NITROX the Problem
The primary issue here is the introduction, even temporarily, of pure oxygen into the scuba cylinder. Pure oxygen can create flammable or explosive situations that would not be possible in regular atmospheric air. Substances that would not burn in atmospheric air burn freely in the presence of pure oxygen. Routine, everyday substances become ignition sources in a pure oxygen environment. This was demonstrated clearly in the space program fire of Apollo I, where Velcro, a popular and common substance, became explosive when used in a pure oxygen environment during capsule testing.
In the scuba cylinder and its valve, the issue is the presence of hydrocarbons, or oil. When exposed to pure oxygen, hydrocarbons have a much lower flash point. To safely use a scuba cylinder for partial-pressure filling of Nitrox, special precautions must be taken to ensure that all hydrocarbons are removed from the cylinder and valve prior to use in the Nitrox filling operation.
What is hyper-clean air and can I fill my non O2 clean cylinder with it?
Oxygen compatible air and modified grade E air are both basically the same as standard Grade E air (standard scuba air), but are required to contain much lower oil/hydrocarbon levels. Indeed Oxygen compatible air is virtually free of hydrocarbons and contaminants. This is to ensure that when Oxygen clean tanks are filled we minimize the contamination to the cylinder and valve by using this "extra/hyper clean" air.
Many dive shops use an extra filter to create Oxygen compatible air, and this is fine. The problem arises when the air is not actually tested to this standard. All dive shops are required to test their air several times a year, but many fail to test their Oxygen Compatible Air.
The easiest solution would be to have all the air coming from the compressor tested to the Oxygen compatible air standard. The standard is easy to achieve with standard breathing air compressor systems if one is diligent about filter and oil changes and uses synthetic compressor oil. Simply always testing to this standard means all the air coming from the compressor is Oxygen compatible and of the highest quality.
Any scuba cylinder can be filled with Oxygen compatible air. However, if a dive shop charges more for this service then you are wasting your money if your tank is not already O2 clean since standard Grade E air is just fine for non O2 clean tanks.
Keeping Your Cylinder Oxygen Clean
Maintaining the cleanliness integrity of an oxygen cylinder is also critical. Any subsequent introduction of air that is not "oxygen compatible air" contaminates the cylinder and it is no longer suitable for partial pressure filling.
Most scuba stores will not fill a Nitrox banded cylinder with normal air, unless that air comes from a Nitrox production facility, in which case it is identified as 21% Nitrox.
However, the practice of filling Nitrox cylinders with standard breathing air (CGA Grade E) is becoming more common place. This presents a clear problem to a facility that subsequently completes a partial-pressure fill on that cylinder.
We caution our customers to be very careful when obtaining Nitrox fills, or when obtaining air fills or top ups, in their Nitrox cylinders. Cleaning is expensive, and we don't want you to waste additional money because you get an "improper" fill in your oxygen clean cylinder.
Should I treat my O2 clean cylinders differently?
Yes. You should do everything possible to ensure they stay O2 clean, and only ever fill them using O.C.A (Oxygen Compatible Air)
Standard scuba air, although very clean and dry, may still contain trace amounts of hydrocarbons and other contaminants. If allowed to collect on the internal surfaces of cylinders, regulators and valves, these create a risk of fire and/or explosion. This is a risk that O2 cleaning and the use of Oxygen compatible air should eliminate. But if you get a standard scuba air fill your O2 clean setup may become contaminated and this is potentially dangerous if you subsequently get a Nitrox mix at a dive shop using partial pressure blending where your equipment will be exposed to 100% Oxygen.
Oxygen Compatible Air, on the other hand, is virtually free of hydrocarbons and contaminants. By making certain that the internal surfaces of cylinders, regulators and valves only come in contact with Oxygen-compatible air, or gas mixtures created using Oxygen-compatible air, you effectively eliminate the risk of fire or explosion, and maintain your equipment's O2 service rating.
It is impossible for a dive operator to determine, simply by looking at cylinder markings that indicate the cylinder was O2 cleaned within the past year, that a cylinder and valve have not been exposed to air or gas mixtures that would invalidate its O2 service rating. Thus, it is the responsibility of the divers who own or use this equipment to:
- Only allow this equipment to come in contact with Oxygen compatible air or gas mixtures, or
- If the equipment does come in contact with standard scuba air, to remove any markings that indicate it is O2 clean, and not represent to dive operators that any such cylinders are suitable to be filled with Nitrox through in-cylinder partial-pressure blending.
Nitrox divers often have their cylinders refilled with air for dives in which having Nitrox may not offer any appreciable benefit. If you do so, and if your cylinder is O2 service rated, you must use only Oxygen Compatible Air.
Be Vigilant About O2 Clean
Eventually all pieces of equipment that once were oxygen cleaned will degrade and become contaminated. The problem is that we just don't know when the contamination threshold has been crossed to create a potential dangerous situation. So regular servicing and O2 cleaning of your equipment is a must and it is recommended that any time contamination is suspected, to have the equipment serviced. Hoping that a piece of dive equipment is still clean enough is a thought that only belongs in utopia and fantasy land.