the AM after a double shot of espresso ;)
Reminds me of pushing my palms against my eyes when they're closed -> A Light Show ;D
Kirk
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Thank you Bob ... my mind now is one with the charts ;)
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Hey Guys,
Thanks Bob -> I'll re-read in the AM after a double shot of espresso ;)
Reminds me of pushing my palms against my eyes when they're closed -> A Light Show ;D
Kirk
ROFL!!!! that hurt me. I had to try it to remind myself what it was like. my gut hurts. LOL
Bob, you sure you don't want to open up an air gun academy of some sort. Your nack for explanation and attention to detail is seconded by none that I've seen. Great work!
Bill
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Bob, you always makes my head hurt... in a good way! :) and its been too long since I've dug into one of your threads.
So, looking at the last graph, I can have a range of plenum/pressures to get the desired 30FPE. Given the fixed airtube (135cc) volume, and value for the space used for the reg, how would you find the optimum res to plenum ratio? The optimum being the most number of shots before falling off reg with an assumed 3K psi fill.
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Bob, sorry about my post about plenum size didn't go so well with you. I will learn "one day" on how to ask correctly.
The data you supplied was exactly what I was looking for. There is absolutely no where what you have done is referenced (anywhere). It is much appreciated.
It looks like from your data, (please correct me if I am wrong) that 2 times the barrel volume will give you the correct plenum volume and adjusting the pressure of the regulator will determine the fpe at the most efficient setting or what I would call the most shots before falling off regulation.
Thanks again Bob.
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i had to increase the magnification setting of my browser to 200% so my eyes wouls stop crossing, the sense of the graphs became clear to me...
excellent visual explaination of the plenum volume to pressure relationship that I only had limited understanding of from reading alot of your previous work...
Thank You Sir for your continued efforts to understand and explain the physical operating charactoristics of these deceptively simple arms
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Sorry, barrel volume has really nothing to do with it except indirectly.... The amount of air used for the shot is the key.... Most PCPs, once running efficiently, have an efficiency of 1 FPE/CI, and it is that volume of air used that determines the pressure loss for a given size plenum.... Yes, a .177 uses less air than a .357, but it also produces less FPE.... That is why by using the FPE to determine the plenum size needed is an easier calculation than bringing barrel length, caliber, pressure, and all the other variables into it.... For example, one guy might want a 12 FPE setup for FT, while another a 36 FPE setup for hunting, but both are using a .22 cal 24" barrel, and a tank (reservoir) pressure of 3000 psi.... The barrel volumes are the same, but the hunter needs a plenum 3 times as big....
There is only a limited range of setpoint pressures that make sense.... they have to be low enough below the fill pressure to give a reasonable shot count, and yet high enough to produce the power you want.... For the FT shooter above, you might pick 1200 psi for a setpoint, while the hunter might need 1800 to get the power he needs.... Determining the starting setpoint is the first thing to do when considering a regulator, and it is based on how the gun shoots unregulated.... Generally, you use a pressure just slightly higher than what is required for the fps/FPE you want in the gun you start with.... If an MRod shoots 850-900-850 from 3000 psi down to 1600, and you want to shoot 850, you would use about 1800 psi and a plenum of 1cc per FPE.... If you must use a smaller plenum, then you will move the setpoint up even more....
FuzzyGrub, your question is the exact problem faced by anyone installing an "in-tube" regulator.... The regulator and the plenum both take up volume inside the reservoir, reducing the air available.... The more power you want, the higher the setpoint, and the larger the plenum, and pretty soon you have such a small reservoir left, and so little pressure difference between "full" and "setpoint" pressures your shot count is dismal.... That is EXACTLY why in-tube regulators are more suitable to low-medium power applications, you have to trade off power and shot count.... It is possible to build a high-powered, regulated PCP, but you need to design it from the ground up, with a large plenum, and an even larger tank (bottle) feeding that to get any kind of a useful shot count.... An example is my "Grizzly" .30 cal repeater, which has a 90 cc plenum, fed by a 22CI (360cc), 3000 psi tank regulated to 2200 psi output, and gets 14 shots @ 100 FPE.... for an efficiency of 1.15 FPE/CI....
