noob question about gas canisters

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Hello,  I'm not sure if I should say something or not.  So I bought a 40CF canister at IOC, and then figured out that the local supplier only does exchanges, and won't fill up the tank I bought.  Fine.  I got it home before I realized that the tank they gave me was actually smaller than the one I gave them.  The markings are the same (D.O.T. 3AA2015), and it's got the same diameter (7").  According to  this site, a 40CF should be about 22" tall, and the canister they gave me is 21" tall.  So now I don't know exactly what I had from IOC.  I was bigger than what I exchanged it for, but not by enough to qualify it as a 55CF.  Do canisters of the same capacity just tend to be of slightly different dimensions?Thanks.
Reply:It's basically the same, probably made at a different time by a different company.HH 187Miller Bluestar1EAHP AlphaTig 200X
Reply:As said different manufacturers make cylinders that vary in size, but usually contain roughly similar amounts of gas. Exact diameter, wall thickness and height all affect the volume, as does working pressure. (D.O.T. 3AA2015), tells me that you have a steel cylinder rated at 2015 PSI. If you got a 3AA2265 cylinder, the size would be different slightly but would probably contain the same amount of gas roughly. Most of the O2 I get is in 2015 or 2265 cylinders, but occasionally I get a 2400 one. They all list out as the same on the invoice...No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:And I have found that just because your tank is rated at 2015, you may only get 1800psi. And the volume will be less than what the tank would have been if had been filled to 2015.There's a lot to it and generaly when I get my fills at Airgas, there is a sticker on the tank that says how much is actually in the tank. I learned a lot becoming a certified tank filler for scuba diving. I am certified for Tri-Mixes. Oxygen, Helium, and compressed air.Lincoln Power Mig 210MP MIGLincoln Power Mig 350MP - MIG and Push-PullLincoln TIG 300-300Lincoln Hobby-Weld 110v  Thanks JLAMESCK TIG TORCH, gas diffuser, pyrex cupThermal Dynamics Cutmaster 101My brain
Reply:Yep this time a year things a lot of things can affect fill pressures. If they store the tanks outside in the cold, or inside a warehouse thats 50deg. If they are filling fast, or slow. If they fill to the "plus" rating on the tank or not, and where the tank is stored after it was filled.A few years ago I noticed an interesting thing based on this. I'd filled a dive cylinder in the fall, and put it away. In preparation for a trip, we had some pool time scheduled in February, and I'd loaded the tanks the night before. Waiting for the pool to open at 7am I was rigging my gear, and noticed my tanks read 3100psi. Seemed low from what I had remembered filling them to in the fall, but it didn't really mater, I just thought maybe I'd bumped the valve a bit loading them. About an hour later in the pool the pressure was up to 4100psi! Outside air temp 14 deg. inside pool temp 90 deg. I knew this happened, but had never seen such a dramatic swing in pressure in such a short period of time. Even though I'd been using gas, the tank fill went up as the cylinder and contents warmed and expanded..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan