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Thread: How much air?

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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?

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

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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.
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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

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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.
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  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.

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    Quote Originally Posted by angrybear View Post
    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.
    Is your gun regulated or not? I'm guessing it is.
    I've read a rule before of about 1cc to 0.5cc of air consumption per ft/lbs in a regulated gun so that must be wrong?
    Edit:that is plenum volume not air.
    Last edited by arnie2b; 24-03-2019 at 10:06 AM.

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    When we did the calculations using digital gauges on the Steyr .177 running at about 11.5 ft-lb it was using about 200cc at atmospheric pressure. (From memory)

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    Quote Originally Posted by arnie2b View Post
    Is your gun regulated or not? I'm guessing it is.
    I've read a rule before of about 1cc to 0.5cc of air consumption per ft/lbs in a regulated gun so that must be wrong?
    No it's fast flow .

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    Reading the replies & comments with interest. I like the idea of catching the air in a baloon to measure the resultant volume. Might try a modification to that with a plastic bag as being less constrictive it wont exert much pressure on the air inside thus reducing its volume a fraction.

    Rich mentioned that a PCP might use a bit more air than a springer.....is this because the pressure increase with a springer is more efficient at getting the pellet moving? I suppose the pressure build up with a springer is much more gradual than with a PCP. Then if a PCP is a bit louder is this 'wasted air' expanding / venting after the pellets left the barrel, if so why isn't the dose of air reduced a bit, or is this what a regulated PCP is?

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    Quote Originally Posted by trajectory View Post
    Reading the replies & comments with interest. I like the idea of catching the air in a baloon to measure the resultant volume. Might try a modification to that with a plastic bag as being less constrictive it wont exert much pressure on the air inside thus reducing its volume a fraction.

    Rich mentioned that a PCP might use a bit more air than a springer.....is this because the pressure increase with a springer is more efficient at getting the pellet moving? I suppose the pressure build up with a springer is much more gradual than with a PCP. Then if a PCP is a bit louder is this 'wasted air' expanding / venting after the pellets left the barrel, if so why isn't the dose of air reduced a bit, or is this what a regulated PCP is?
    A pcp will use more volume because the air is the only energy source, a break barrel has a chuffing great spring

    Remember school physics ? - the amount of energy that exists in the universe is finite all we can do is change the form of that energy,
    So you transfer energy (which you got from food) from your body to compress the spring, which stores that energy in the spring tension, when released the spring transfers that stored energy in to the air as both pressure, heat & recoil, the air in turn transfers the energy in to propelling the pellet which in turn transfers it to the target.

    The pcp only has the energy stored within the air pressure, so no effort from you & no recoil but the pay off is more air is used per shot.

    A regulator (very basically) is a valve between the main store of air in the bottle & the single shot store in the firing chamber,it uses spring & air pressure to only allow a pre-set pressure in to the firing chamber rather than the actual pressure in the bottle so the pellet always has the same pressure pushing it across the whole bottle charge.

    Air is "wasted" because the pellet has to be accelerated up to the required velocity within a very short time & barrel length and the transfer system is not all that efficient due to the need for the pellet to be free to move & exit the barrel.
    Last edited by angrybear; 24-03-2019 at 11:25 AM.

  12. #12
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    With a PCP you start with air at high pressure and that pressure falls from the moment the firing valve opens.

    A springer starts with air at NTP and initially compresses this air when the piston is propelled forward. Now, the work done by the spring heats the air, and anyone who has pumped up a bicycle tyre knows that the pump gets warm, and the harder you pump the warmer it gets.

    The compression is done so quickly in a springer that this heat is not taken up by the metalwork, and it results in the air temperature rising. This increase in temperature leads to an increase in pressure, substantially more than the simple compression from swept volume to head volume would predict. Thus the springer effectively gets more work per cc of air than a PCP does.

    It's called adiabatic compression.
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    I bought a 10m target rifle and modified the reg to use it for FT. The original reg had a very small air space within it. I made new parts for the reg to increase the volume to about one cubic centimetre and easily upped the power enough to use it for FT. Not sure of reg pressure but I think most are normally in the 50-60 bar range?
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    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).
    .

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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!

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