Variations with temperature

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

  1. Archer50

    Archer50 Active Member

    Joined:
    May 24, 2015
    Messages:
    102
    Location:
    Perth
    After seeing some of the threads about changes in velocity with temperature and having some surprising chrono results, I did a few tests on my standard Lg110 and my Lane regulated HFT 500. In each case I filled the guns to 150 bar and let them acclimatise for at least an hour at each change of temperature (they seemed to need that long to settle) . I used JSB Exacts and 20 shot strings through my LMBR R2A chrono. I have the full results, if anyone is interested, but here is the summary graph:

    Temperature graph.jpg

    There are not enough results to be sure whether the graphs are straight lines, or as the trend lines suggest, some sort of curve, but the size and general shape of the effect is very clear, and remarkably consistent between the two guns - note the %-age change per °C which is identical to 4 decimal places - coincidence or some sort of 'law'?

    I knew that PCP's tend to run slower in colder temperatures, but I am surprised at how big the effect is. I would be interested to hear any general comments, particularly if anyone else has done something similar. i am especially interested to know if variations of this sort of scale apply to all PCP's and exactly what causes it.

    Alan
     
    Chalkie, Jesim1 and Steviek like this.
  2. RobF

    RobF Administrator Staff Member

    Joined:
    Mar 4, 2010
    Messages:
    12,754
    Location:
    New Forest, Hampshire
    Club:
    Parkstone Gun Club, South Dorset FTC, Southampton Buccaneers
    Nice.

    We can all have a crack at this to get some more data.

    One question, are your hammers running dry or are they lubed?
     
  3. Archer50

    Archer50 Active Member

    Joined:
    May 24, 2015
    Messages:
    102
    Location:
    Perth
    The hammers are running dry. Out of interest, I've done some preliminary tests on my HW100, which runs wet, and it looks like the the effect is much smaller - watch this space :cool:
     
  4. nemesis

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

    Joined:
    May 19, 2010
    Messages:
    1,257
    Location:
    Yorkshire
    If you redo the test again measure the O.D of the barrel, say from 26deg to 2 deg.
     
    DaveRob likes this.
  5. AlexS

    AlexS Active Member

    Joined:
    Sep 10, 2013
    Messages:
    108
    Location:
    Germany
    Oh, what coincidence, the mean velocity of the air particles that propel the pellet has also an increase by roughly 0,17 % per °C.
     
  6. RobF

    RobF Administrator Staff Member

    Joined:
    Mar 4, 2010
    Messages:
    12,754
    Location:
    New Forest, Hampshire
    Club:
    Parkstone Gun Club, South Dorset FTC, Southampton Buccaneers
    I think you're overblowing the effect of the expansion/contraction of steel.

    The thermal coefficient of steel is 0.0000072 per degree give or take.

    So if you say we want to see how much steel contract from 20 down to 0 we first multiply that by 20 which is 0.0000072 x 20, which is 0.000144.

    Now we take that 4.5 mm bore and we multiply that by 0.000144 by 4.5mm which gives us 0.000648 mm. In other words the 4.5mm gets just under 7/10,000th's of a mm , or 0.6 microns smaller in 20 degrees drop.

    I know that sort increase in tightness is something that could register on a northerner's wallet but the average pellet isn't going to notice. You'd probably alter the dimension of a barrel more with harsh language.
     
    Chalkie, EV2UK, Keith t and 2 others like this.
  7. Darron

    Darron Dwarf Slayer

    Joined:
    Aug 2, 2015
    Messages:
    1,298
    Location:
    Aldridge West Midlands
    Club:
    Blackbrook FTC
    You won’t be able to measure any difference unless you work for NASA
     
  8. Jesim1

    Jesim1 Active Member

    Joined:
    Mar 23, 2016
    Messages:
    320
    Location:
    Wigan/St Helens
    Club:
    BOAC, BGC
    Thanks for this Alan, really interesting and strangely it was on my mind just a few days ago.

    I had been working on two guns - a Daystate MK4is and a Daystate Regal, both in .177, the MK4is was crono'ing in my house (22deg ish) at 11.3fpe and the Regal at 11.6fpe, both with JSB heavies - perfect! The next day I was at the range accuracy testing them and noticed over the few hours I was there that both guns dropped their zero by 1/4 to 1/2 an inch at 35m (ish). It was no surprise to find that both were now shooting at around 10.5fpe when I checked them again - and it was now 4/5 degrees with the guns having been out the car for the time I had been there.