Bob
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Bob, I am not trying to make you crazy but is it 1fpe/cu in or 1 fpe/cc ?
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Ok, I am now really confused.
let's take my .357 marauder with a 215 cc "stock" reservoir. Shooting at 100 fpe for and average of 100 fpe for 6 shots and with a 3000psi charge (16" barrel). If I where to regulate it according to 1cc per fpe, I would need 100cc of plenum.
The confusing part to me would be that it is almost half the reservoir size and give only 2 shots at 100fpe according to the pressure drop across the valve. That don't make sense to me.
How does that work ?
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Hey Shorty - 1CC per 1 FPE ;)
All 3 of my regulated (.177 cal.) PCP's can shoot CPUM (10.5 gr.) pellets about 890/915 fps. This is between 18/20 FPE.
I have made all my plenums between 18/20 cc's. I probably could gotten away with 15cc's, but I have chosen to stay near 20cc, because in the future I may want to shoot JSB Monster or Beasts 13/16 gr. with the same regulator set-point and just a more hammer strike.
I could raise the regulator's set-point to give a higher velocity (which would help a smaller plenum), but have decided to sacrifice a few cc's on the high pressure side to ensure I have enough plenum for maxing power (FPE's). If you have to drill a vent hole in the air cylinder for reg. venting and this was done using a 10cc plenum, your stuck and don't have the plenum capacity for great FPE flexibility.
The only downside to the 1FPE/1CC plenum rule of thumb is you loose some air on the high pressure side of the regulator and shot count. The plenum volume discussion is more concerned with Power than Efficiency. So IMHO, it's best to give yourself some flexibility in power at the expense of shot count. You can then otherwise tune the gun for efficiency to bring up the shot count.
The "Otherwise Tuning" would entail Air Flow management (valve, transfer, barrel ports and valve throat adjustments), Valve Poppet adjustments, Hammer Strike management (Spring Pre-load, Hammer weight and Hammer travel adjustments). All these tweaks can get your efficiency up to 1.25/1.50 (FPE/CI), thus increasing shot count near 30 to 50%.
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shorty.... The EFFICIENCY is calculated in FPE/CI, and can vary from about 0.3 to 1.5, with most PCPs running around 1.0 - 1.2.... That is the amount of air (at 1 bar) used from the tank to create a shot, divided into the FPE for that shot.... or the total volume in CI used for a shot string, divided into the total FPE of the string....
The PLENUM SIZE should be about 1 cc (of HPA at the setpoint pressure) per FPE.... and yes, for a gun shooting 100 FPE, you want about a 100cc plenum.... Note above, I used a 90cc plenum for my Grizzly.... I would have used 100cc if I could have, but didn't have the room.... The result of the slightly undersized plenum was I had to increase the setpoint from 2000 psi to 2200 to get the 100 FPE I wanted at decent efficiency.... High powered PCP are VERY difficult to regulate, just because of the large plenum required to maintain the pressure to produce the big FPE numbers.... Trying to regulate an MRod (for example), with 215 cc of reservoir, and produce 100 FPE, will leave you with exactly the problem you face.... just a few shots, because half the reservoir ends up being plenum or regulator.... As I said above, in-tube regulators are great for low-medium power, pretty much useless for high power....
Using an MRod as an example, let's say it's a .30 cal, designed to produce 100 FPE (like my Grizzly).... The reservoir is 215 cc, the regulator will take up about 15cc (assuming an inch of length), and you have a 100 cc plenum, so your remaining reservoir is only 100 cc (6.1 CI).... Let's say you can achieve your 100 FPE at a pressure of 2000 psi (in the unregulated gun), so you choose a setpoint of 2200 psi to make up for the pressure loss with a 100cc plenum, compared to the 215cc reservoir.... That means you have 800 psi of pressure drop available from the 3000 psi fill to the 2200 psi setpoint, which is (800/14.5) = 55 bar.... Multiply that by the reservoir volume of 6.1 CI, and you have a total of 336 CI of air (at 1 bar) available before the reservoir pressure drops below 2200 psi.... If the gun shoots at an efficiency of 1.00 FPE/CI, each 100 FPE shot will use 100 CI of air (at 1 bar), so you will get 3 shots.... If you can get the efficiency up to 1.20 FPE/CI, then you could get 4 shots....
