Help understanding plenum size

[Vmaxpro]

Im a little confused on something I was hoping someone could clear up? On my regulated bantam magnum, the plenum volume from what I understand is about 2.5cc. At 150 bar I can make 60fpe. Yet all I read is how it needs .5-1cc per fpe to make that energy yet Im not even remotely close. What am I not understanding here?

[Motorhead]

Within a confined space .. ( plenum in this case ) higher the pressure greater is the total volume of air to do the work.
if your plenum was larger you would be able to use a reduced pressure and still make the same power.

Not familiar with the gun you have, but if plenum is a LOT less than 1/2cc per ft lb of output power your likely operating closer to a dump shot and not taking sips as is more common.

FWIW
Scott

[Vmaxpro]

Ok, now that makes sense. Thank you! Sorry, Im not trying to reinvent the wheel or anything just learning as I go. I just leaned on the hammer spring and your correct, I can almost get the reg to drop to zero upon shooting.

[rsterne]

I am also not familiar with your gun, but just the valve volume in your gun is probably more than 2.5 cc.... That is an extremely small plenum, but assuming you are correct and it is only 2.5 cc, you have to add the valve volume to get the volume of 150 cc air available for the shot....

The 1 cc per FPE recommendation for plenum volume is to get reasonable efficiency without having to run more regulator pressure than you would have to have in an unregulated PCP at the same power.... You can drop that to about 1/2 cc per FPE if you increase the setpoint about 10%.... A smaller plenum than that will require even more regulator pressure....

Bob

[Nvreloader]

Guys

Question for you,

Does it hurt anything, if you have a plenum of larger cc, than the recommended minimum amount?,
ie; you need around 30 cc and have 80 cc, reg set point is 1500 psi, on a 13/3000 HPA tank,
14 gr CHP @ 975-950 fps.

Tia,
Don

[nervoustrigger]

From the performance and consistency points of view, there is no such thing as too much plenum.  The only negative is that it is volume that might otherwise be put to better use as the high pressure reservoir (for higher shot count).  Certainly so for an in-tube regulator.  For a regulated bottle, it just means bigger and heavier than necessary.

[Psipumper]

I don’t understand some basics.
Does a unregulated system have a  plenum chamber?
The regulator back fills the plenum during the shot cycle. The rate of back filling varies as in high flow or can the regulator react slowly to make the plenum more like a dump valve?

[Vmaxpro]

I don’t understand some basics.
Does a unregulated system have a  plenum chamber?
The regulator back fills the plenum during the shot cycle. The rate of back filling varies as in high flow or can the regulator react slowly to make the plenum more like a dump valve?

Mines seems to be “slow” , if I have my hammer set too heavy it will dump the plenum and I can hear it fill back up for a few seconds

[nervoustrigger]

No, an unregulated air rifles does not have a plenum.  I suppose you could think of the reservoir as a plenum but that muddies the waters.

A regulator will refill the plenum fairly slowly compared to the few milliseconds the valve is knocked open when you pull the trigger.  In other words it does not contribute to the pressure delivered to the pellet.  For all practical purposes, the only air available to accelerate the pellet is that which is initially in the plenum.

And if you think about it, it's actually undesirable for the regulator to flow fast enough to contribute because that would mean you have the variable pressure from the reservoir being injected behind the pellet.  Not a good thing considering the whole point of a regulator is to ensure a consistent pressure.

[Ribbonstone]

Haven't found it to make a whole lot of difference in bottle fed HPA's...the high pressure bottle volume is the same short tube or long tube (which is basically the plentium).

Cheap guns have some advantages for expeiments: can afford to buy two and make them as identical in most respects as possible.  For that matter, to doube check results, can "swap the tops" (the breech and barrel) or "swap the guts" (valve/striker)  and run the test again.

Nothing is perfectly euqal, the variations can't be 100% controled,   it's about as equal a playing field as possible.

So one short tube.22 QB HPA and one long tube .22 QB HPA..both carbine length barrels. Only valve mod (for both) was to remove the filter.  Lost track of exactly when I did this...feels like about 8 years ago.



Found the short tube to be easier to get air efficient,but leveled out on power quickly.  Most of the QB79 power mods include ways of increasing the plentium, and there is evidently good reason for that if power is a goal.

The long tube version was much harder to get to be efficient in air use at all levels of power, but had a much higher power possible.

From here, could have modded them UNEQUALLY and compared them...didn't.

Went with what they showed to be their strong points/natural tendencys.  Adjusted the .22 QB79 to low power (12-13 foot pounds per shot) but +200 shots per fill (850psi output tank).  I still use that one.

