How much of the plenum is used?

[MACTEN]

I really have no idea how much of a plenum like 60-110cc is used when firing. I know there are variables like dwell, hs, sub 12 or fac etc but ...I wonder if an airgun is tuned to standard velocities for a given pellet like 18.1 in .22 and going for 900 fps how much of a 110 cc plenum is emptied when fired? It can't be the whole thing on a modest tune and reg setting and having had to fire several shots to empty even smaller plenums made me wonder.

 

[Firewalker]

I really have no idea how much of a plenum like 60-110cc is used when firing. I know there are variables like dwell, hs, sub 12 or fac etc but ...I wonder if an airgun is tuned to standard velocities for a given pellet like 18.1 in .22 and going for 900 fps how much of a 110 cc plenum is emptied when fired? It can't be the whole thing on a modest tune and reg setting and having had to fire several shots to empty even smaller plenums made me wonder.

I have no idea of the math for the plenum, I'm the wrench turner guy but I find that the plenum only empties a small fraction of its volume with each shot.

 

[thomasair]

A somewhat reasonable tune might close the valve when the pellet is half way down the barrel. You can calculate the volume of air at the average pressure and deduct that from the plenum size.

Mike

 

[thomasair]

A 24" 22 cal barrel has around 15-16cc of volume.

 

[AlanMcD]

I've never bothered to figure it out, but it would be easy to do if you know the volume of the reservoir and plenum and used the data over multiple shots or a fill, and thus could work out the average usage per shot. We can work in "units" that define the amount of air used, pretty much normalized for pressure - I like "bar-cc" as that unit, and it is the volume of air multiplied by the pressure in bar. It is not perfect, as it treats the air as an ideal gas, but as long as we work under about 3500 psi it holds fairly true.

For example, I have shot string data handy on a double regulated bottle-fed Marauder I built that can show how easy the math is. It has a 480cc bottle on it, and a ~35cc plenum on a Huma regulator set at ~155 bar in it as the second one (the first is a Ninja in the drop block to smooth out the pressure to the Huma, but we can ignore that one as all shots are above the Huma set point). The math is straight forward . . .

On the 40 shot string, the bottle started at 3200 psi and dropped 950 PSI, or 65.5 bar. That give us 480cc x 65.5 bar = 31,440 bar-cc of air used over 40 shots, for an average of 786 bar-cc per shot.

As long as the pressure is above the regulator set point, the regulator will average holding 35cc x 155 bar = 5,425 bar-cc of air.

So on an average shot, the valve releases 786/5,425 or about 14.5% of the air it holds. That would put the low point of the pressure before the valve closes at ~133 bar. Of course this ignores the pressure drop across the valve and speaks only to the average value, but it gets you close.

Hope that helps. You can figure this out for any gun for which you know the volumes and data.

 

[bigHUN]

I still have the original=first Edgun Edmu digital gauge on my L2. I was using earlier years on my FX's and when the L2 surfaced I mounted there.
In the beginning I replaced the battery like 2-3 times playing with these features. And btw that first gauge was a b***h to replace the battery.
The microchip inside was/is calculating the amount of air per shot - starting from pressure, number of shots, ending to pressure. Longer you able to shot per fill the volume for averaging is larger and the calculation is more defined.
And I got sick of replacing that battery years ago, now waking up the screen only to check the tank level.
So pretty much there is a way to calculate or measure.

 

[CanMike]

Isn’t the purpose of a larger plenum to keep the pressure constant without pressure drop. Volume used should be a small fraction of actual plenum size.

 

[Peashooter]

Well even with my FAC pea shooters, and me slinging "little " weights with just 60 BAR, and rifles have what i would call significant size plenums, well every shot i do hear the regulator work, so clearly a large enough pressure drop in each shot.
A .177 barrel 700 mm long, it is after all some volume that need to be filled.

I have played with the idea of filling up the plenum with something, to see if that have a effect.
On my Vulcan 3, turning the plenum pipe around to super large big caliber plenum, it worked but i have to admit it also felt a bit funny so i soon changed it back to the " small " orientation.

BTW same Vulcan 3 came in a no plenum at all ( i assume Sub 12 ) or just the 3-4 CC you would find behind the regulator at the very back of the pipe, and it also worked just fine even at FAC power levels.
Got a FAC plenum pipe + REG spacer from AGT to restore my Vulcan 3 to what it should have been.
terrible notion of several brands dumping SUB12 rifles on the Danish FAC market, my FX Maverick came in the same way only shooting SUB 12 at full hammer spring and 90 BAR on the regulator, though regulator screw HEX not blocked like i understand they are in the UK.

 

[JimD]

It seems like it depends on how long the valve is open and how freely the air flows into the barrel. The valve should close before the pellet exits the barrel to avoid wasting air and making a lot of noise. But a larger plenum makes it possible to get a target velocity at a lower regulator setting which will leave the valve open longer.

I think the size of the plenum should reflect the fpe of the gun. A 20 fpe airgun does not need a 60cc plenum in my opinion and may be difficult to get a good tune for. But it is hard to get a good tune for a 50 fpe airgun with a 15cc plenum.

 

[weevil]

The most efficiently tuned airguns can make 2fpe for every cubic inch of air used. Most make 1-1.5fpe per cubic inch of air.

However, plenums are often measured in cc and of course they are pressurized, so it makes more sense to convert numbers to bar-cc.

The most efficient example (@2fpe/cu. In.) will use 8.2bar-cc of air per FPE.

A 50cc plenum with reg set at 100bar holds 5000bar-cc of air.

So, if your shot makes 50fpe, the very efficient AG will use 410bar-cc of air for that shot, which is less than 10% of the total air in the 100bar, 50cc plenum.

CanMike explains why this is the case…