You can also calculate the pressure drop in that 100 cc plenum during a shot.... You are starting with 100 cc of air at 2200 psi in the plenum.... At an efficiency of 1.00 FPE/CI, and a 100 FPE shot, you use 100 CI of air at 1 bar.... However, the air in the plenum is at 2200 psi, which is (2200/14.5) = 152 bar, so the volume of air you will draw out of the plenum for a shot is (100 x 16.4 / 152) = 10.8 cc of air at 2200 psi.... You will then be left with (100-10.8 ) = 89.8 cc of air at 2200 psi, expanding into that 100 cc space, so the pressure after the shot will be (2200 x 89.8 / 100) = 1976 psi.... and the average pressure during the shot will be 2088 psi.... If you decrease the plenum size to 50 cc, but still need that same 10.8 cc of air to make the shot.... the air remaining after the shot will be (50-10.8 ) = 39.8 cc, expanded into that 50 cc space, so the pressure will only be (2200 x 39.8 / 50) = 1751 psi.... and the average pressure during the shot will only be 1976 psi, instead of 2088 psi.... You would have to increase the setpoint from 2200 psi to 2310 psi to compensate for the smaller plenum.... Going smaller than 1/2cc per FPE for the plenum will require even greater increases in the setpoint....
Using the 50 cc plenum, and a 2310 psi setpoint, we can calculate the possible number of shots for the MRod example above.... The reservoir will now be 150 cc (9.1 CI), but the pressure drop will only be 690 psi (48 bar).... making the total volume of air available (9.1 x 48) = 437 CI at 1 bar.... At an efficiency of 1.00 FPE/CI you will now get 4 shots of 100 FPE.... and if you can get the efficiency up to 1.20 FPE/CI you should get 5 shots.... So, in this case, using the smaller plenum, of 1/2 cc per FPE instead of 1 cc per FPE, and increasing the setpoint to compensate, should give you one extra shot.... As Spiralgroove pointed out, however, should you ever wish to push the FPE higher by using a heavier bullet, you won't have the same flexibility as with the larger plenum.... Your actual results will depend on what setpoint you require to get 100 FPE while maintaining usable efficiency, of course.... I used numbers which worked for my Grizzly, which should be similar, providing you can get the airflow you need....
Bob
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FuzzyGrub, your question is the exact problem faced by anyone installing an "in-tube" regulator.... The regulator and the plenum both take up volume inside the reservoir, reducing the air available.... The more power you want, the higher the setpoint, and the larger the plenum, and pretty soon you have such a small reservoir left, and so little pressure difference between "full" and "setpoint" pressures your shot count is dismal.... That is EXACTLY why in-tube regulators are more suitable to low-medium power applications, you have to trade off power and shot count.... It is possible to build a high-powered, regulated PCP, but you need to design it from the ground up, with a large plenum, and an even larger tank (bottle) feeding that to get any kind of a useful shot count.... An example is my "Grizzly" .30 cal repeater, which has a 90 cc plenum, fed by a 22CI (360cc), 3000 psi tank regulated to 2200 psi output, and gets 14 shots @ 100 FPE.... for an efficiency of 1.15 FPE/CI....
Bob
So, for the medium power of the 30fpe, why is the 30cc (1cc per fpe) better than 1/2 cc per fpe? Yes, it takes 190psi more, but it is unclear to me why that is significantly worse. Now, going to 2cc per fpe (60cc), looks like the pressure would only drop to close to 1900psi. That is easy to see that you ate up the majority of your volume with little psi gain, and would be a poor choice. I suspect the optimum may be somewhere between 1/2 and 1 cc per fpe, but don't know how to graph or prove or dis-prove it.
