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the air in the cylinder gets very high as well. Aluminum transfers that thermal en- ergy fairly effi ciently which is why you can feel it so eas- ily after fi lling your cylinder. The good news, however, is the aluminum will never get as hot as the air inside the cylinder so you don’t have to worry about getting burned. According to Paul’s math, an aluminum cylinder start- ing out empty will get up to about 100ºF, which is only about as warm as a hot tub. On another note, you’ve


probably experienced the opposite effect of all this when emptying your cylin- der. It cools down when you let the air out quickly. The same laws of thermodynam- ics and ideal gas law are working in the other direc- tion.


So, why do we care about


this and since when did USA Shooting News become a science magazine? The real reason for writing this ar- ticle is to save people from spending money or worry- ing their cylinders are faulty and leaking. While that is a possibility, we would recom- mend letting your cylinder sit for a few days before decid- ing it was leaking. Once the temperature has stabilized, it really shouldn’t change pressure after that. We would also suggest shooters


fi ll their cylinders as early be- fore the match as possible. I prefer to fi ll mine right after training or as soon as I’m done for the day. That way it is ready to go the next morn- ing and I don’t have to worry about it before my match. If you fi ll it immediately before shooting, the pressure in your cylinder will be chang- ing rather quickly while you are shooting as it cools back down to room temperature. While the regulators in our air rifl es should be able to handle this, every tenth of a point counts these days and I don’t want to leave room for any chances. One last note I’d like to make is that some cylinders are made of steel. The temperature change in- side these cylinders is the same as the ones made of aluminum; however, it won’t be as easy to feel on the out- side. Even if you don’t really


care why your cylinder gets hot while fi lling it up, at least now you can impress your friends by explaining what is going on while standing in line to get air at the next match. If anyone wants to know


more about all of this or has questions please feel free to email me at henryhank- gray@yahoo.com or Paul at rpborthw@yahoo.com.


Cylinder pressure after cooling to room temp. 200 190 180 170 160 150 140 0 25 50 75 100 125 150 Cylinder Pressure Before Filling (bar) 175 200


Effects on a cylinder filled to 200 bar


Cylinder Air Temperature


Pressure Before Fill (bar) 1


75


100 125 150 175 195


Immediately After Fill (˚F) 278 183 156 132 109 88 72


Pressure After Cooling


to Room Temperature (bar) 144 165 172 179 186 193 199


About the Authors Henry “Hank” Gray is a Sergeant First Class assigned to the U.S. Army Marksmanship Unit in Fort Benning, Georgia. He has


a B.A. in Biochemistry and has shot competitively for 23 years, including numerous World Cups and two World Championship teams. Paul Borthwick, Ph.D. is a Mechanical Engineer, who owns a small engineering company near Baltimore, Maryland. He cur-


rently is engaged with the National Institute of Standards and Technology studying climate change issues. Paul has been in- volved with competitive smallbore and air rifl e for more than 40 years.


46 USA Shooting News | March 2015


Final Cylinder Pressure (bar)


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