Altitude and high temps?

[Madd Hatter]

I've got a question for those brainiacs that prowl the pages of GTA. What would the fps affect be on a springer between sea level and say 70f and 3000 ft and 105f?

[fderry]

I live at 7000' in Colorado.  I bought a HW30s from AoA a few months ago and compared their chrony numbers with my chrony numbers at my house.  I'm not sure what altitude Gilbert, AZ is at.  I don't have any weather data from either test.  Here are the numbers:

AoA - H&N Field Target Trophy 8.64 gr (HW30s)

Average      564
High         574
Low         525
Extreme Spread        49
Standard Dev        19

H&N Field Target Trophy 8.64 Shot at 7000ft Altitude (HW30s)

Average      520
High         530
Low         504
Extreme Spread        26
Standard Dev          9

[Madd Hatter]

What were the temps at? I know the the air pressure here at that temp was right at 13.03 PSI. Sea level it's 14.7 psi so that's right around a 1.7 psi drop in pressure. So the pressure is 88.7% of normal sea level pressure. I wonder if that correlates to to 88.7% of sea level vel? 

[Madd Hatter]

What were the temps at? I know the the air pressure here at that temp was right at 13.03 PSI. Sea level it's 14.7 psi so that's right around a 1.7 psi drop in pressure. So the pressure is 88.7% of normal sea level pressure. I wonder if that correlates to to 88.7% of sea level vel? 

By the way, that's the pellet I was testing.

[Madd Hatter]

I live at 7000' in Colorado.  I bought a HW30s from AoA a few months ago and compared their chrony numbers with my chrony numbers at my house.  I'm not sure what altitude Gilbert, AZ is at.  I don't have any weather data from either test.  Here are the numbers:

AoA - H&N Field Target Trophy 8.64 gr (HW30s)

Average      564
High         574
Low         525
Extreme Spread        49
Standard Dev        19

H&N Field Target Trophy 8.64 Shot at 7000ft Altitude (HW30s)

Average      520
High         530
Low         504
Extreme Spread        26
Standard Dev          9

Gilbert AZ is aprox. 3550 so just about half your alt.

[rsterne]

I would think the FPE would follow the pressure, so the velocity drop would be the square root of the ratio of pressures (about a 6% loss).... Springers heat the air, so the high ambient might work for you....

Bob

[Madd Hatter]

But the air density is less which would equate to a even higher elevation? Higher elev. even lower FPS correct.

[Techie]

I read that ambient air pressure decreases about 3% for every 1000 feet of elevation.  In my experience chronographing springers and pumpers at varying Colorado (high) elevations, I find that velocity also decreases by about that same amount.  So... at 3000 feet elevation I would expect a springer to shoot about 9% slower than at sea level.  I don't find air temperature to have much effect, other than if the gun itself is hot or cold, which could lead to thicker or thinner oil or grease having an effect on gun performance.

[rsterne]

Atmospheric air density and pressure are interchangeable when one goes down, so does the other.... I based my guess of a 6% velocity loss on your 12% pressure loss.... I'm not that familiar with the thermodynamics of springer, so if others are saying that the velocity loss follows the pressure drop, I won't argue.... Generally in PCPs, FPE is proportional to pressure, that's what I was using for a baseline....

Bob

[fderry]

Robert - I do not have any weather data from the AoA test nor my initial test.  If you are interested I can re-run my test but it would be flawed.  My HW30s has had 300+ pellets run through it now.  When I did the original test AoA fired 6 of their pellets through the rifle and I shot the next 10 pellets from my tin of pellets for my chrony testing so no break in at that time.  Still too many variables.  Different chronys, no weather data available and can not confirm if AoA and I used pellets from the batch as examples.  My true altitude, based on GPS, is 7134'.

I found an interesting blog that talks about some testing done a while back.  This may interest you - https://www.pyramydair.com/blog/2008/02/shooting-airguns-at-altitude/

[Madd Hatter]

Thanks for the replies;-) I did some cronoing yesterday and my results were lower using the same pellet and less than what would have expected with some FFT 8.5?  h and n pellets. I got aroud 25 fps less with Baracuda 10.65 pellets but it was with a different crono than I originally did my shot string on. Do not have the original anymore as I killed it!!! Lol