FWIW: I only have one reg gun, and it is a 177 mrod, where I basically just followed Scott's recipe. I am getting 80 18.5fpe shots at 1600psi set point. The only plenum is a hogged out gauge port and valve. I measured the volume and will have to find the numbers but don't believe it was 1cc per fpe. I know, significantly different than this disco model, but I'm just trying to relate the math to practical application.
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Good stuff, you never fail to make me learn a bit more about how these things work. By the way, I get as much content from your writings, probably more, as I do from your graphs. Well expressed.
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You would have to work out the specifics for your individual gun, and you can really only do that if you know the efficiency.... Let me give you another example, this time with a QB.... I did a regulated setup at 1500 psi, 13CI 3000 psi tank, same valve, barrel, breech and bolt, but in one case used a QB79 tube and in the other a QB78 tube.... The QB78 shot 40 shots at 31.1 FPE, which was 0.90 FPE/CI.... and the QB79 did 39 shots at 26.4 FPE, which was 0.74 FPE/CI.... I then cut the QB78 tube to make it half way between, and it did 40 shots at 30.5 FPE, which was 0.85 FPE/CI.... The QB79 had a plenum plus valve volume (yes, the valve volume counts) of about 10 cc, the QB78 was about 50 cc, and the half tube '78 was about 30 cc.... It is pretty obvious that it was the best compromise.... In this case, increasing the setpoint pressure to compensate for the small plenum of the QB79 was not possible, because of safety considerations, where the regulator had to be fitted with a 1.8K burst disc, and going higher than 1500 psi would have popped the disc.... If you had the ability to increase the setpoint safely, such as you do in an in-tube installation, then it may have been possible to get as high a total FPE (ie same number of shots at the same FPE) with a plenum that is 1/2 cc per FPE.... It will all depend on the efficiency you can get with the two different configurations....
There is another thing which influences how big the plenum should be, and that is the setpoint pressure itself.... Let's compare a setpoint of 100 bar (1450 psi) to 200 bar (2900 psi).... Let's assume that the shot consumes 400 cc (24.4 CI) of air at 1 bar.... That is 4 cc at 100 bar, but only 2 cc at 200 bar.... Now let's assume the plenum is 20 cc.... In the 100 bar system, the air left after the shot will be (20-4) = 16 cc, and the pressure after the shot will be (100 x 16/20) = 80 bar, a 20% loss.... However, in the 200 bar system, the air left after the shot will be (20-2) = 18 cc, and the pressure after the shot will be (200 x 18/20) = 180 bar, a loss of only 10%.... Therefore, as the setpoint pressure increases, the size of the plenum you need to minimize the pressure loss decreases.... If you are running a 4500 psi tank, regulated down to 3000 psi, you can use a smaller plenum.... However, most regulated systems have a setpoint pressure of 1000-2000 psi, with the majority between 1400-1800.... so that is the pressure range I based my 1 cc per FPE recommendation on.... Remember, there is no downside to having a bigger plenum from a performance point of view.... but there are definitely situations where running a plenum that is only 1/2 cc per FPE, at a higher pressure, may give you more shot count.... particularly in low-medium power applications.... but IF AND ONLY IF doing that gives you a larger reservoir to draw from (eg an in-tube regulator).... If you are after high power (relative to the caliber) you are generally better off using 1 cc per FPE.... There may be some situations, however, such as very low power setups (eg. a 500 fps target rifle), where a plenum even smaller than 1/2 cc per FPE, running at several hundred psi higher pressure, may be the best choice.... One additional advantage in this use is that tiny sips of higher pressure are quieter....
I guess the best way to sum this up is as follows....
