about volume

about volume

Joined: December 14th, 2010, 5:08 am

June 19th, 2012, 3:14 pm #1

so adding volume in the valve helps keep the average pressure stay the same during the shot, how much of this volume actually needs to be directly in front of the valve exit path to the barrel? for example: the as2250 has its air tank on the back and attached to the valve by a small tube. therefore much of the volume is constricted from the being immediately available upon firing. or am i wrong? is there some kind of formula for determining the optimum valve volume to barrel volume at a given pressure?
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Joined: November 28th, 2002, 6:26 pm

June 19th, 2012, 5:19 pm #2

...freely available to the valve is of little value in sustaining the pressure that propels the pellet. But, in the absence of effective hammer debouncing, this is often doen deliberately, to reduce the amount of propellant that's squandered by hammer bounce, after the pellet has already departed the muzzle. When the hammer is allowed to bounce, there's a tradeoff between efficiency in pushing the pellet against inefficiency due to wasted gas.

Only when the hammer is adequately debounced is there a clear and unambiguous advantage to greater valve volume, where more is always beter.

Steve
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Joined: December 14th, 2010, 5:08 am

June 19th, 2012, 5:47 pm #3

so if i were to add an external reservoir i should still increase the volume in the valve

is there a formula for what i mentioned earlier?
Last edited by Dnadnnoid on June 20th, 2012, 5:30 pm, edited 1 time in total.
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Joined: November 28th, 2002, 6:26 pm

June 19th, 2012, 8:18 pm #4

...intended MV, power, caliber, propellant, pellet, etc.

But lacking that info, for just a rough rule of thumb, experts like James Perotti have suggested a minimum figure of half the barrel bore volume, which is approx...

Minimum_Valve_Volume = 0.39 x Caliber2 x Barrel_Length

Steve
Last edited by pneuguy on June 19th, 2012, 8:18 pm, edited 1 time in total.
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Joined: February 17th, 2010, 3:30 am

June 20th, 2012, 1:25 am #5

so adding volume in the valve helps keep the average pressure stay the same during the shot, how much of this volume actually needs to be directly in front of the valve exit path to the barrel? for example: the as2250 has its air tank on the back and attached to the valve by a small tube. therefore much of the volume is constricted from the being immediately available upon firing. or am i wrong? is there some kind of formula for determining the optimum valve volume to barrel volume at a given pressure?
but that's not strictly the valve volume, but the volume available for the valve to draw from easily.... eg. easier flow into the valve than out.... It's based on the concept that the larger the volume available, the higher the pressure will be at the valve seat DURING the shot cycle.... There are a large number of variables, but here is a graph showing why I use that as a goal....

[/IMG]

This graph is based on the average pressure for different sized resevoirs for the valve to draw from between a 2000 psi regulator and the valve seat, for a gun that is generating 1 FPE/CI of air used at 1 bar (ie an efficiency of 16.4 Barcc/FPE).... Cutting the volume in half (from 50 cc to 25) loses about 8% of the average pressure, whereas doubling it to 100 cc only gains about 3%.... Reducing the valve volume to 15 cc would lose about 16% of the average pressure, and if it was just 10 cc, the loss would be about 28%....

While the details of the curve will depend somewhat on the specifications of the gun, the trend should stay more or less intact.... It doesn't mean that you can't get by with a smaller valve.... but what it does mean is that as you reduce the valve (and reservoir) volume you will have to increase the starting pressure to maintain your FPE goal....

Bob
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Joined: December 14th, 2010, 5:08 am

June 20th, 2012, 4:15 am #6

thats amazing!
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Joined: November 28th, 2002, 6:26 pm

June 20th, 2012, 4:57 pm #7

...than typically found in anything but PCPs and bulk CO2.

For example, the volume of a 392 valve is only about 5cc.

Steve
Last edited by pneuguy on June 20th, 2012, 4:59 pm, edited 1 time in total.
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Joined: February 17th, 2010, 3:30 am

June 20th, 2012, 6:41 pm #8

but the volume available for the valve to draw from easily during the shot cycle.... and yes, it's MUCH larger than what you find in most guns, particularly regulated ones.... The solution, if you can't fit in that much volume, is to use a higher pressure to accomplish the power desired.... Here is an example of how the volume affects the performance in a hotrodded HPA version of the QB78/79.... I used all the same parts for both tests, swapping out ONLY the tube to change the volume between the regulator (1500 psi) and the valve.... 18.1 gr. JSB Exact Heavy pellets were used.... There were 3 washers behind the hammer spring for increased preload....

Tube.........QB78....QB79
Volume.......53cc....8cc

Low...........872......805
High...........885......817
Avg fps.......880......811
Avg FPE......31.1.....26.4
No.Shots.....40.......39
Total FPE....1245.....1031
PSI Used.....1550.....1550
Vol Used.....1390.....1390
FPE/CI........0.90.....0.74
Barcc/FPE....18.......22

As you can see, the extra volume of the QB78 tube allows it to produce 18% more power at over 20% more efficiency.... The volume quoted is my best estimate, as the valve was slotted to open it up on the inlet side, and it includes the valve plus tube volumes.... Using my spreadsheet to calculate the average pressure during the shot, the QB78 works out to about 1420 psi, while the QB79 has to deliver its shot from an average of only about 1000 psi.... Those are based on the volume of air used per shot, and assuming that the regulator response time is too slow to provide any top up.... I have lots of other examples, but this one was done with no variables except the tube volume....

This is the reason all of my recent HPA conversions use a reversed tank mounting block to provide a large plenum of regulated air for the valve to draw from.... In the above test, I didn't play with hammer strike to optimize the efficiency of either gun.... I later optimized the QB78 for 60 shots from a 13CI tank at 32 FPE on 1500 psi, for an efficiency of 1.2 FPE/CI (13.7 Barcc/FPE).... Here is a photo of the completed rifle....

[/IMG]

It is certainly possible to develop more FPE from less volume.... at the expense of increasing the pressure required to do that....

Bob
Last edited by rsterne on June 20th, 2012, 7:36 pm, edited 1 time in total.
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Joined: November 28th, 2002, 6:26 pm

June 20th, 2012, 9:48 pm #9

...with my remark because I'm sure you noticed that Trevor's original question was about "optimum valve volume."

Hopefully I prevented (a little) confusion instead of creating (more of) it. Anyway, that was my intent.



Steve
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Joined: February 17th, 2010, 3:30 am

June 20th, 2012, 10:13 pm #10

because unless the valve is severely restricted/closed on the inlet.... as in a pumper (with a check valve), or using Carbon Dioxide (CO2) with a piercing pin in the way.... then the tube has some contribution to the volume.... In some cases, the tube IS the volume.... like in a Pre-Charged Pneumatic (PCP) where the valve has a wide open inlet side.... Then there are in between cases like the Disco where just in front of the valve is a gauge mounting block with a 1/8" hole in it restricting the flow into the valve....

I think Mr. Perotti's "half the bore volume" certainly has some merit.... It's larger than most of the valves out there, from what I've seen.... After all, if you want more power, in most cases, you can just increase the pressure.... The QB79 I tested would be pretty close to that volume....

Bob
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