    So as well as giving you the detail that your findings are wide spread to other guns, it struck me at the time not to sell a cold gun if crono'ing it in front of the buyer, but more importantly not to adjust the power of a cold gun, as PC Plod will no doubt be checking it in the comfort of a warm office:eek::oops:

    Great bit of work, thank for sharing.

    James
     
  9. Archer50

    Archer50 Active Member

    Joined:
    May 24, 2015
    Messages:
    102
    Location:
    Perth
    Thanks James. On the basis of my results, if your gun is like either of these two and you set it to the mid-770's in your garage at 4 or 5 degrees, you will be on very dodgy ground at 20-odd degrees.

    I also wonder if the temperature effect accounts for some of the highly technical discussions you hear so often over the chrono at HFT/FT shoots. You know the sort of thing: Your f-ing chrono is f-ing well f-ed! I set my f-ing gun last night and it was f-ing fine! :mad: Yes, but it was cold last night.... :rolleyes:

    Alan
     
    RobF likes this.
  10. Jesim1

    Jesim1 Active Member

    Joined:
    Mar 23, 2016
    Messages:
    320
    Location:
    Wigan/St Helens
    Club:
    BOAC, BGC
    Very true Alan

    And just to make things worse - today I got my cylinder filled at a gas retailer and they filled it with NITROGEN :eek:

    I can't do the maths on this one I'm afraid, so if anyone can shed any light on whether this would be affected by temperature to any extent that would be good, but my thinking was that being very similar to air (about 3% less dense) there would be a negligible difference, and being an inert gas it would if anything be more stable, but not so as you would notice - kind of like they do with car tyres? :rolleyes:

    James
     
  11. Pavel

    Pavel New Member

    Joined:
    Dec 17, 2018
    Messages:
    19
    Location:
    BG
    should be a less sensitive cloud temperature. about 70% of the air in the atmosphere is nitrogen. What's wrong?
     
  12. tillygti6

    tillygti6 Tilly's gun stocks

    Joined:
    Jul 30, 2011
    Messages:
    612
    Location:
    herne bay
    Dont do it.... if you do, atleast film it so we can say I told you so.

    Seriously though, altho on paper inert mixed with oils it can become all sorts of dangerous
     
  13. Archer50

    Archer50 Active Member

    Joined:
    May 24, 2015
    Messages:
    102
    Location:
    Perth
    Also, the use of any gas other than air or carbon dioxide is illegal without an FAC -

    www.gov.uk/government/uploads/system/uploads/attachment_data/file/518193/Guidance_on_Firearms_Licensing_Law_April_2016_v20.pdf

    2.48 By virtue of section 48 of the 1997 Act, firearms using compressed carbon dioxide as the power source are treated as air weapons and, if not regarded as ‘specially dangerous’ (over 6 foot lbs in the case of a pistol or 12 foot lbs in the case of other air guns and air rifles) are thus exempt from the firearm certificate procedure. Firearms using other gases are not so exempt.
     
    Last edited: Jan 4, 2019
  14. Jesim1

    Jesim1 Active Member

    Joined:
    Mar 23, 2016
    Messages:
    320
    Location:
    Wigan/St Helens
    Club:
    BOAC, BGC
    OOOOOoooooo……… I'd never heard of that? :eek:

    TBF though, testing my gun for power is one thing, but actually testing the gas in it? Well, I won't lose sleep once I've put my gun over the crono just to make sure there is no difference. With a top up from 150 bar to 300 it should only mean a reduction of O2 of around 7%, and as someone alluded to already - air is 78% Nitrogen to begin with :p

    James
     
  15. frank

    frank Reactive Member

    Joined:
    Oct 7, 2014
    Messages:
    258
    Location:
    lincoln
    Club:
    lincs hft
    James, Your nitrogen fill should not cause you any problems and certainly isn't dangerous, it is the oxygen in air that supports combustion, nitrogen is non combustable, it is also put in car tyres as its supposed to be less temperature sensitive, probably of benefit on an f1 car but of little value in the family runabout,
     
