What is O2 cleaning?
O2 cleaning is the preparation of your cylinder and valve for the introduction of pure (100%) O2 into the cylinder. This does not however mean you are necessarily breathing 100% oxygen from the cylinder. One method of blending Enriched Air Nitrox, called Partial Pressure Blending, involved first introducing a volume of 100% oxygen into the cylinder and then adding standard air to the cylinder to have a final blend of EANx at your desired mixture.
For example: To create a standard AL80 cylinder of 32% Nitrox using Partial Pressure blending, you would first add 411 psi of 100% O2 and then add 2589 psi of air - the final product would be 3000 psi of 32% Nitrox.
O2 cleaning is the process of removing any hydrocarbons and other contaminents that are incompatible with oxygen and present a high risk of fire during the filling process. O2 cleaning is a multi-step process that involved carefully scrubbing the cylinders insides and threads and disassembling and carefully cleaning the valves individual parts to remove any traces of possible contaminents before thoroughly drying the insides, all the individual parts, reassembling and finally certifying the cylinder as O2 clean. For steel cylinders, there is an additional process of treating the insides with a rust inhibitor between the cleaning and drying stages.
O2 cleaning is a time consuming process that requires specialized equipment and chemicals and is always an additional expense to the visual inspection.
Do you need to be O2 clean?
In most cases no. Most filling stations these days use a process of continuous blending which mixes the O2 and air in a chamber before filling a cylinder with it, so in this case, if you are diving Nitrox blends of 40% or less - your cylinder does not need to be O2 clean.
There are however a few dive shops and filling stations that continue to rely on partial pressure blending to make Nitrox. If you cylinder is not O2 clean, you will not be able to get a nitrox fill from these locations. For recreational divers, not being O2 clean may mean that one or two dives that would have been traditionally done on Nitrox will be done on plain old air, but at a cost of between $50 and $70 per cylinder, the expense of O2 cleaning for minimal benefit may not be worth the time and effort of having your cylinders cleaned.
If you are diving richer oxygen mixtures than 40%, often in stage/deco bottles, then O2 cleaning is absolutely necessary.
Ok, I am O2 clean, now what?
If you have gone to the trouble and expense of having your cylinders O2 cleaned, you now have a responsibility of maintaining that O2 clean status. This means carefully choosing which filling stations you get your air from and doing your part by using ONLY oxygen safe lubricants on your cylinders and regulators. Silicone lubrcants ARE NOT oxygen safe/compatible and should never be used in conjunction with oxygen clean cylinders, valves or regulators. Lubricants like Tribolube, Christolube or Krytox should be used exclusively when equipment needs to be oxygen clean.
To keep your status as oxygen clean, you should only be getting air fills at stations using Grade D or modified Grade E air. These stations have filter systems in place that limit the potential hydro carbons and contaminents to very low levels and are considered to have oxygen compatible air. Any filling station should have air testing done regularly and have air quality reports posted where you can see them. Filling your cylinders from a compressor that is using simple Grade E air will violate oxygen cleaning standards and void the oxygen clean status of your cylinder(s).
Maintaining the oxygen clean status of your cylinders is a serious safety issue for both yourself and the techician(s) filling your cylinders and serious fire risk and even explosions can result from using oxygen with improperly cleaned or contaminated equipment. It is also important to note, that micro-combustions can occur inside the closed loop of a fill system that may not be noticeable during the filling process but could leave dangerous contaminents in your cylinders and in the air you will be breathing at depth and this is another reason that ensuring your maintain your oxygen clean status carefully.