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  1. #1
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    How much air?

    Might be a silly question but I was wondering about how much air a PCP uses v a springer.

    For instance if a PCP & springer both produced 11.5 ft/lb & the springer has a known cylinder length & diameter the volume of air behind the pellet can be calculated. With a PCP a 'dose' of compressed air is released from the reservoir but what volume would this 'dose' of air occupy at the same temperature & pressure as the air in the springer?

    Does one type use more air than the other to achieve the same result? Any fluid mechanics, physisists, or theorists got any ideas?

  2. #2
    bossmugler is offline More breaking even than breaking bad
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    Interesting post. I look forward to the BBS collectives answers.

  3. #3
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    Yes, the PCP will use more air than the springer, depending on various factors probably twice as much. One reason why they are usually louder.
    www.shebbearshooters.co.uk. Ask for Rich and try the coffee

  4. #4
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    If you put a balloon over the barrel of your PCP, collect one shot's worth of air in it, then measure the volume by displacing water in a measuring jug you won't be too far out. From memory it is usually somewhere between 150 and 300 cc, depending on the gun, the power etc.

    Alan

  5. #5
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    That's a little high as an estimate, but you said it's from memory.

    The last rifle I tested using the balloon method was running a bit warm power-wise and was using 140cc per shot.
    www.shebbearshooters.co.uk. Ask for Rich and try the coffee

  6. #6
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    It's not hard to work out.

    I fill my FAC rapid to 210 bar, 400cc bottle, so 210 x 0.4 = 84ltrs of air
    I get 55(ish) full power shots before they drop off at 120 bar, 120 x 0.4 = 48ltr
    84 - 48 = 36ltrs used
    55/36 = .654
    so 654cc's per shot .25 at 42ftlb
    or there abouts.

    .177 is least economic with air, .25 most economic.

  7. #7
    Fluffybuck is offline Member of the .25 cal fan club
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    Quote Originally Posted by trajectory View Post
    Might be a silly question but I was wondering about how much air a PCP uses v a springer.

    For instance if a PCP & springer both produced 11.5 ft/lb & the springer has a known cylinder length & diameter the volume of air behind the pellet can be calculated. With a PCP a 'dose' of compressed air is released from the reservoir but what volume would this 'dose' of air occupy at the same temperature & pressure as the air in the springer?

    Does one type use more air than the other to achieve the same result? Any fluid mechanics, physisists, or theorists got any ideas?
    The air reservoir volume of my HW100K is about 105cc. At 200bar pressure that's 21 litres of air crammed in there.
    From 200bar down to 90bar it gives about 100 shots.
    Therefore it uses about 100cc of air per shot.

    Springers or rammers probably use between one-third and half that, as measured by swept volume (stroke length x piston area).
    .

  8. #8
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    Quote Originally Posted by trajectory View Post
    Might be a silly question but I was wondering about how much air a PCP uses v a springer.

    For instance if a PCP & springer both produced 11.5 ft/lb & the springer has a known cylinder length & diameter the volume of air behind the pellet can be calculated. With a PCP a 'dose' of compressed air is released from the reservoir but what volume would this 'dose' of air occupy at the same temperature & pressure as the air in the springer?

    Does one type use more air than the other to achieve the same result? Any fluid mechanics, physisists, or theorists got any ideas?
    yeah there's a big difference - plenty of posts on this forum about the bore and stroke of tx200s and hw97's, something like 26mm bore x 80mm stroke, about 40cc of air.
    A 12 ft PCP would need something like 3 times that much air, maybe a bit more for a .177.

    Note that doesn't mean springers are necessarily 3 times more efficient than PCPs though!

    BB

  9. #9
    Fluffybuck is offline Member of the .25 cal fan club
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    Quote Originally Posted by brassbanjo View Post
    ....Note that doesn't mean springers are necessarily 3 times more efficient than PCPs though!....BB
    But it does mean that for a spring gun the barrel length required to extract all the energy from the air pulse is much shorter.
    A 12ftlb springer only requires about 10" barrel for .22 and 13" for .177. Rammers slightly shorter than that. PCPs about double that.

