Shooting Science

BY SFC HANK GRAY, U.S. ARMY MARKSMANSHIP UNIT AND PAUL BORTHWICK, PH.D

Under Pressure: Why You Can’t Get Your Air Cylinder Full

A little while back, I was standing in line to get my air cylinder fi lled at a competi- tion and I struck up a con- versation with fellow shooter Paul Borthwick about how warm the cylinder gets after fi lling it up. I’m sure anyone who shoots air rifl e has expe- rienced this phenomenon. Have you ever wondered why this is and what is happen- ing inside the cylinder? How about noticing that if you let your cylinder sit a couple hours and cool back down, it is no longer “full?” Is there something wrong with your cylinder? Is it leaking? As it turns out most of these questions can be answered with a little bit of math and science using some thermo- dynamics and the ideal gas law.

Due to Paul’s expertise

in this area (he has a Ph.D in Mechanical Engineering specializing in Fluid Me- chanics and Thermodynam- ics) and my background in biochemistry, we decided to delve a little deeper into those questions just for “fun.” Paul worked out the thermo and math to develop a little chart describing what one should expect from their cylinder upon fi lling it. He even went as far as record- ing some experimental data to verify the results with a Feinwerkbau 800 air rifl e cylinder.

Without getting too deep

into the calculations, here’s an explanation for what is happening with the cylin- der. The ideal gas law states PV=nRT; where P is pressure, V is volume, n is the amount of gas (air), R is the “ideal gas constant” and T is tem- perature. If things change on one side of the equation, then they must also change on the other side to keep ev- erything equal. When you begin to fi ll your

cylinder, the air already in it is compressed by the new air causing the tempera- ture to rise. This increase in temperature along with the additional air being added to the cylinder causes the pressure to increase. You

get to watch this happen as the gauge on your cylinder increases towards 200 bar. Most of the time you will also feel the cylinder start to get warmed by the air inside. After

you remove your

cylinder from the “fi lling station,” the volume (V), amount of gas (N), and ide- al gas constant (R), do not change. That means as your cylinder cools back down to room temperature,

the

pressure must also go down to

keep everything equal

in the ideal gas law equa- tion. The result is eventually your cylinder will no longer show “full” at 200 bar even though you haven’t taken a single shot.

Take a look at the chart

where we assumed the cyl- inder was at 70ºF before fi ll- ing and notice the warming effect on cylinders of varying levels of “emptiness.” There really is a pretty substantial drop in fi nal pressure once the cylinder has cooled; es- pecially if your cylinder was really low on air. You might even consider fi lling your cylinder twice, a few hours apart, after air travel since it should be totally emptied out for fl ying. A completely empty cylinder fi lled to 200 bar will level out at just 144 bar once it has fully cooled — that’s barely enough to get through a match com- fortably!

One might also notice the temperature of March 2015 | USA Shooting News 45

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