Why does a bigger plenum allow the same results with lower reg pressure?

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Pale_Rider

Pale_Rider

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Feb 16, 2019
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This seems to be one of those esoteric things that is only understood by Veteran AG tuners or mechanical engineers. It’s the big craze now, slap on a larger plenum and boom! your gun breathes more efficiently. It’s like magic to me with my novice understanding of air pressure. So I did a quick search and came across a post by one of my favorite airgunners, Bob Sterne AKA R.Sterne(GTA). It’s one of those threads that you’ll need to read 2 or 3 times to fully digest it. But once you do, you’ll walk away with a deeper understanding of how air pressure works. Sure wish Bob was posting over here. Guy has more knowledge than the Vatican’s library. Enjoy the read: https://www.gatewaytoairguns.org/GTA/index.php?topic=97924.0
 
Basic physics, you can up your psi or up your volume of air at the same psi.
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Thanks for the insight but I wasn’t wondering HOW to increase velocity by upping pressure. I wanted to know the WHY. An in depth explanation as to what’s at play when using a larger plenum with lower reg but achieving the same results as higher reg pressure, smaller plenum 
 
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Far from an expert but I would view it this way. It takes a certain volume of air to move the pellet at a given velocity. A larger plenum may give that certain volume at a relatively lower pressure compared to the smaller plenum even if the smaller is at a relatively higher pressure. One could raise pressure on the smaller plenum to gain the needed volume but limits (pressure and I assume mechanical) would be reached at some point. I would guess that if one could utilize unlimited pressure then it would make no difference but that isn't the case physically. Seems all would be dependent on the RELATIVE plenum size and pressure of each. Just on opinion and possibly all wrong.
 
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Far from an expert but I would view it this way. It takes a certain volume of air to move the pellet at a given velocity. A larger plenum may give that certain volume at a relatively lower pressure compared to the smaller plenum even if the smaller is at a relatively higher pressure. One could raise pressure on the smaller plenum to gain the needed volume but limits (pressure and I assume mechanical) would be reached at some point. I would guess that if one could utilize unlimited pressure then it would make no difference but that isn't the case physically. Seems all would be dependent on the RELATIVE plenum size and pressure of each. Just on opinion and possibly all wrong.
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You’re closer to an expert than I am! Haha. Thanks for your insight on how it works. 
 
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OK;

Velocity is a function of average pressure over time. Imagine your plenum is the same size as your barrel. If your pressure in your plenum is 2 BAR then it contains twice as much air as your barrel. If your pressure is 4 BAR then it contains four times as much air as your barrel. Now imagine your plenum is twice the volume of your barrel. If your pressure is 1 BAR it contains twice as much air as your barrel. If your pressure is 2 BAR then it contains 4 times as much air as your barrel. Simple enough.

Pressure is stored energy. More pressure, more energy. When the pressure leaves a vessel the vessel cools off. This is energy leaving the vessel as heat. One liter of air stored at 2 BAR contains the same amount of energy as two liters of air stored at one BAR. This is true in a vacuum. It is also true when there is no differential in pressure because the energy being considered is actually heat.

To keep things simple, imagine the following is taking place in a vacuum. Imagine a barrel that is plugged at the far end with a pellet. Suppose you put the air into that barrel from a plenum that was 1/4 times as big as the barrel and was pressurized to 4 BAR. The pressure at the moment the pellet leaves the barrel is 1 BAR. This is also the average pressure in the barrel over the time the pellet traveled down it. Now imagine the same situation but the plenum is 1/2 the volume of the barrel and the pressure is only 2 BAR. At the moment the pellet leaves the barrel the pressure is still 1 BAR more importantly the average pressure in the barrel over the time the pellet traveled down it is also 1 BAR. Larger container, lower pressure, same velocity.

You can scale this to a plenum which is any size with any initial pressure. Imagine a plenum that is the same size as the barrel and is pressurized to 2 BAR. At the time the pellet exits the barrel the pressure is reduced to 1 BAR because the volume of the whole system (plenum plus barrel) has doubled. Suppose you increase that plenum to twice the size of the barrel and keep the pressure the same. At the time the pellet exits the pressure is 4 BAR/3 (33% higher). The average pressure over the time in the barrel is 33% greater. We could reduce the pressure and wind up with a velocity equal to the first example in this paragraph. With the same plenum (twice as big as the barrel) we can reduce the pressure to 3 BAR and end up with 1 BAR of average pressure at the time the pellet exits the barrel (3/3=1).

So what matters is average pressure over time and that number is always the same as the muzzle pressure.

I'm sorry I tried to make this easy to digest but it is hard to talk about, without doing calculus.


 
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Here's Gile's explanation, pretty easy to understand as he describes it.

jk



https://www.youtube.com/watch?v=6HKXV1E3SIg

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A few minutes into the video and I’m thinking “ I already know all this stuff” then bam! 3:43 into the video and he gives an easy to understand explanation of WHY a larger plenum can give same results on with lower pressure. The way I understand it after watching that video, is the larger plenum allows the gun to take a longer sip of regulated air pressure required to send the projectile down the barrel. So the reg isn’t called into action to regulate more are from the reservoir while the valve is still open, resulting in a steady sip of regulated air pressure. 
 