If you can achieve a plenum size of 1 cc per FPE, there is little point in going larger, the gains in power are too small.... If you are using an in-tube regulator, however, going that large may reduce the reservoir and shot count, so you MAY be better off using a plenum as small as 1/2 cc per FPE, running at a slightly higher pressure.... Only rarely, in very low power applications, would it be worthwhile going less than 1/2 cc per FPE.... If in doubt, use the largest plenum that will give you a decent shot count, up to 1 cc per FPE.... with a minimum of 1/2 cc per FPE....
Bob
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Bob, it will take me some more time to walk thru and understand the example, but the summary was clear! :)
The model only is calculating based on chamber size, and does not model the true effect of a reg that will refill the plenum, if I'm understanding correctly. Is the reg dwell time to refill the plenum significant longer than the valve dwell?, that it doesn't impact the calculation? I think I've seen people talk about reg cycles that are in seconds, vs millisecond timing on valve dwell, so that would seem to make sense.
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Correct, the refill time on the regulator is MUCH slower than the shot cycle, which is 1-2 mSec.... I have had gauges in the plenum, and you can see the pressure drop and refill on the gauge, it often doesn't stop moving for over 1 second.... For all practical purposes, the only air available for the shot is what is inside the valve and plenum.... To make it worse, if the feed hole between the plenum and valve is smaller than the exhaust port of the valve, you may not even keep the pressure in the valve you should have in the calculations.... I strive for an entrance hole in the valve AT LEAST as large as the hole in the throat of the valve, not counting the area of the stem of the poppet, to make sure....
Bob
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Hey Bob,
If all of my (.177 cal.) shooting is in the range of 18/25 FPE (105 bar set-point), it sounds like I do not need an 18/20cc plenum ???
Without doing the Math, (all thing considered) my gut says I could likely go down to 15cc's, bump up the regulator a whisker and use the lost 5cc of air for (optimal) shot count.
What do you think?
Kirk
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Hey Bob,
So in your Marauder example, if you were to go with a 75cc plenum instead of 100cc or 50cc:
- Pressure during the shot would be increased to 2,042 psi vs. 2,088 psi vs. 1,976 psi
- Pressure after the shot would decrease to 1,883 psi vs. 1,976 psi vs. 1,751 psi
- Setpoint needs to be increased to 2,250 psi vs. 2,200 psi vs. 2,310 psi to bring pressure during the shot back to 2,088 psi ---------------->>> Right?
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What I feel you guys are missing is that the DWELL of any given set up effects what the pressure drop is from ANY given plenum pressure.
Whats more important is how quickly and with minimal losses the pressure that is within plenum gets in behind pellet while at rest and still seated within breech.
While there will always be this collapse of pressure within plenum during the firing cycle ... it is most important how efficiently the transference is between storage and being able to use it in launching the pellet with minimal pressure drop yet maximum flow to a point where said pellet is well on its way 1/4 to 1/3 the barrel length. ( Ideally )
If valve is still open beyond this pellet travel distance little to no increase in velocity happens ... yet pressure within plenum continues to collapse further :P * Which was the subject of this thread 8)
DWELL we speak of often yet few really grasp IMO ???
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Yeah Scott,
I think Bob was just trying to isolate plenum size changes (only), in lieu of other tuning variables to help Shorty and others like me ::). You're right, were dealing with pure "plenum" theory here, irrespective of any other tuning forces that will further complicate matters.
Unfortunately, you 8) and Bob 8) have PhD's in this stuff, the rest of us have signed up for "PCP tuning" 101 or 201 at Coalmont University.
In my Marauder example above, if I have not changed anything except Plenum size, do I need to think that much about a dwell change going from a regulator setpoints of 2,250 psi vs. 2,200 psi vs. 2,310 psi ?
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Scott is correct.... I chose a slightly longer than usual dwell to make the numbers larger and the graphs easier to read.... For a more efficient tune, the valve will be closing sooner, and the pressure drop will be less.... On the other hand, the MRod numbers I gave in the text in Post #14 use calculated pressure drop, not dwell, but assume an efficiency of 1.00 FPE/CI.... If the gun is more efficient, it will use less air per shot, and again the pressure drop will be less.... However, in both examples, the TREND continues to apply....
