The Kestrel/Density Altitude/POI Shift thread

Discussion in 'Hunter (HFT) & Field Target (FT)' started by RobF, Sep 27, 2011.

  1. RobF

    RobF Administrator Staff Member

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    Cue rolling eyes from the back of the class of something else technical i've purchased for shooting, but the phenomenon of POI shift which seems to correlate with weather has had me head scratching for years. Aside from the impact temperature can have on a scope's ability to tell a true range, or the effect it has on an airgun's ability to deliver a constant muzzle velocity, there seems to be the possibility atmospherics have something to add to the muddle that can send POI way off.

    In Italy the springer delivered shots over the crono with less than 3ft/s variation total over 3 days,
    the scope never ranged over or under, yet I saw 1.5 MOA of shift during the comp (meaning the scope would have had to misrange by 10yds in some cases or invent a new zenith in others)... either the barrel was raising in the heat and dropping in the cold(er) air or something else was happening.

    Pete M put me onto a cheap source of the Kestrel 4000, a few years ago these were £400, now commonly found for £250, at 1/2 price on the bay it seemed like a good deal. These combine humidity, temperature, barometric and altitude data to come up with a benchmark figure called Density Altitude. This is because although air temperature changes density, air pressure can change it the other way, which means there can be a time where density can be unexpectedly denser or lighter especially when altitude as well varies. The idea that a cold damp morning has denser air than a hot one may not always be true. So Density Altitude gives us a reference point to work with, and the Kestrel can measure and calculate this.

    It seems to have impact in long range full bore shooting, http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/ is a useful link, and there is a commercial outfit custom building them for specific loads in the US. As FT seems to be a short long range shooting competition, it's plausible the same effect could happen to FT shooters. Whilst some shooters never seem to have a problem, I suspect that those that have done a few visits abroad have seen movement, and some who live in wildly changing climates might see it far more often.

    So hopefully this thread will be a chance to correlate the info out there and scale it down for airgun purposes, and see if the effect is real and has an impact, or is another bit of technobable that can be ignored, or a bit of both.

    So far though, the Kestrel seems simple to use. The manual is tiny and about 30 secs of reading you are off and using it. So I am now praying for some wildly varying weather and some shifting POI to accompany it... so expect good weather and stable zero points for a while ;)
     
  2. villiers

    villiers Self appointed antipimp

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  3. PeteM

    PeteM New Member

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  4. rich

    rich Active Member

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    I was impressed with Pete's piece of kit on Sunday. Our ground is at about 500 feet above sea level. Pete's gizmo said it was about 1100 feet equivalent at about 0930, and by the time the sun had been out for an hour or two we were at a heady 1500 feet. I felt quite light headed. :)
     
  5. NJR 100

    NJR 100 Because I`m AWESIME !!

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    Its windy enough at 500 ft, would expect to be aiming at next lane at 1500 ft :eek:

    I think you have used too many pallets rich :D
     
  6. rich

    rich Active Member

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    Just a few.

    [​IMG]

    :)
     
  7. rich

    rich Active Member

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    Thing is Simon, at 1500 feet compared to 500 feet, the air density is less (given the same temperature and RH) so the pellet would experience less wind drift. But you knew that already and you were just being a torment. :D:mad::D
     
  8. RobF

    RobF Administrator Staff Member

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    Just been playing with it without shooting... today has probably seen the widest spread. DA has gone up from 480 to 1200 just now, seems humdity has played a part as that has dropped a lot even though the temp has only moved a few degrees.

    The other day it dropped from 1300 to 750 as the sun went down and the evening cooled.

    Early days, but it will be interesting to see what the DA is like when I do see a shift. Gonna take the springer up the range tonight and see what's what :)
     
  9. villiers

    villiers Self appointed antipimp

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    Have seen the DA doing the same , baro has been pretty stable am at 25m above sea level and have been comparing my number from Heathrow airport which is very near to me.My Kestrel is not telling me lies.;)
     
  10. TOOL

    TOOL Independent FT Pumper

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    Up north we use these Kestrel's, mainly for catching your tea.
     

