Scuba cylinders that are used for Nitrox/Trimix must be specially cleaned for use with oxygen.  This is because nitrox mixes are typically prepared by emptying the cylinder first, then adding 100% oxygen to the required amount before topping up with air to get the right mix.   Oxygen cleaning must be carried out every 18 months due to the risk of contamination. 

In order for a cylinder to be filled with Nitrox / Trimix it must have a valid oxygen test label.  If this is not present, or it is out of date, then the cylinder will not be filled with Nitrox/Trimix. 

During the oxygen cleaning process, the cylinder is washed, cleaned to remove all traces of hydrocarbons, and the valve refitted with oxygen compatible (viton) o-rings.