The long tube did some time with a higher pressure output tank (1150psi)and a  long barrled .25 upper. Actually worked quite well...until I got power-hungry and converted that into a 3K PCP tube.

So no math/charts...pretty much just going along by experiments.  Do think tanker HPA's (and likely all regulated PCP's)  shoot only from the air volume in the plentium. 

With a small plentium, they limit out in power rather suddenly (won't do a bit of good to smack the valve stem any harder) and "blance" (best energy for air use) SLOWER than many would like.  Choices for power hungry folks would be to increase the peltium (hollow out some stuff)or increase reg. pressure (likely both).

 With a large plentium,they keep going in power adjustments father,but tend to be air hogs.  If you don't mind an air hog (and with compressore filling easier/cheaper now),that might not be an issue.  Although I will remind you that "extra"air does make for "extra" noise.

[Motorhead]

I don’t understand some basics.
Does a unregulated system have a  plenum chamber?
The regulator back fills the plenum during the shot cycle. The rate of back filling varies as in high flow or can the regulator react slowly to make the plenum more like a dump valve?

An unregulated gun has the ENTIRETY of the air supply to draw from being nothing sits ahead of the valve but open space at high pressure. ( tho some guns such as AA use shot pots )

[Psipumper]

I don’t understand some basics.
Does a unregulated system have a  plenum chamber?
The regulator back fills the plenum during the shot cycle. The rate of back filling varies as in high flow or can the regulator react slowly to make the plenum more like a dump valve?

An unregulated gun has the ENTIRETY of the air supply to draw from being nothing sits ahead of the valve but open space at high pressure. ( tho some guns such as AA use shot pots )

The unregulated is pretty simple then. The guideline of 1cc per fpe plenum size is a normal in a regulated gun. The actual size would need to account for caliber and barrel length plus some? Since the tuning is done with hammer strike and the entire plenum isn’t used for the shot , size isn’t that critical but should  be larger rather than small? Does that sound close for a standard setup?

[Ribbonstone]

Sometimes it helps to simplfy to the abstract.  Good news is that air acts nicely proportional in the volume/pressure respect.

LEts takethat 2.5CC plentium volume (which I also think is a low estimate..at the very least the internal volume of the valve should be added to that).  BAsically 2.5cc's is about what you'd get if a standard Crosman valve was butt-welded onto the end of the regulator.

But will use 2.5cc's as the abstract example of the ralationship between volume and pressureachieved during a shot.

Regulator set at 2000psi.

IF you were to use  2cc's of that plentium air per shot, would start off at 2000psi and fall by 80%  to 400psi.  Average used would be 2cc's at 1200psi.

Crank up the spring tension(and lets ignore the dwell time/length of valve opeing for the time being) and maybe you get to use 90%of the air. That would be 2000psi to 200psi, for an average of 2.5ccs at 1100psi.

Lets leave everything else the same and just magically double the volume to 5cc's.

Using 2cc's  would drop 2000psi to 1200psi, for an average of 2cc's at 1600psi.

Using 2.5ccs  would drop 2000psi to 1000spi, for an average of 2.5cc' sat 1500psi.


Complicating the abstration as little as possible, lets say you have 200cc's of 3000psi air on the recieving end of the regulator.

So you have 200cc's of air  over  the 2000psi regulator set pressure that can be used.

And pretend everything worked uniformly perfectly with no waste.

So if you move 2cc's of air with each shot, should get 100 shots per fill.  If you move 2.5cc's of air each shot,should get  80 shots.

The question becomes (and the usefulness of Bob's "knee graph" on regulation) what is the velocity energy difference between (using the last 5cc plentium example) squirting out 2cc's/1600psi/100 shots and squirting out 2.5cc's/1500psi/80 shots?


The obvious answer is that to be EQUAL in efficency, the 2.5cc's per shot would have to earn 20% more power per shot than the 2cc air ejection(20 foot poundsX100 equal to 25 foot pounds X80).

Regulation seldom works out as easy as the abstract..realaity butts-in and have to consider valve dwell,air passage size, etc.

[Motorhead]

Many regulated guns in the smaller calibers ( .177  .20   .22 ) do just fine at 1/2 cc per ft lb power.  Having more just makes it a tad easier to make BIG power if leaning on the perspective calibers potential.
Down side being, as you increase plenum volume you LOSE high pressure storage volume.  No free lunch there 

[rsterne]

Scott is correct, unless there is a restriction between the reservoir and the valve, the entire reservoir in an unregulated PCP acts as the plenum.... The larger the reservoir is, and the less air used per shot, the less the pressure drops during the shot.... The same applies in a regulated PCP in the plenum.... The smaller the plenum is, for a given amount of air used, the greater the pressure drop during the shot.... Regulators, even "high flow" ones, cannot maintain the pressure in the plenum during the ~ 1 mSec the valve is open....

