You must be familiar with the ring shaped part of every cylinder, which is situated at the top for holding it. You will find the expiry date written in one of the three strips. You will find a character from A, B, C or D along with two digits.
A means First Quarter (January to March)
B means Second Quarter (April to June)
C means Third Quarter (July to September)
D means Fourth Quarter (October to December)
You will also find two digits after the above-mentioned character. The two digits actually represent the year. If you see the written character as B 13, you should not use the cylinder after April 2013. Cylinders after their expiry date are risky so one should always check the expiry date of the cylinder.