    Many airguns have a barrel that isn't the optimum length for efficiency because other factors such as excessive overall length, excessive forend weight, cocking leverage or even cosmetics also influence the design.
    .

  10. #10
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    Pcps are more efficient for me as I don’t have to use any muscle power. The tuxing compressor does it for me.
    Air is readily available if you have your own compressor just like with a springer.

  11. #11
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    Quote Originally Posted by brassbanjo View Post
    yeah there's a big difference - plenty of posts on this forum about the bore and stroke of tx200s and hw97's, something like 26mm bore x 80mm stroke, about 40cc of air.
    A 12 ft PCP would need something like 3 times that much air, maybe a bit more for a .177.

    Note that doesn't mean springers are necessarily 3 times more efficient than PCPs though!

    BB
    65.3cc's by my maths ,

    As I said above, in a PCP the energy that you have put in to the springers spring when you compress it cocking the rifle for each shot, does not exist so that stored effort/energy has to come from extra compressed air.

    For a given pellet in a given bore & barrel length, the energy (from the compressed air) required to reach a specified muzzle velocity will be exactly the same ( vol A), so the shot is equally efficient.

    BUT to replace the energy from the spring you need another 150%ish of vol A (vol B).
    All vol B does is push vol A in to the breach it then exits the muzzle as wasted air once vol A has shot the pellet out.

    In a gas ram vol B is contained inside the ram, but You again have to impart the energy in to the gas by physically cocking the rifle to compress the gas for each shot.

  12. #12
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    Quote Originally Posted by angrybear View Post
    65.3cc's by my maths ,

    As I said above, in a PCP the energy that you have put in to the springers spring when you compress it cocking the rifle for each shot, does not exist so that stored effort/energy has to come from extra compressed air.

    For a given pellet in a given bore & barrel length, the energy (from the compressed air) required to reach a specified muzzle velocity will be exactly the same ( vol A), so the shot is equally efficient.

    BUT to replace the energy from the spring you need another 150%ish of vol A (vol B).
    All vol B does is push vol A in to the breach it then exits the muzzle as wasted air once vol A has shot the pellet out.

    In a gas ram vol B is contained inside the ram, but You again have to impart the energy in to the gas by physically cocking the rifle to compress the gas for each shot.
    Check your maths

    Volume of a cylinder = π x R² x L

    3.1416 x 1.3² x 8 = 42.27 cc

    That's mine.

    I think there's an assumption that air is like a liquid and that the pellet is pushed all the way down the barrel by a consistent volume. That's not the case and it's a highly complex system to do with pressure waves and flow, which aren't the same things.

    I agree the comparison with springers and other plants is flawed because of the stored energy of the power plant (Spring/Ram)

  13. #13
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    Quote Originally Posted by RobF View Post
    Check your maths

    Volume of a cylinder = π x R² x L

    3.1416 x 1.3² x 8 = 42.27 cc

    That's mine.

    I think there's an assumption that air is like a liquid and that the pellet is pushed all the way down the barrel by a consistent volume. That's not the case and it's a highly complex system to do with pressure waves and flow, which aren't the same things.

    I agree the comparison with springers and other plants is flawed because of the stored energy of the power plant (Spring/Ram)
    26mm x 80mm or as we want it in cc's 2.6 x 8 so - pi x D x L

    3.142 x 2.6 = 8.1692 x 8 = 65.3536cc's swept volume
    Last edited by angrybear; 26-03-2019 at 07:35 PM.

  14. #14
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    Quote Originally Posted by angrybear View Post
    26mm x 80mm or as we want it in cc's 2.6 x 8 so - pi x D x L

    3.142 x 2.6 = 8.1692 x 8 = 65.3536cc's swept volume
    3.142 x diameter = circumference. Circumference x length = surface area of the cylinder in square cms.

    Try 3.142 x radius x radius x length for volume in cubic cms.

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