Kirk, I didn't check your numbers, but you seem to have exactly the right idea.... a plenum in between will work in between....
Bob
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Thanks Scott & Bob,
So a plenum change from 1cc per 1fpe to .75cc per 1fpe at very high (2200 psi) setpoint, makes the setpoint increase -> insignificant (2,200 psi vs. 2,250 psi). Dwell maybe a tad shorter because more psi is on the poppet.
However, how linear is this (above) relationship when it is applied to a 15 cc plenum change (from 20 cc plenum) at 105 bar setpoint. Hence, can you comfortably get away with a 15cc plenum for a (.177 cal.) 18/25 fpe gun?
Or ......... have I now traveled off the Reservation :-\
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I did a quick search but didn't find any reports on reg'ing a 22 disco, or at least a single tube one at 30fpe. Curious to how many shots it does get at that power. My guess would be 20-25.
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You will notice very little difference between 1 cc per FPE and 3/4 cc per FPE if the gun is low-medium power, OR if the setpoint pressure is high (2000+)....However, if your gun is relative high power (ie thirsty for air) and/or the pressure is lower, then you will see a fair difference in power and/or efficiency.... As I said, each gun is an individual case when it comes to plenum sensitivity.... Ultimately, if you are after maximum power, regulators are NOT the way to go, as you need the full tank pressure to achieve even a few max. FPE shots.... Big Bores are the typical example of this.... but it applies to any gun pushed to its FPE limit....
One alternative is to design the gun to shoot as an unregulated PCP and then tether it to a regulator, as is common with long range shooters.... It is common for them to tether to about a 3600 psi regulator (to get an ES of 1% or less), and when untethered the gun will give 2-3 shots within a tight 1-2% ES.... This means quite a large plenum (reservoir) to provide enough air for that, eg. the .257 boys are pushing about 200 FPE.... Those 3 shots would use about 600 CI of air (at 1 bar), which at 3625 psi (250 bar) is (600 x 16.4 / 250) = 40 cc, which would be a 20% drop in pressure in a 200 cc reservoir when untethered for those three shots.... ie from 3625 psi to 2900 in just 3 shots.... When tethered, the gun is shooting at the velocity of the first (high pressure) shot of that tight 3-shot string, and hence as efficiently as it can at the power level chosen....
A regged single tube Disco at 30 FPE would be a very poor choice.... If the efficiency was 1.00 FPE/CI, you would be using 30 CI per shot.... With a 30cc plenum and replacing the gauge block with a regulator, you would have about 100 cc (6.1 CI) of reservoir remaining.... To get 30 FPE, with a ported valve and large ports, might take a 1500 psi setpoinit using 18 gr. pellets, so that would only give you about 500 psi (35 bar) of "headroom" (difference between setpoint and fill pressure).... That would only give you (6.1 x 35) = 214 CI of air available (at 1 bar), so you should get about (214 / 30) = 7 shots.... IF you replaced the valve screws with 10-32s to make sure you could safely use a 3000 psi fill, your headroom would triple to 1500 psi and your shot count to 21-22 shots.... Alternately, if you used a Disco Double at 2000 psi, you would get about 15 shots.... and a DD at 3000 psi would give you about 45 shots....
Bob
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I thought I would do another example for you.... This one is typical of what you might get if you regulated a stock .22 cal Disco, using a 2000 psi fill and 14.3 gr. pellets, aiming for 22 FPE, which is pretty much stock power.... For the 22 cc plenum, I chose 1400 psi as a setpoint, which gave a dwell of 1.6 mSec at 59% mechanical efficiency (very typical for a stock Disco) and had the valve closing at exactly 25% of the barrel length, when the pellet had travelled 6" down the barrel.... I then adjusted the setpoint pressure to keep the fps and FPE constant (828 fps = 21.75 FPE) with the three plenum volumes of 1 cc, 1/2 cc, and 1/4 cc per FPE (ie 22, 11, and 5.5 cc).... Here is the graph....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Plenum%20Volume%204_zpsjr16xyae.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Plenum%20Volume%204_zpsjr16xyae.jpg.html)
Once again, the title of the graph gives the plenum size and setpoint pressure required to produce 22 FPE with that plenum volume.... With the 22 cc plenum at 1400 psi, the efficiency was 1.09 FPE/CI, the 11 cc plenum required 1535 psi but delivered 1.11 FPE/CI, and the 5.5 cc plenum required 1810 psi and delivered 1.16 FPE/CI.... Just looking at those numbers, the smaller plenum looks more efficient, but that is only part of the story.... Let's look at the shot count they will produce....
