As the temperature of a gas begins to increase the molecules will begin to expand. If this occurs in a diving cylinder the pressure exerted against the internal walls will increase.

Because the cylinder is an inflexible container it will appear as a rise in the internal pressure of the tank.

A good rule of thumb if filling a cylinder or diving in cold environments is that every +/- 1 degree C will transfer to approximately 0.6 Bar pressure change within the cylinder.