Reducing air volume

Discussion in 'Steyr' started by RichWafta, Nov 16, 2018.

  1. Adam

    Adam Active Member

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    Charles's Law rather than Boyle's Law
     
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  2. RichWafta

    RichWafta Second time around

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    Thanks for giving the explanation in laymans terms Rob, I now understand what you have been trying to get over to me.
    This may well be the negative side to reducing the volume and if that is the case, I will try and find something else to occupy me over the winter ;)
    I'm thinking back to some of the older guns we used to tune where one of the tweaks was to put a tapered grub screw into the side of the port. This would allow fine tuning of the power by restricting the port. (Don't Air Arms still do this?) Now with the Steyr, we find that it works best when the port is opened fully and we now rely on the firing valve closing quickly enough to restrict the power, which obviously works, but does use a lot of air.
    The question is, why the change from restricting the port, which Steyr and other manufacturers do as standard, that allows just the right amount of air to pass through to the barrel for the required power, to unrestricted which has proven to give better efficiency?

    Which brings me in a circle back to attempting to restrict the amount of air. If the effect of reducing the volume makes it more temperature sensitive, will the regulator control this change? Its set for 86Bar now, so will it still give 86 when the temperature goes up? Hmmmm!!
     
  3. hmangphilly

    hmangphilly Floppy Quick Phil

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    Yes
    It will still give.86


    To start with
     
  4. SteveC200

    SteveC200 Active Member

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    Given that with a temperature increase there will be a corresponding pressure increase in the plenum, would that only be for one shot if the gun has been left for a while? The next shot will be back to 86 bar won't it? If you're taking a shot every 30 seconds or so, would there be much temp shift between shots?

    Is there any downside to a large plenum, apart from packaging?
     
  5. hmangphilly

    hmangphilly Floppy Quick Phil

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    Yes it will go back to what the regulator is set out until the air inside had a chance to warm up and expand .

    There is a downside to a reservoir that is too big in that it may not allow the regulator to cycle properly because there will be an insufficient drop in the pressure
     
    Last edited: Nov 19, 2018
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  6. RobF

    RobF Administrator Staff Member

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    Define a while ;) You don't shoot every 30 seconds, I'm fairly certain you stop shooting after the 2nd shot for a bit, let your partner have a go, maybe 2. Time that.
     
  7. SteveC200

    SteveC200 Active Member

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    You might not. I do, particularly in a showdown. :p
     
  8. RobF

    RobF Administrator Staff Member

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    In a showdown I normally run to about 4o secs for both sitting, and it can be less in positionals. But I'm still allow my partner to take shots in between that.

    If you time how the periods in between shooting you'll see it's longer than 30 secs. I know this because when we test reg behaviour we simulate competition timings both for the 2nd shot being fired and the longer wait between shots.
     
  9. RichWafta

    RichWafta Second time around

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    When I do my testing, I'll go 45 minutes between shots to simulate Worlds 2018 LOL
     
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  10. SteveC200

    SteveC200 Active Member

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    OK, 40 seconds then. It was just a number off the top of my head as an appropriately short amount of time between shots. :)



    Anyway, been having a play with a gas laws calculator, turns out I found it quite interesting, who knew? :D So for a given temperature rise the pressure increase is the same whatever the volume is. 3cc or 200cc, doesnt matter. What does make a difference is the initial pressure. An 80 bar start point will have less pressure increase for a given temperature rise than a 110 bar start point. There's not much in it though, starting with 90 bar and 20degC and rising to 32 degC its only about 0.4 bar difference to a total 4 bar increase. So I guess that if temperature stability is the goal then a low pressure/large volume plenum is preferable to a high pressure/small volume plenum.

    Combined Gas Law Calculator
     
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  11. RobF

    RobF Administrator Staff Member

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    Steve, does your partner shoot a lane in, walk to another and you start shooting in 40 secs? No.

    The assumption with using the gas law is that the temperature rise the is the same for a different amount of gas with the same input energy. That isn't so. As with all things the less of it you have the less energy is required to heat it up. Big rooms in a house of the same construction as a house with smaller rooms takes longer to heat up. Kettle full of water takes longer to boil than 1/4 full. Put a 2 inch bar of steel in a flame, doesn't burn. Stick steel wire in, burns nicely.

    The reason I say this is that some guns have 7.5 times less CC than others. And the one with the smallest CC is a renowned shifter...
     
  12. SteveC200

    SteveC200 Active Member

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    Rob, there is an airgun world outside of FT. :)

    Yes, the rate of change is longer with a larger volume but that total change is the same. So we need to find out what that rate is and whether it makes a significant difference for a range of small volumes.
     
  13. RobF

    RobF Administrator Staff Member

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    No such thing :D

    The rate of change is important, because you either get there or you don't. As you say you could be discharging a shot in 20 secs, or as Rich says, 20 mins. So it's better to have something with a slower rate.

    Apart from regs not tripping, I can't see any benefit to having a smaller volume. I can see plenty with though.
     
  14. RichWafta

    RichWafta Second time around

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    You're sat on the course with the sun shining. Temperature is 20 odd degrees. The action cycles and the reg does it's job and shuts off at 86 bar. How much is that pressure going to rise in the plenum chamber? Maybe before we work out the answer to that, we need to consider the temperature of the air before it goes through the reg as well as after???
     
  15. hmangphilly

    hmangphilly Floppy Quick Phil

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    That's odd

    I can change the pressure in an 85 ish bar 15 ish cc steyr reservoir by over a bar in A few seconds ., well maybe a minute by warming the action and breathing heavily on it ( yeah I know )
     
    Last edited: Nov 20, 2018
  16. SteveC200

    SteveC200 Active Member

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    Using that calculator it worked out roughly 0.3 bar increase for every 1 degC. If you're holding the reg in your hands I guess it's possible to get 3 degCs of heat transfer pretty quickly. Don't suppose you've got an infrared thermometer? ;)
     
  17. NeilM

    NeilM Well-Known Member

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    There are a lot more variables here than meet the eye.

    Temperature; if you move high pressure air through small transfer ports quickly it generates heat, quite a bit. That heat then dissipates through the surrounding metal, which all expands and then starts to contract... at different rates.

    Air Pressure; My experience is that the plenum chamber does not fully empty, this is I assume by design it is a combination of the hammer, its stroke, its weight the hammer spring, the size of the exhaust valve the pressure to shut on the valve from the return spring (if there is one) the additional pressure of the air flooding from the cylinder through the reg into the plenum.

    More Temperature; the air entering the reg from the cylinder is itself passing through a very small opening, this helps to slow that flow down but at the same time it generates heat in the body of the regulator, more expansion and contraction.

    I question if you can ever get to the bottom of this by theory alone, I think experimentation is the only way.
     
  18. hmangphilly

    hmangphilly Floppy Quick Phil

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    Sure
    I'll take a look
     
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  19. Monty

    Monty Senior Moment

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    Didn't know you could multitask, testing regs and making a phone call at the same time. Impressive!:D
     

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