In pumpers which use a dump valve, we talk about the valve size as a percentage of the barrel volume.... and values between 20-50% of the barrel volume are typical.... In a PCP, however, we almost never use a dump shot, the valve is closing when the pellet is only 10-50% of the way from the valve to the muzzle (depending on tune).... Therefore, relating the plenum volume to barrel volume is not the best way, you need to relate it to the amount of air used per shot....

Most properly tuned PCPs have an efficiency of between 1.0-1.6 FPE/CI.... That means that the amount of air used per FPE is in a rather narrow range, so to keep the pressure drop during the shot to a minimum, we can related the plenum volume to the FPE.... I'll use a 50 FPE gun shooting at 1.19 FPE/CI as an example.... That means it uses (50/1.19) = 42 "standard CI" per shot (42 CI of air at 1 bar pressure)…. Let's choose an MRod, with a 215 cc reservoir, and 3 plenum sizes, 50 cc, 25cc, and 10 cc.... and start with a pressure of 2000 psi (138 bar) in each case.... That 42 CI of air, compressed to 138 bar, takes up (42 x 16.4 / 138) = 5 cc.... Now you see why I close that efficiency, not only was it "typical", but it will make the math easy to follow....

Unregulated - reservoir volume 215 cc @ 2000 psi.... Starting volume 215 cc, with 5 cc used for the shot.... Pressure after the shot 2000 x (210 / 215) = 1953 psi.... Average pressure = (2000 + 1953) / 2 = 1977 psi....

Regulated - 50 cc plenum @ 2000 psi.... Pressure after the shot 2000 x (45 / 50) = 1800 psi.... Average pressure = (2000 + 1800) / 2 = 1900 psi....

Regulated - 25 cc plenum @ 2000 psi.... Pressure after the shot 2000 x (20 / 25) = 1600 psi.... Average pressure = (2000 + 1600) / 2 = 1800 psi....

Regulated = 10 cc plenum @ 2000 psi.... Pressure after the shot 2000 x (5 / 10) = 1000 psi.... Average pressure = (2000 + 1000) / 2 = 1500 psi....

As you can see, with a plenum that is 1 cc per FPE, the average pressure during the shot is only about 4% less than unregulated.... Using a 1/2 cc per FPE plenum, it is 10% less.... and with a plenum that is only 1/5 cc per FPE, you have 25% less pressure available to produce the shot.... Since FPE is proportional to pressure (for a given gun setup), it is pretty easy to see why plenum volume is so important.... You can get away with 1/2 cc per FPE if you increase the setpoint pressure by about 10%.... Going smaller than that loses power and efficiency quickly....

Bob

[Vmaxpro]

Haha Im sorry guys its closer to 16cc. 30 yrs in machining and not much metric time for me!

[rumbleUU]

I understand the most you all are saying, but maybee someone here can explain what I asked before, regarding FX Crown, wich is regulated, and surely must apply any regulated PCP with same "normal" internals....... WHY does the power/speed drop when increasing hammertension after finding that sweetspot?

Example: On the FX Crown, you set hammertension pretty high(MAX on the wheel), full airflow on the powerwheel, reg pressure lets say 140 bar, plenumsize ?(but dont matter for now), and the way to go is to decrease the hammertension until the speed starts dropping, then stop, no more airwaste, and efficient for that pellet and reg pressure and the speed is max with best efficency, BUT from here, WHY does the speed drops if you increase more and more hammertension??? I dont get that, I can accept it doesent get any higher just wasting the extra air in the plenum and the speed stays the same, but the speed dropping? dont get it

[rsterne]

Likely the hammer is bouncing off the back of the valve as you increase the hammer spring preload.... I have seen that on some guns.... If it hits the back of the valve hard enough and bounces off, the dwell gets shorter instead of longer....

Bob

[rumbleUU]

Likely the hammer is bouncing off the back of the valve as you increase the hammer spring preload.... I have seen that on some guns.... If it hits the back of the valve hard enough and bounces off, the dwell gets shorter instead of longer....

Bob

Ok, but this also applies for unregulated?

I need to get more fact of this before believing it, especially on the fx crown, just seems so odd, more hammertension= lower velocity,,,,,,,, either way, there is no purpose of increasing hammertension further, but was just curious

[rumbleUU]

Been thinking, maybee to much

But how in earth could same hammerweight just traveling a bit faster=more energy bounce of quicker then same hammerweight traveling a bit slower, dont make sence???