Cold weather shooting
The other day I posted how well my gun was shooting and wanted to shoot a full string to see how good it was. Well this time it was not 40 degrees instead it was 25. The last time I did a full chrony string I was getting 45 shots in 30fps spread or less and a max. velocity of 890-895fps with the JSB 15.9g. Today I got 40 shots with a 33fps spread and a max. of 873fps with 15.9g JSBs. So I lost a few shots and 20fps. I let my gun warm up, refilled, went outside and shot over the chrony before it had a chance to get cold, and I got 879 882 885.
What kind of changes do you guys see in your guns when it gets below freezing vs warmer weather?
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gimmick
I did a little testing a few weeks ago to see how much the pressure changes with air temperature. I got an average of .14% per degree. So .14×10(degrees)=1.4% If you fill to 2,500psi you should see close to a 35psi drop. With my rifle that is only 3 shots worth of air so it’s not that much of a change. Now if it was a 30 degree drop you might want to over fill some. These are just my estimates using my handpump guage.
Yeah, I figured as much. My interest is in how much it shifts per 10F change as a % in psi.
I fill my tanks to 2500~2600psi as that seems to be my sweet spot (no chronograph). If moving from 75F to 105F bumps me to 2800 or 3000psi then I need to know as I valve lock in those higher numbers which makes it a useless gun for an impromptu hunting shot.
So is there any ballpark experience in how HPA changes with temp change? I did some searching on the web and both Airgun and SCUBA sites don’t seem to indicate how much it changes.
I’m sure you know the thing where stuff expands and contracts with temperature change. The air is doing that. Higher temperatures move the molecules farther apart and increase pressure.
Probably a good idea to fill a bit under 3000 just to be safe.
I slightly broader question. How does HPA pressure change with ambient temperature? i.e. a tank at 2500psi in 75F becomes XXXXpsi at 100F and YYYYpsi at 32F
Its a broad question because in Texas we go from 75F to 105F in the shade when moving from indoors to outdoors. Should I underfill by 100psi or more to ensure an approximate pressure outside?
I usualy pump outside or pump slightly over my regular pressure. Its a pain in new england with all of are temp changes here.
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Jeff, thank you!
So for a 75F->105F raise I’m looking at a minimum of 100psi extra. So for field work during the summer I need to fill to 2500psi and let the temp raise it to 2600psi or 2700psi which is approaching my partial valve lock limit.
For a car at 140F from 75F I’d be looking at a 300psi extra. So the conventional wisdom of 2400-2600psi fill with a medium power wheel has some significant advantage in hotter climates.
And from 75F to 35F its approximately 140psi less, meaning a little extra helps.
Thanks again!