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OK;

Velocity is a function of average pressure over time. Imagine your plenum is the same size as your barrel. If your pressure in your plenum is 2 BAR then it contains twice as much air as your barrel. If your pressure is 4 BAR then it contains four times as much air as your barrel. Now imagine your plenum is twice the volume of your barrel. If your pressure is 1 BAR it contains twice as much air as your barrel. If your pressure is 2 BAR then it contains 4 times as much air as your barrel. Simple enough.

Pressure is stored energy. More pressure, more energy. When the pressure leaves a vessel the vessel cools off. This is energy leaving the vessel as heat. One liter of air stored at 2 BAR contains the same amount of energy as two liters of air stored at one BAR. This is true in a vacuum. It is also true when there is no differential in pressure because the energy being considered is actually heat.

To keep things simple, imagine the following is taking place in a vacuum. Imagine a barrel that is plugged at the far end with a pellet. Suppose you put the air into that barrel from a plenum that was 1/4 times as big as the barrel and was pressurized to 4 BAR. The pressure at the moment the pellet leaves the barrel is 1 BAR. This is also the average pressure in the barrel over the time the pellet traveled down it. Now imagine the same situation but the plenum is 1/2 the volume of the barrel and the pressure is only 2 BAR. At the moment the pellet leaves the barrel the pressure is still 1 BAR more importantly the average pressure in the barrel over the time the pellet traveled down it is also 1 BAR. Larger container, lower pressure, same velocity.

You can scale this to a plenum which is any size with any initial pressure. Imagine a plenum that is the same size as the barrel and is pressurized to 2 BAR. At the time the pellet exits the barrel the pressure is reduced to 1 BAR because the volume of the whole system (plenum plus barrel) has doubled. Suppose you increase that plenum to twice the size of the barrel and keep the pressure the same. At the time the pellet exits the pressure is 4 BAR/3 (33% higher). The average pressure over the time in the barrel is 33% greater. We could reduce the pressure and wind up with a velocity equal to the first example in this paragraph. With the same plenum (twice as big as the barrel) we can reduce the pressure to 3 BAR and end up with 1 BAR of average pressure at the time the pellet exits the barrel (3/3=1).

So what matters is average pressure over time and that number is always the same as the muzzle pressure.

I'm sorry I tried to make this easy to digest but it is hard to talk about, without doing calculus.

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Oldspook coming thru with the thorough knowledge! Thanks! Got me wanting brush up on calculus now haha
 
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During the brief time the valve is open, a pressure drop occurs in the plenum. With a larger volume of air at the ready, there is less of a pressure drop. Therefore the average pressure available to accelerate the projectile is higher.
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Short and to the point. This pretty much sums up that thread. Thanks for the explanation
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Yes, my vote for nervoustrig, the other answers given are wrongish or incomplete.
 
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Thanks guys, good reading, I shall however need to read oldspooks a few more times to nail it down completely.
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You should read some of rsterns stuff. If you read it and can’t grasp it, work on airguns for a solid year then come back and read it again. Then you will understand half of it. Then spend another year in the hobby and read it again. If you’re lucky you will finally get it. Rstern is a wealth of knowledge but wow, I was tempted to record some of his stuff to audio and play it while I was sleeping to grasp it. I’m good now but stuff with these guns can get pretty deep.
 
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How can an old unregulated 1500psi max fill Mac1 USFT do what it does?
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Perfect valve dwell is my guess(perfect: hammer weight, Hammer spring and valve spring strength/length, port size, valve stem length)
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The GTA forum has loads of technical info lead by Rstern (Bob).

The small transfer port is the key for a very flat shot curve in an unregulated PCP. This is of course most useful for target airguns where high power is not a requirement.
 
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How can an old unregulated 1500psi max fill Mac1 USFT do what it does?
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I own one, my primary field target gun. VERY VERY VERY accurate. Fill to 1450-1500 and shoot it down to 1250psi. Around 45 good, consistent shots. 

I have a .22 barrel and got it up to 28fpe by opening the transfer port and adding weight to the hammer. That is the highest fpe I've ever gotten out of it (quickly went back to 19.5fpe .177 because FT is what this gun is supposed to do). 

I think of the long tubed (unregulated) USFT sort of as the original power plenum, that whole tube is the plenum, ie the pressurized air directly available for each shot. So, the gun can get a nice consistent string in the 19.5fpe area by relying on lots of low pressure air, vs a smaller amount of high pressure air. 

At some point in my digging I kinda remember coming across someone estimating the tube volume of the full-length version like mine to be roughly 800ccs. That's a lot of air at the same pressure. So, no regulator, per se, but lots of immediately available air for the shot, and all that air is in one "compartment" of the same pressure (vs a regulated gun that has two "compartments" of different pressure air: a certain, and relatively, small volume of the "correct" pressure of air between the reg and the back of the pellet, and another larger compartment of non-regulated air upstream of the regulator.
 
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