Variations with temperature

Discussion in 'General Airgun Chat' started by Archer50, Jan 4, 2019.

  1. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    Can I just check I'm understanding this.. are you all agreeing that a PCP will lose approximately 1.5 fps for every degree C the temperature drops?
     
  2. Welsh Wizard

    Welsh Wizard FT: You Give, You take! Staff Member

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    no
     
  3. Archer50

    Archer50 Active Member

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    I am reporting what I found with the two guns I tested, but I'm not quite sure what to make of the results, particularly how far they can be generalised.

    I've run a few more tests today, going from 6 to 27 degrees, and results for the regulated HFT 500 are very close indeed to a straight line change of about 1.3 fps per degree C throughout the temperature range. The lower temperature results were very close to those already published, and the new, higher temperature results were 785 fps at 25 degrees (786 predicted on trend) and 790 fps at 27 (789 predicted on trend). I am still testing the Steyr.
     
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  4. Jesim1

    Jesim1 Active Member

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    The two guns I noticed it with a few days ago, but TBH was not actually looking for at the time, were an electronic MK4is and a Regal, so one "kind of regged" electronically and one not. My first thought was that the regged gun, especially an electronic one, should keep the power the same as it can adjust it's self on the fly so to speak, and perhaps the un regged gun would suffer more. On further consideration however it dawned on me that we are dealing with a difference in "air density" rather than actual pressure, and that is what is causing the majority of the power drop.

    Between this thread and another on scope cant I'm actually having to think for a change:D
     
  5. frank

    frank Reactive Member

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    Can I just check I'm understanding this.. are you all agreeing that a PCP will lose approximately 1.5 fps for every degree C the temperature drops?

    That would roughly agree with the results I found, cylinder at about 22 degrees fps 773, cylinder at about -18 fps 716,
    57 fps difference 40 degree spread =1.425 fps for each degree C, but that is a figure based on a high and low temperature reading so the actual progression may not be linear
     
  6. RobF

    RobF Administrator Staff Member

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    No, I've seen them move much worse. And better...

    What we have to remember though is there are measuring errors and other issues which can bias our findings.

    Here's an example from last year. That said we didn't run it to 0 degrees so perhaps the change could have got worse.

    There will be some who say they don't see it, because they don't. If their gun shifts one way and the scope another, then they can cancel each other out. Things like not waiting for a gun to sit for an hour to settle or other factors we know mitigate shift can skew results and bias theories further. It's another reason why if you want to solve this stuff then doing the hard work is something you yourself can only do... There's not much point in knowing what someone else's gun does and if you know what you're does then it doesn't matter, that's all that counts. There's a bit of a bragging thing with kit in that the person next you somehow is better off because their's doesn't do it. That could be the case, but I'd say it's the case they also own their own chrono... :D





    IMG_8647.JPG IMG_8646.JPG
     
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  7. tillygti6

    tillygti6 Tilly's gun stocks

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    If you really want a bit of head scratching,
    Run some cold ,room temp and warm pellets through it.

    Something like the accupel, a harder lead mix pellet was less
    Affected in the hot.
     
  8. simona

    simona Active Member

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    Interesting to see the post where the cylinder went in the freezer. This suggests that the variation is to do with the actual air temp rather than hammer drag or anything like that.

    I'm not a big chrono merchant. I used to see my Dominator drop a bit low in the cold but I've not noticed it with my Steyr at all and that's been the case with two or three different scopes on there over about 4 years.
     
  9. RobF

    RobF Administrator Staff Member

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    I think it's air temp, but can see how cool air from a cylinder can be heated by a hot gun...

    Airguns... :D
     
  10. frank

    frank Reactive Member

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    That is why the cylinder went in the freezer, I wanted to get an idea if the power drop was caused by the gun cooling down or the compressed air temperature, I think like Rob other factors come into play but just trying to identify one factor
     
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  11. C.Eaton

    C.Eaton Confirmed Anschutz Nut...

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    Personally I still think that the biggest factor in velocity change with PCP's is to do with the hammer and valve springs being affected by temperature certainly on modern Anschutzes.
    When you consider that even a minor amount of adjustment on the hammer, i.e. an eighth of a turn in will increase the velocity by 10fps, then it stands to reason that the expansion caused by heat on the spring will cause a similar effect.
    Yes I know that the whole gun is affected by temperature, but the spring is thinner and more susceptible to temperature than the receiver.

    I guess the only way to know for sure is to rig a rifle with a variable heat source on the springs and run some tests.
     
  12. Nomads HFT

    Nomads HFT Well-Known Member

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    I'm not so sure about that. The spring mean diameter will also increase with a temperature increase, making it softer, and its modulus of elasticity will decrease with an increase in temperature.

    Could go either way for all I know.
     
  13. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    Phew! I thought for a moment there was a thread on temp variation and everyone was agreeing. I thought I'd wandered onto a different forum for a moment.
     
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  14. Jesim1

    Jesim1 Active Member

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    But what about my Daystates? They are electronic with no hammer spring, it's an electronic solenoid which hits the valve, even the trigger is a switch, so a lot less mechanical movement to be affected? So as it's been mentioned the expansion of metals over, say 20 degrees, is literally nothing, then it goes back to the main change is probably the air density caused by temperature change?

    We could go back and forth all week on this, and at some point you have to go with your own opinion, but I'm leaning towards a fairly linear drop in fps of around 1.2 to 1.5 fps per degrees C dropped from UK normal temps - so lets say 28 down to around 3/4 degrees? Above and below those figures I'm sure other factors will start to come into effect, but in the real world we don't see a lot of comps going on outside those temps so it's less of an issue for us.

    James
     
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  15. RobF

    RobF Administrator Staff Member

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    As above Col, the expansion/contraction is tiny over tiny things, some way off the order of magnitude we'd see affecting them. So if it is them, then it's something else apart from that.

    That Anschutz we had in we made some changes on. It was pretty bad at shifting, but 'we' didn't touch the springs or hammer... at the end 'we' got the shift down to about 30% of where it was when we got it over the same range.

    I'd like to dig into the modelling for the theory a bit deeper to suggest why that is, because my theory could be nonsense, but we did see a change in it's behaviour.
     
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  16. frank

    frank Reactive Member

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    Don't fret Brian,Situation on the forum is as normal
     
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  17. C.Eaton

    C.Eaton Confirmed Anschutz Nut...

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    If we as a species agreed on everything all the time I guess we would have died out a millennia ago sussing out why a club is more effective held at the end instead of the middle.
    By the same token, it probably was ‘our’ ancient ancestors having that same argument and doubtless it will be same in the future discussing the finer points of handheld rail gun velocity...
     
  18. Welsh Wizard

    Welsh Wizard FT: You Give, You take! Staff Member

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    Mine don't shift, but then I have been shown how to correctly Chrono a Gun!
     
  19. Archer50

    Archer50 Active Member

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    Would you like to share?
     
  20. LittleJack

    LittleJack Active Member

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    Does any of this testing really matter and is it worth you while? When you turn up to a shoot if you have a problem on the zero range, a quarter turn in your transfer port will sort anything out surely?
    I don't understand when people super test guns?
     
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