Drop in power

Discussion in 'General Airgun Chat' started by mak1, Feb 2, 2019.

  1. RobF

    RobF Administrator Staff Member

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    We can work it out with the maths physics type thing... we did it recently for a barrel.

    Or we can be lazy and use this calculator :D

    Thermal Expansion Calculator

    Stick it in metric. If you want to know the coefficient for lead click the link and it will tell you. Make sure you have the one for metric. Lead is 27.2 x 10-6 or in more simple terms, 27.2 divided by 1,000,000, which is 0.0000272

    So if we assume a pellet is 4.5mm wide at 20 degrees, it will be 4.49755mm wide at 0 degrees or 2 n a half 1000th's of a mm smaller.

    Our steel barrel contracts by 1/1000th of a mm over the same temperature.

    So the speed drop theory relies on the difference of 1/1000th of a mm, or AKA 1 micron.

    Now if we are of the mind that our pellets vary by more than a mic, we should see wildly fluctuating speeds then at constant temperature...
     
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  2. RobF

    RobF Administrator Staff Member

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    Stick some in the fride Dan, and some in your trouser pocket with pack of Gummybears... then chrono :D
     
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  3. Dan Smith

    Dan Smith Active Member

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    Ok I surrender!

    My plan for a temperature controlled pellet pouch are now in the bin....
     
  4. Adam

    Adam Well-Known Member

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    There will always be some drop in power in PCPs in cold weather, but the HW100 can suffer more due to the amount of grease on the hammer shuttle. Years ago when I had an HW100 I set up the scope once in a long greenhouse and it was pretty warm inside (which is the point of greenhouses) and it was doing 11.5. The following day at the club it happened to be pretty cold and I was shocked that it was now doing just over 10 fpe. I degreased it and that reduced the variation quite a lot.
     
  5. hmangphilly

    hmangphilly Floppy Quick Phil

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    I dunno mate .
    I have no idea why Dan thinks that .

    I was just suggesting an answer to your question to him , "Why do think the temperature of the pellet is a factor?"


    For what it's worth , It seems quite feasible to me that a pellet might change it's dimensions through temperature .
     
  6. hmangphilly

    hmangphilly Floppy Quick Phil

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    Don't listen to him Dan ..stick 'em up yer jumper .

    Athough Chalkie might suggest somewhere else
     
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  7. Nick G

    Nick G Active Member

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    I suppose the cold pellets will have an effect, I recently found that wet pellets in my springer travel faster down the barrel than dry ones, this caused a large shift in poi as the pellet was leaving the barrel at a different part of the recoil cycle .
     
  8. nemesis

    nemesis 55yrd standing expert, or was it 8 or 9?

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    Like I've said before, it's your barrel.
     
  9. RobF

    RobF Administrator Staff Member

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    Yep, if you count it shrinking by 1/1000th of a millimeter (or 1 μm)

    Things that are small, measured in Microns below

    [​IMG]
    Logic dictates your powder puffing would slow your pellets down enormously... being 5x as much as the shrink on a barrel from 20 - 0 s degrees.
     
  10. C.Eaton

    C.Eaton Confirmed Anschutz Nut...

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    I knew someone that used to 'warm' pellets in his mouth before shooting.
    He's not around anymore so maybe just stick to pulling them straight out of the pouch...;)
     
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  11. DeanB

    DeanB Active Member

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    Not sure how you calculated the 1/1ooo of a mm for a steel tube. Try....Thin Circular Ring - Temperature and Expanding Radius
    Lots of useful stuff at the site. I doubt the differences in expansion of the barrel and pellet are causing the change in pellet speed. Also, do we know the actual composition of the material used to form pellets? Then possible to look at hardness. Based on earlier postings sounds like the gun mechs are the issue.
     
  12. Chalkie

    Chalkie New Member

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    I can't believe we are discussing temperature variations causing changes to pellet and barrel dimension of microns affecting accuracy
    The pellets and barrels are not manufactured to micron tolerances. We are in the realms of watchmaking
     
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  13. RobF

    RobF Administrator Staff Member

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    Can you check it for me?

    Using that calculator a 4.500 mm tube went to 4.499mm

    That's 1/1000th of a mm.

    I'm using;

    the coefficient of linear thermal expansion of steel as 11 x 10-6 or 0.000011

    the diameter of the ring as 4.5mm (d1) which seems to be the inner diameter on that calculator

    and -20 degrees (from say room temp to zero)

    That's what it gives me, which is what the last one gave me.
     
  14. Dan Smith

    Dan Smith Active Member

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    what if it just gets an ickle bit softer, more pliable, might just snuggle down into the rifling a bit snugger - just a teeeeny bit maybe? I'm grasping at straws here!
     
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  15. Nomads HFT

    Nomads HFT Well-Known Member

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    If anything, colder temperature should make lead harder, less pliable, and increase the energy needed to get the head and skirt to conform to the rifling. Alone, that would make little difference in a PCP, but in a springer, it would delay pellet start, reduce the time that the piston and pellet were traveling in the same direction (when the pellet gains most energy), and cause a reduction in muzzle energy.

    Back to the OP. To get that kind of muzzle energy reduction purely from a fall in temperature between a PCP being charged with air and shot would require a temperature shift in the region of 30C, so it's probably a combination of this, plus the striker grease others have mentioned.
     
    Last edited: Feb 5, 2019
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  16. RAIDER

    RAIDER Member

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    We go by the temp at 55 yds were adding 5clicks @ 10’ and under.
    50 yds 4 45 3 40 2 back to normal dial @ 35 yds
     
  17. DeanB

    DeanB Active Member

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    I used slightly different thermal expansion coefficients and found change in pellet dia of 2.6 micron and change in barrel of 3.5 micron. So about 1 micron difference between them.
     
  18. RobF

    RobF Administrator Staff Member

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    I can't make either that... what coefficients did you use?
     
  19. spartan

    spartan Member

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    i've been doing some testing on my own gun regarding power dropping when the weather gets cold, my gun drops around 15 to 20 fps on a cold day, i checked all the obvious things and no joy, but what i found out today i would say is the cause, i've been testing the reg using a digital pressure gauge, with an average temperature of around 15 degrees in my shed, and a reg pressure set at 95 bar all is well, but what i did notice, i left my action in the shed overnight with the gauge attached and the reg pressure had increased to around 98 bar, now the temp in the shed was probably close to freezing overnight, and not much above 5 degrees when i checked it again, upon trying it on the chrono a power drop of 15 to 20 fps, obviously harder for the valve to open against the 3 bar increase, when the action was took back in the house the reg pressure dropped back down to 95 bar, and everything was fine again, its got to be the alloy and brass and steel parts moving at different ratios in the reg, it only take a couple of thousands to make a fair difference to the reg pressure, i dont think theres a lot i can about it if i'm honest, i might try making the whole reg assembly out of brass instead of indifferent materials, see if that makes any difference.
     
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  20. nemesis

    nemesis 55yrd standing expert, or was it 8 or 9?

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    I shot in minus 4 at the weekend with no power drop at all now that I have sorted the barrel.;)
     

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