    SteveC200 likes this.
  16. frank

    frank Reactive Member

    Joined:
    Oct 7, 2014
    Messages:
    258
    Location:
    lincoln
    Club:
    lincs hft
    May be of interest to some sad soul,
    couple of the lads at the club noticed the power over the chrono dropping as the temperature got colder,
    I set my hw100 to 11.2 indoors at room temperature went to the club on a wed night and after the gun had got cold put it over the club chrono, got 10.9
    suspected it might be the club chrono, so,
    without adjusting anything I took the gun home allowed it to warm up and put it over my chrono, back to 11.2, left the gun outside for an hour, back on chrono 11.0, brought gun back in and allowed to warm up but put the cylinder in the freezer for an hour,
    back on chrono with the gun at room temp but the cylinder very cold got 9.6, allowed every thing to warm back up, back to 11.2, go figure
     
  17. Jesim1

    Jesim1 Active Member

    Joined:
    Mar 23, 2016
    Messages:
    320
    Location:
    Wigan/St Helens
    Club:
    BOAC, BGC
    These are exactly the findings this thread is pointing to, and TBH until now I've known about it but never gave it much thought. People often go on about their scope being temperature sensitive - really:rolleyes: or are they barking up the wrong tree...…:oops:

    James
     
    Last edited: Jan 5, 2019
    Archer50 and bow like this.
  18. RobF

    RobF Administrator Staff Member

    Joined:
    Mar 4, 2010
    Messages:
    12,754
    Location:
    New Forest, Hampshire
    Club:
    Parkstone Gun Club, South Dorset FTC, Southampton Buccaneers
    James, correct, the car tyre thing that nitrogen isn't affected by temperature is a myth. Nitrogen is quite close to an Ideal Gas and obeys the Ideal Gas Laws, which air also follows close enough to say in layman's terms it does as well.

    The reason why car tyres are swapped with nitrogen is that it purges moisture, and moisture varies with different fill supplies with air, and moisture causes the biggest variation with predicting pressure changes with temperature. It's not that big of a deal but where you might be trying to predict pressure changes very precisely it can upset that if your fill has a different humidity. The other by product is it reduces corrosion and doesn't support combustion, things that also become more important in racing situations or important situations like aeroplane tyres.
     
    Jesim1 likes this.
  19. RobF

    RobF Administrator Staff Member

    Joined:
    Mar 4, 2010
    Messages:
    12,754
    Location:
    New Forest, Hampshire
    Club:
    Parkstone Gun Club, South Dorset FTC, Southampton Buccaneers
    Not so much as barking up the wrong tree but if you look at a wood you might be looking at the wrong tree because there's more than one. There's ways to separate the potential issues, but on a course where you don't actually know what you're shooting at and can't control the situation it becomes much more difficult to ascertain which problem you're facing.

    I tend to think there's 3 types of shift to do with temp in FT... power (ie your velocity changes), ranging (ie your ranging on a known fixed distance changes it's read out) and mechanical (the gun bends, normally to being pushed around by an unstable stock material, like wood). I've experienced all 3, and not all scopes move the same way, but velocity tends to go down in cold and up in heat (although springers reverse this often as the seal heats and cools and provides more or less friction). Knowing this I would say that if people should expect to see shift, if they shoot over a range of conditions and don't change their rig.
     
    biwain, Jesim1, Archer50 and 2 others like this.
  20. Nomads HFT

    Nomads HFT Well-Known Member

    Joined:
    Sep 27, 2015
    Messages:
    430
    Location:
    Worcester
    I acclimatised a couple of PCPs and their charge cylinder to a range of temperatures between 10C and 39C, charged the rifles to the same pressure at each temperature, and recorded the muzzle energies. There was a strong correlation between the muzzle energy and charge temperature in degrees Kelvin (K = degrees C plus 273).

    Thus, a PCP that gave 11.4 ft. lb when acclimatised and charged at 20C gave 11.4/293x273 (10.62 ft. lb.) when acclimatised/charged to the same pressure at zero C. The relationship is not strictly linear, because it does not extrapolate back to 0 ft. lb. at zero K but, within the narrow range of temperatures in which we shoot, it’s very close.

    If the rifle is charged at one temperature, but shot after being acclimatised to a different temperature, the change in muzzle energy will be a little more marked because the temperature change will cause a change in pressure, but calculating using the change in degrees K will at least put you in the right ball park.

    The rifle that gave 11.3 ft. lb. when charged at 22C in the quoted post would give ~11.3/295x277 (10.6 ft. lb.) at 4C. Pretty close.
     
    Tone, poth, RobF and 1 other person like this.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice