FPS affected by elevation and humidity?

[smokingpancake 25]

I've noticed my guns chronoing at high fps at D-Day Adventure Park. I've had this happen with several AEGs. For example, chronoing at 390 fps at my home elevation of 3600', but chronoing at 415 at DDAP which is about 600' elevation. May be something to do with the change in air density or the increased humidity swelling the orings to give a better seal. Anybody else run into this issue?

[Mikey575 41]

Remember mine last year? My masada went from 388 w/.20 here, to 422 w/.20 when we got there.

[Guest Specs]

It might be a difference with air pressure within the gun and the moisture levels in that air. Though I would think that increased water in the air would decrease FPS, so that's an interesting find.

[Mikey575 41]

It might be a difference with air pressure within the gun and the moisture levels in that air. Though I would think that increased water in the air would decrease FPS, so that's an interesting find.

Yeah I was extremely surprised last year, thank god for quick change spring gearboxes.

[Dave 781]

Lower altitude = higher atmospheric pressure

 

Easy to read chart is a bit lower on the page; I recommend clicking the image.

[smokingpancake 25]

Yep, higher atm pressure at lower elevation, thus denser air. Factoring this in, the gun should actually shoot slightly slower since there would be more drag on the Bb due to the "thicker" air. The increased humidity factor makes more sense so far.

[Mikey575 41]

My Block.II is shooting 380 right now, hopefully it doesn't jump up like the Masada did.

[moonball43 33]

If the air is more dense, would there not be more behind the BB?

[Wolf 11.10 7]

I'm pretty sure that Moonball is right.  The increased atmospheric pressure means that the air being pushed by the piston to fire the bb will have more actual mass to compress, making for a higher pressure in the piston and thus a higher FPS, but it also will most likely lead to decreased performance over longer ranges because of increased drag on the bb.

[moonball43 33]

Holy crap!! Mark this day on the calendar!!!! I got something right!!!! I'm going to go drink a beer now!!!

[smokingpancake 25]

Yes, but it would be traveling at a slower speed due to the denser air in front of the bb too. This would create a larger pressure buildup in front of the Bb and slow the piston's travel speed.

[Wolf 11.10 7]

The difference being that the air in front of the bb has an easy escape through the muzzle, making it similar to a cylinder with really horrible compression, which would counteract the denser air in the piston a slight amount, but the net would be an increase in velocity.  I'm not saying you're completely wrong, it does play into the equation, but the air density would be the reason for the increased muzzle velocity.

[smokingpancake 25]

You guys may be right, but I'm starting to think there are too many variables to say for sure (air mass in cylinder, compressible fluids (air), piston velocity, bb weight, etc.). Makes me wish I still had access to the simulation software I used to work with a while back.

One way or the other, folks from the mountains might have some issues at the chrono this weekend.

[Wolf 11.10 7]

Like Mikey said, thank god for quick change spring gear boxes.  I'm already shooting a bit hot with a shortened m120 so I'll be using about half of one down there.

[Mikey575 41]

Like Mikey said, thank god for quick change spring gear boxes. I'm already shooting a bit hot with a shortened m120 so I'll be using about half of one down there.

Why not short stroke or change the cylinder? I was at 425 with an M125 short stroked 2 teeth (amazing compression) so I short stroked one more tooth and moved my full cylinder to a type 1(?) ported cylinder.

[Wolf 11.10 7]

Alright, thanks. I'll try that.

[Dave 781]

You guys may be right, but I'm starting to think there are too many variables to say for sure (air mass in cylinder, compressible fluids (air), piston velocity, bb weight, etc.). Makes me wish I still had access to the simulation software I used to work with a while back. One way or the other, folks from the mountains might have some issues at the chrono this weekend.

 

 

You don't need simulation software, the equations are pretty simple.  P=F/A, F=M*A, F=-k*x (Hooke's Law),

 

Weight, air density, gas compressibility, size of piston, spring constant, etc. are all known variables.

 

The only one I can't recall offhand is air drag, but it's only a ~14" barrel or so; I can't imagine it would make a difference at muzzle.

[Doc Dodge 666]

[SWO_voodoo 0]

just gonna leave these here

http://mackila.com/airsoft/atp/01-b-01.htm

http://mackila.com/airsoft/atp/05-b-01.htm