How do I calculate how much air is in a plenum?

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MadCowCrazy2

MadCowCrazy2

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Sep 11, 2021
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Jomala, Åland, Finland
I'm trying to figure out how to calculate how much volume of air is inside a plenum.
Example Fx Panthera 700 has a "Magnum XL Plenum (156cc)". I believe the air shotgun looking rounds for the AEA Defender holds 3.7cc of air.

So how do I calculate how much air is inside? I need to know the psi/bar right? I don't know the pressure inside the Panthera but I do know the air pressure in the AEA Defender should be 300bar or around 4350psi.

I know 1 bar = standard atmospheric pressure. So if you were standing inside a large tank at 1 bar you would notice no difference from standing outside.

So do you take the 300bar x 3.7cc = 1110, is this air volume in cc? So would this be 1110cm³ of air? CC is cubic centimeter afaik.
So this little thing can basically hold 1 square yard/meter of air but the size means it's compressed to 300bar?
Screenshot 2023-04-03 004457.jpg



What air pressure do airguns normally shoot at? From what I know it's usually around 100-125 bar (1450-1812 psi). I'm wondering how much air normally expelled. What about break barrels, is there a way to calculate how much air is expelled? Could you tie a balloon to the end of the gun, fire it and hold it shut as fast as possible to see how much air is expelled?
 
MadCowCrazy2 said:
So do you take the 300bar x 3.7cc = 1110, is this air volume in cc? So would this be 1110cm³ of air? CC is cubic centimeter afaik.
So this little thing can basically hold 1 square yard/meter of air but the size means it's compressed to 300bar?
View attachment 345259
Click to expand...
One cubic meter is 100x100x100 = 1,000,000 (one million) cubic centemeters.
 
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rcs9250 said:
Here’s a link.
In the INITIAL VOLUMES and FINAL VOLUME boxes, touch the blue letters to find the description like cc’s psi etc.

www.omnicalculator.com

Boyle's Law Calculator

Use this Boyle's law calculator to estimate a gas's pressure and volume in an isothermal process.
www.omnicalculator.com www.omnicalculator.com
Click to expand...
I'm trying to figure out who to use the tool, you need to fill in 3 values to get the 4th one. Initial pressure at 1 bar for base atmosphere. Final pressure for pressure inside the plenum and final volume for size of the plenum. This will then change Initial volume to show have much volume at atmospheric (1) pressure is expelled with each shot. Does this seem correct? So a 3.7cc air container would hold 1110cm³ of air at 300bar?
So the calculation is 300x3.7=1110cc which is 1.11L³ of air or 0.264 US gallons³. So when the AEA Defender is shot it expels 1.11cm³ of air at 300bar to propel the projectile.

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Only problem is I need to know 2 values to be able to get the 3rd one (Initial pressure is always 1 bar).
I have no idea what pressure is inside the plenum of the FX Panthera but if it was 125bar then the volume of air for 1 shot would be 19.5L³ or ≈5gallon³
Seems like quite a lot, anyone know the pressure inside the Panthera plenum?
2.jpg
 
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AlanMcD said:
But the Panthera is not expelling all the air in the plenum with each shot - you need to know how low the pressure drops before the valve closes, and that would give you a good estimate of the air usage. It is probably only dropping 10 to 20 bar, depending on the tune / energy in the shot . . . .
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Yeah, I realized this. So it's basically impossible to know how much air is used because the plenum will fill up as soon as you fire.
Does the plenum empty if you empty the gun? I guess one way would be to remove the airbottle and screw in a plug and then fill the rifle. This should fill the plenum and you could then fire and see how much air is used.

Or tie a balloon to the end of the barrel and shoot. Problem is balloon would probably explode from the quick pressure change. Maybe tie a large garbage bag to it.
 
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sb327 said:
The air used per shot can be determined by the pressure drop of the MAIN reservoir/tank. Of course you need to know the volume of it plus any other volume before the reg. The volume after the reg matters not, so long as the reservoir pressure is above reg set pressure.

