You may know every cylinder has a testing date or the expiry date. It is written just beside the seal. You must see this date before taking it's delivery. You will see the letters A, B, C and D written along with date. It is the expiry date of the cylinder.
Letter A means from January to March. B means from April to June. C means from July to September and lastly D means from October to December. If you use the cylinder after this date, then it can be dangerous.
Do you know cylinder is temperature sensitive?
With the increase in temperature, gas may expand leading to the bursting of cylinder. So, to control the temperature, there is a hole on the bottom of a cylinder for the passage of air and to maintain the low temperature on the bottom side.