Oxygen Cleanliness & Compatibility FAQ

Oxygen & Nitrox

About O2 Cleanliness & Compatibility

Yes and no; this simple question has a complex answer because there is no widely recognized scuba diving standard for what constitutes suitability for oxygen service. Hypothetically, the equipment we sell could be used with oxygen if we describe it as Nitrox Ready, otherwise you should assume air only service. However, from the point of view of stringent U.S. aerospace guidelines and Compressed Gas Association handling standards for oxygen, nothing in general use in recreational diving is suitable for oxygen service. Using oxygen for diving requires you to make an informed decision to accept for yourself the risks of handing the oxygen. To help form your opinion about the answer read our article About Oxygen Disclaimers.

According to our suppliers, their Nitrox Ready products are oxygen compatible and oxygen clean from the factory, prior to initial use, for compressed gases greater than 23.5% oxygen. (Air is defined as containing from 19% to 23.5% oxygen.) However, most suppliers in the dive industry specify their Nitrox Ready products for use with gas mixtures containing a maximum of 40% oxygen as the upper limit. Although controversial, some dive industry professionals are of the opinion that Nitrox Ready products do not have to be dedicated to oxygen service and may be used interchangeably with CGA Grade "E" air as long as the Nitrox Ready product never contacts gas mixtures in excess of 40% oxygen.

To better understand the issues associated with handling oxygen rich compressed gas, please read Understanding Nitrox Ready.

The 40% specification has become widely used in the recreational diving industry to reduce risk to safety of the diver and limit manufacturer liability, yet indicate a product is (at least initially) oxygen compatible and oxygen clean. For more information please read About Oxygen Disclaimers.

Since Air is defined by the CGA as containing 19% to 23.5% oxygen, describing nitrogen-oxygen gas mixtures up to 23.5% as Nitrox is just an obscure way of describing ordinary Air. Gas mixtures greater than 23.5% oxygen are what most people assume to be Nitrox, and of course there is also pure Oxygen. To avoid conflict with various dive supplier labeling and any misunderstanding about exactly what constitutes Nitrox, Dive Gear Express sometimes uses the phrase "suitable for service with compressed gases containing greater than 23.5% oxygen" to describe products that are both oxygen compatible and oxygen clean prior to initial use, especially in reference to cylinders and valves.

Our suppliers tell us their equipment described as Nitrox Ready is already oxygen clean from the factory. The Compressed Gas Association standards state that equipment handling any Nitrox mixture in excess of 23.5% oxygen shall be cleaned to their standard as if the gas is pure oxygen, there is no lesser standard. That requirement is why Nitrox Ready equipment has already been oxygen cleaned by the manufacturer. SCUBA equipment is labeled 'NITROX READY' or '40% MAX OXYGEN' instead of 'Oxygen Service' because the manufacturers are managing the risks, both to your safety and their liability, associated with oxygen-related fires and explosions by specifying their SCUBA products for use only with breathing gas mixtures no more than 40% oxygen.

Nitrox Ready and Oxygen Service equipment from our suppliers is not just 'kinda sorta clean' or 'premix clean', it is oxygen clean. The practice by shops and individuals of aftermarket cleaning for new dive equipment that is already oxygen clean from the factory is pointless at best and dangerous if done improperly. Prior to initial use, no amount of aftermarket cleaning can make a Nitrox Ready product more oxygen clean nor can cleaning it ever override the manufacturer specified limits. The manufacturer can't stop you from using their equipment with breathing gases and pressures that exceed their specifications, but doing that requires you to make an informed decision to accept for yourself the risks of handing the oxygen.

There is no visual inspection method to be sure an O-ring is oxygen compatible, only specialized materials testing equipment can make that determination. To ensure oxygen compatibility, O-rings must be specified as oxygen compatible at time of purchase and then segregated from non-compatible O-rings. There are no O-ring color coding standards and thus color or appearance is not an indication of the material or quality. O-rings are available in nearly any color, although most commonly seen in black regardless of material.

Even though most fluorocarbon elastomer (aka FKM, aka FPM, aka Viton) O-rings are black, the loose FKM O-rings sold by Dive Gear Express are oxygen compatible and usually brown or green; simply to make them easily distinguishable from more common acrylonitrile butadiene rubber elastomer (aka NBR, aka Nitrile, aka Buna-N) O-rings which are usually black and not generally considered suitable for oxygen service. However, the oxygen compatible O-rings inside equipment (valves, regulators, hoses, service kits, etc.) are almost always black. O-ring materials also can not be distinguished from each other by their surface appearance, more a function of the manufacturer than the material. NBR is incorrectly said to have a shiny surface and FKM a matte surface. This is not a reliable indicator, and certainly not true if the O-ring has been lubricated.

Just to be clear, the COLOR and SURFACE APPEARANCE of the O-ring is MEANINGLESS regarding their compatibility with oxygen.

The inlet and outlet on our oxygen compatible DGX Premium O2 Valves are oxygen clean copper metal alloy (aka raw "brass") which will very quickly exhibit some discoloration and spotting after cleaning that that is not present with electroplated chrome finishes typical of other SCUBA valves. Raw brass that is not coated with some kind of sealer (which would defeat the purpose of the valve) will discolor very rapidly, thus appearing aged or used.

If you consider the cosmetics to be important, and maximum compatibility with pure oxygen is less of a concern, then we recommend choosing our standard Nitrox Ready DGX Premium Valves over the DGX Premium O2 Valves.

DOT regulations currently limit the oxygen fill pressure of aluminum and composite cylinders to 3,000 psi at 70F. There are no similar DOT fill pressure limits for compressed oxygen gas in steel cylinders, but as a practical matter pure oxygen fills exceeding 3,000 psi are highly unlikely, regardless of cylinder composition or service pressure. In the US, almost all bulk compressed oxygen available to local dive shops is aviation grade in steel DOT-3AA-2400 cylinders with a CGA-540 valve at a service pressure of 2,640 psi. Since most oxygen fills by local dive shops are done using the cascade method, a typical local dive shop pure oxygen fill in a rebreather or decompression cylinder will be between 1,800 psi and 2,400 psi unless the shop has an oxygen booster.

The DOT considers any gas containing more than 23.5% oxygen to be pure oxygen for the purposes of handling. Thus, knowledgeable fill station operators will not fill aluminum cylinders with Nitrox higher than 3,000 psi. This means the 3300 psi "compact" or "neutral buoyancy" aluminum SCUBA cylinders will receive a short fill. Again, there are no such restrictions on Nitrox in steel high pressure SCUBA cylinders with a service pressure of 3442 psi. Although there are some older steel SCUBA cylinders with a service pressure of 3500 psi, recent DOT regulations limit the transport of such cylinders to a maximum fill pressure of 3442 psi as well.