Dave
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Is there a way to calculate how much air a break barrel uses?
For PCP I guess you would need to fire like 10 shots and count how many shots it takes for the tank to go from say 250bar down 240bar or whatever the next mark may be. A single shot might not even be noticeable.
 
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MadCowCrazy2 said:
Yeah, I realized this. So it's basically impossible to know how much air is used because the plenum will fill up as soon as you fire.
Does the plenum empty if you empty the gun? I guess one way would be to remove the airbottle and screw in a plug and then fill the rifle. This should fill the plenum and you could then fire and see how much air is used.

Or tie a balloon to the end of the barrel and shoot. Problem is balloon would probably explode from the quick pressure change. Maybe tie a large garbage bag to it.
Click to expand...
It sounds like you want to know the physical volume of the plenum more than the amount of air that is used. That is best determined with the gun apart, when you can measure the diameter and length of things and do the calculations. Alternately, you can actually measure the volume with water and a graduated cylinder - I did that with all the plenums on my regulated guns. Also, you can get an approximate estimate on the pressure drop while firing by watching the gauge; the actual usage will likely be a bit more, but it will get you in the ballpark.

But the best way to figure out the air usage per shot is to know the volume of your reservoir, and use the pressure differential over a known number of shots off it, rather than trying to mess around with the regulated space. The air usage is easier to calculate that way as the air pressure is read after the shots.

As for the spring gun, as already said the air volume used is the swept volume of the piston - but you also have to take into account that the air is heated massively by compression, which increases its pressure, and also will ignite some of the lubes via dieseling, and that adds to the pressure atthe time of the shot. Once it all cools back to to normal, you are back to the swept volume of air, but I would not be surprised to see that the air charge "acts" like it is four or five times bigger than that on the pellet due to the other factors.
 
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AlanMcD said:
It sounds like you want to know the physical volume of the plenum more than the amount of air that is used.
Click to expand...
What I'm trying to figure out is how much air is expelled from the airgun to shoot the pellet/slug. I thought the plenum was a good way to calculate but as pointed out above that doesn't matter as what does is the amount of time the air valve is pushed open when you pull the trigger.

Reason I want to know is so I'd know how big a plenum I'd need for a custom air rifle that expels ALL the air that is inside the plenum.
 
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Well that's a bit different . . . you'd be using what is known as a dump valve, much like most multi pump airguns do. Our PCPs do not use these valves - they have a limited dwell to conserve air for future shots.

Setting aside the difficulty of having a PCP that fires through such a valve, and having that valve close once the pressure in the plenum reaches close to atmospheric without dumping extra air, why would you be looking to do that? Most of the air that you expel would be "wasted air" in terms of efficiency of the shot - more than the roughly last half of the air released would have no impact on the pellet being shot, and would just vent out the barrel making noise . . .

If I wanted to do this, I'd probably take a multipump gun and fit it with a regulated air reservoir, and simply install a valve that allows me to refill the shot reservoir after the gun has been shot and the valve reset. But it would waste so much compressed air, and add a step to shooting over it over a conventionally regulated PCP, so there would be no good reason that I can think of to do this.

If this is a thought exercise, I would base it more off a multipump than a regulated PCP . . . .
 
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If dumping the entire plenum, it’s handy to know volume of plenum and volume of barrel.

Then you can find out pressure at bullet exit (-losses which are numerous, accounted for in Bob’s equation below). PV=pv

(Starting pressure + exit pressure)/2 is average pressure.

Bob Sterne’s formula
FPE Goal (air) = Bore area (sq.in) x Pressure (psi) x Barrel length (in.) / 24 .... where the constant you divide by, 24, changes inches to feet, and includes the 50% factor....

Bob reminds us this is a goal that is difficult to achieve.

Dave
 
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sb327 said:
FPE Goal (air) = Bore area (sq.in) x Pressure (psi) x Barrel length (in.) / 24 .... where the constant you divide by, 24, changes inches to feet, and includes the 50% factor....
Click to expand...
Is Bore area the caliber? So a 6.35mm at 300bar in a 1000mm barrel would look something like= 0.25*4350*39/24=1767
Does the above math mean the FPE Goal would be 1767 fps?
 
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