Instruction: Answer the given question by extracting information from the supporting document.

Input:
Question: I need to take this on a plane.  How do I convince the airline that it is empty?
Document: I purchased this cylinder for my co2 system on my aquarium. I am very happy with my purchase. I take this in to my local welding shop and they fill it for me in less than 5 minutes. Cost me $10.00 to fill. The date on the tank was only a few months old so I have a long time before I need to have it tested. This tank looks nice and is very well made. I would definitely buy one of these again.
It's hard to review an empty gas tank because when you take it to your gas supplier, they take this tank and replace it with another full tank. It was really hard to let this one go - when brand new, this aluminum tank is a thing of beauty. Fortunately they gave me one that was quite comparable. I think this nice tank shamed them to not give me a crap one in exchange!
I can't really review the actual product since it ships empty and I have to exchange the tank for a full one. My recommendation would just be to go and find the place that will do the swap and buy the canister there. Buying a nice shiny new one is pointless when you have to trade it in right away. The only reason I'm giving it three stars is because I missed the clause where it says that is empty- I feel I have to take some accountability for my oversight.
I like this tank just as it was online. note the top part is plastic and could break and you will need a tool if this happens.
High quality, with a brand new hydro stamp. Unfortunately I had to surrender it to get it filled (my local vendor would only do an exchange). Overall, purchasing this was cheaper than purchasing it locally, so it was a great deal.If you need a cylinder, this one is great even if you have to exchange it.
To be entirely honest, I simply traded this empty bottle for a full bottle at my local CO2 emporium. To it's credit, the folks that wound up with it were very pleased and had no qualms about taking it in exchange.
I bought this to make my own carbonation system and it's just fine. My local gas supplier has an exchange program, so you just bring in your empty tank (it needs a current hydro-test date of course) and they give you a full one for just the price of the gas. I used this as my trade in.
It holds co2...works as intended so I cannot complain! I did not have any leaks in my tank. Remember guys, these tanks ship empty (its the law) so before you order one, make sure theres a place around you that can fill it. A paintball, welding supply store or a firehouse supply store will be able to do that for you.
A CO2 tank is a tank is a tank - I'm not sure what I saved - maybe $0-$20+. The fact that AigGas didn't question the quality of the cylinder rates at least four stars. Plus one star for Amazon selling at a competitive price regarding other vendor's empty 5# aluminum cylinder offerings... But always shop around. :-)
I purchased this empty 5# aluminum cylinder and was asked to trade it in at AirGas for their 5# filled CO2 aluminum tank - Their tank looks exactly like the Amazon tank.So how much did I save? Not sure - The AirGas salesman said the tank is the most expensive part of their transaction - But AirGas does not publish their prices on-line for either filled or unfilled CO2 aluminum tanks.
Answer: At least in the US, there are TSA rules on bringing compressed gas cylinders.. it doesn't just need to be empty, it needs to have the valve head completely removed so the inside of the tank can be examined visually.http://www.tsa.gov/traveler-information/compressed-gas-cylinders