Lloyd's calculator gives me the exact volume of air at 1 bar used to produce each 21.75 FPE shot.... It was 19.9 CI for the 22 cc plenum, 19.5 for the 11 cc, and 18.8 for the 5.5 cc.... The headroom (pressure range available from the 2000 psi fill pressure down to the setpoint) was 600 psi (41.4 bar) for the 22 cc plenum, 465 psi (32.1 bar) for the 11 cc, and just 190 psi (13.1 bar) for the 5.5 cc plenum.... If we assume that the regulator is 1.5" long (occupies ~ 11cc), that a PRod gauge block is used for the larger through hole (adds 2 cc) to insure the air in the plenum can flow easily into the valve, and include 3 cc for the valve volume, the regulator would be virtually sitting against the gauge block and still supply a 5.5 cc plenum (including the valve and gauge block volume), meaning the reservoir would be (135-11) = 124 cc (7.56 CI).... The plenum spacers required for the larger plenums, we will assume are 16 ga. wall tubing (taking up 0.43 CI of reservoir per inch), and therefore 5/8" ID, so the spacer for the 11 cc plenum (5.5 cc internal volume) would be 1.1" long (taking up 0.47 CI) and the spacer for the 22 cc plenum (16.5 cc internal volume) would be 3.3" long (taking up 1.42 CI).... That makes the remaining reservoir the following size; 22 cc plenum = 6.14 CI, 11 cc plenum = 7.09 CI, 5.5 cc plenum = 7.56 CI.... Mulitplying those numbers by the headroom in bar gives the total volume of air available above the setpoint.... For the three reservoirs, that is; 22cc plenum (6.14 x 41.4) = 254 CI, 11 cc plenum (7.09 x 32.1) = 228 CI, 5.5 cc plenum (7.56 x 13.1) = 99 CI.... Dividing those numbers by the volume of air used per shot gives the expected shot count, as follows; 22 cc plenum (254 / 19.9) = 12.8 shots, 11 cc plenum (228 / 19.5) = 11.7 shots, 5.5 cc plenum (99 / 18.8 ) = 5.3 shots....
I realize there are a lot of numbers in the above paragraph, and I hope you were able to follow along.... The bottom line is that because of the small reservoir of a Disco, and the low fill pressure (2000 psi), regulating a stock gun with the intention of producing stock power (22 FPE) with a flat shot string is not a great idea, as you will lose about half the shots compared to unregulated.... There is little difference between the large and medium plenums (1 cc and 1/2 cc per FPE respectively), about 1 shot.... but going to "no" plenum (just the gauge block and valve volume, which is about 1/4 cc per FPE) requires such a high setpoint you lose about half the shots.... Of course if you modify the gun (eg. bigger ports) to produce 22 FPE at a lower pressure (say 1200 psi), you will get a different set of numbers.... The same applies if you reduce the FPE....