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  11. RobF

    RobF Administrator Staff Member

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    Yes, but we don't eat little boys down here... :D
     
  12. AndyIoW

    AndyIoW Member

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    With the weather being at it is could be interesting to see shat it does from Early Saturday morning to the evening Rob as I think Sunday you will not have much chance to check it ;)
     
  13. villiers

    villiers Self appointed antipimp

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    What Drag model for a airgun pellet in a Ballistics program a G1. ?:eek: would you use.
    Just trying to under stand how that bit of lead goes down range.
     
  14. RobF

    RobF Administrator Staff Member

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    Not sure...

    What I did see over the weekend was a 2400 ft shift between extremes, however the only things that really changed dramatically was the humidity and temperature. At my place, saturday morning, it was 600ft, and at newbury it was 1800ft. Sunday morning was much more 'dewy' and that read minus 250 ft, newbury being about the same.

    So what it appears to suggest is that the humidity and temp when they get low can change air density as compared to a warm drier day.

    Unfortunately I wasn't able to shoot at home at the weekend, but tests the night before indoors had the DA at 600ft, and there was no zero shift between that and 1800ft.

    What would be interesting to see is what it's like when we get into the more extremes of winter, when zero shifts are more often seen.
     
  15. villiers

    villiers Self appointed antipimp

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    Will see what its like at the weekend on the Island , will you show me yours if i show mine. Zeroed at Bisley yesterday at a DA of 600 the biggest change i have seen so far is 1000DA for no change in point of impact.
     
    Last edited: Oct 6, 2011
  16. RobF

    RobF Administrator Staff Member

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    Yep, would have loved to stick a few down her when it was really dewy sunday morning and reading -200.
     
  17. RobF

    RobF Administrator Staff Member

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    DA is reading minus 380ft and I am having to add 3/4 Moa to my summer settings all through the ranges.
    Scope is range finding targets the same and the speed is the same.
     
  18. PeteM

    PeteM New Member

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    interesting

    That's interesting Rob, haven't had a chance to do anything more yet.

    Been trying to get the Steyr LG110 working, think I managed it yesterday, will see if it works consistently after that. Then may sell it if it is functioning properly.

    Looking forward to getting the stock on the Airstream MkIII, amazingly consistent rifle. Only been able to shoot the action out of my test rig. Results are HERE
     
  19. greyskullnz

    greyskullnz Non member

    Thoseare similar conditions to late Winter/early Spring here. Morning start to a FT match can see you taking the first 3-4 lanes in those conditions, and then clawing back up to your standard trajectory settings as the ground warms in the sun.......continuing steady until lat afternoon. Standard environmental conditions here at that time of year.

    Each Season is different. The effect of cold dense air on trajectory and the transition to a warming period later in the morning can be more or less pronounced during each Season. Sea Level atmosphere behaviour.

    Interestingly, CG3 don't account for altitudes below sealevel. (ie: -380ft)
    If you plug in just DA into the Altitude box in the "environmental controls" (when its above 0 ft) it gives the exact represetation of your trajectory (if you have set your trajectory according to the correct Environmental conditions at the time), and rezero'd accordingly at the new location. It won't give you a solution for shift during a match......unless you print out a set of variable trajectories that match all the possible DA conditions you could encounter. It is not inconceivable that it could be of great use during a match, if you have your wits about you during a shift in conditions.......and only useful at one of those venues that present the particular problems of DA shift due to their altitude/change in altitude/change in barometric pressure over a match in the first place.

    I'll be getting myself a Kestral early next year to play with, but have been doing it all longhand up to now, so at least the concept is well known and the effects understood. I still think unless you have a grasp of it before you get a kestral type device, it will only confuse you in the short to mid term.

    Cheers
    GS
     
  20. villiers

    villiers Self appointed antipimp

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