So what happens if you fit 10-32 valve screws and bump the fill pressure up to 3000 psi?.... You add 69 bar to the headroom in all cases.... it becomes 110 bar for the 22 ccc plenum, 101 bar for the 11 cc, and 82 bar for the 5.5 cc.... That gives you an available volume above setpoint of (110 x 6.14) = 675 CI for the 22 cc plenum, (101 x 7.09) = 716 CI for the 11 cc, and (82 x 7.56) = 620 CI for the 5.5 cc.... That would result in much larger shot counts, of course.... (675 / 19.9) = 34 shots for the 22 cc plenum, (716 / 19.5) = 37 shots for the 11 cc, and (620 / 18.8 ) = 33 shots for the 5.5 cc plenum.... In this case, the 11 cc plenum is the clear winner.... Why?.... because 22 FPE is quite low powered for a 3000 psi gun in .22 cal, and the relatively small reservoir volume, with an in-tube regulator, favours a larger reservoir with only a slight setpoint pressure increase....
There is a third possibility, that of using a bottle to replace the reservoir, mounting the tank block slightly further forward to allow for different plenum sizes.... This means that the reservoir (bottle) is always the same size.... Let's use the common 13 CI 3000 psi tank and see what happens for our three plenum sizes, using the appropriate setpoints to maintain 21.75 FPE as above.... This will mean that the headroom is the same as in the 3000 psi case above, but the volume will always be 13 CI.... Now, for the 22 cc plenum we get (13 x 110) = 1430 CI of air, for the 11 cc plenum we get (13 x 101) = 1313 CI, and for the 5.5 cc plenum we get (13 x 82) = 1066 CI.... Dividing by the air used per shot, for the 22 cc plenum we get (1430 / 19.9) = 72 shots, for the 11cc we get (1313 / 19.5) = 67 shots, and for the 5.5 cc plenum we get (1066 / 18.8 ) = 57 shots.... For a bottle gun, the 1 cc per FPE plenum is the clear winner....
I hope that the above examples show you that even for the same basic gun, at the same power level, the optimum plenum size can still vary.... For a bottle gun, where the reservoir is always the same size, the larger plenums will always produce the highest shot count at any given power level.... For a gun with an in-tube regulator, somewhere between 1 cc per FPE and 1/2 cc per FPE will deliver the best shot count in almost every case.... The only exceptions will be low power with a lot of headroom, where an even smaller plenum may be the answer....
Bob
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.... IF you replaced the valve screws with 10-32s to make sure you could safely use a 3000 psi fill, your headroom would triple to 1500 psi and your shot count to 21-22 shots....
My ruff order of magnitude was based off a typical 40-50 shot reg string on the 215cc Mrod, and dividing in half. And yes was thinking 3kpsi fill, because I was relating it to my long prod tube gun, which is close to disco volume.
PS: And I see I have more reading to catch up on! :)
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I fully understand. I fully understand. I fully understand that I now have a headache. LOL I also understand I need to send my guns to a professional tuner who will do the job right. This is way above my knowledge and understanding. :o
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First of all special, thanks to Bob for starting this illustrative thread and then thank to Fuzzy Grub (Jhon) SprialGroove (Krick) for asking questions to make the topic more informative.
Motorhead (Scott) chime in and draw the attention of Bob towards "affects of dwell". But as Krik said, when PhD Scholars discuss the matter, it is pretty hard to understand for the ppl like me who are just thinking to sing up for "PCP tuning". So for now, i am just concentrating on the topic. I am sure Bob will explain this aspect of tuning (as pointed out by Scott) with his wonderful graphs and apprehend words in another post.
I am enjoying this informative post very much.
Once again, bundle of thanks to Bob.
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Lloyd's calculator gives me the exact volume of air at 1 bar used to produce each 21.75 FPE shot
Bob, in the reply No. 28 you talked about Lloyd's calculator. I googled it but could not found. May i have the link for the said calculator, please.
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Lloyd's calculator gives me the exact volume of air at 1 bar used to produce each 21.75 FPE shot
Bob, in the reply No. 28 you talked about Lloyd's calculator. I googled it but could not found. May i have the link for the said calculator, please.
It is in the GTA Library : http://calc.sikes.us/ (http://calc.sikes.us/)
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Sorry, I was referring to Lloyd's Internal PCP Ballistics Spreadsheet, which is proprietary, but I am fortunate to have a copy.... The calculators mentioned by Scott are for fills or efficiency....
Bob
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A great read