Eanx

Higher percentages of oxygen for recreational diving are currently prepared in two ways:

Enrichment - partial pressure mixing, continuous blending and pressure swing absorption that add oxygen during the filling process and require oxygen cleaning.

Enhancement - Permeable membrane gas separation that removes nitrogen from the air and requires no cleaning.

Premixed nitrox purchased from commercial gas suppliers is also an alternative, but at this time there are only a few supplies in only a few areas. The US based Food and Drug Administration (FDA) requirements prevent this from being widely available for recreational diving in the US, for example.

The most basic method of partial pressure mixing seems very easy and foolproof. Mathematic calculations, mixing tables, and triangles for EANx32 and EANx36 allow a trained blender to calculate oxygen and air amounts for empty and partially full oxygen clean cylinders. By putting a small amount of oxygen into a cylinder and then topping with air, the blender can achieve fairly accurate results. Before use, the mixture is checked with an oxygen analyzer and corrected as necessary. While this is a simple method of obtaining nitrox, the risk of highly volatile oxygen coming into contact with hydrocarbons and contaminants found in air equipment makes it potentially hazardous. It is also mandatory that cylinders receiving oxygen be cleaned for oxygen service.

Use with oil lubricated compressors requires connecting the existing compressed air system into a double filtration, partial pressure mixing system that removes excess hydrocarbons that might cause combustion when exposed to pure oxygen. By feeding compressed air from the storage banks into a second filtering system and a particulate matter, volatile contaminants are reduced. Specially constructed filter banks are available through companies such as Lawrence Factor Filtration System. The air then passes through nonreturn valves to prevent oxygen in the next part of the system from back flowing into the air system.

A predetermined amount of oxygen taken from a storage flask passes through a high pressure pump and into oxygen service rated storage cylinders. Compressed air is then pumped into the cylinders to bring them to the proper gas percentages. The resulting enriched air is then analyzed for the proper mix and then adjusted accordingly. It can then be used to fill a scuba tank.

Even though most hydrocarbons and contaminants are removed, using an oil based compressed air system with oxygen still presents the possibility of direct contact with oxygen. Equipment must be continually monitored for safety.

The US based National Oceanic and Atmospheric Administration (NOAA) Nitrox Continuous Mixer/Blender system developed by J. Morgan Wells, Director of NOAA’s Diving Program, combines oxygen and air into nitrox mixes that can be delivered into a diver’s tank or storage cylinders.

The blender works by feeding oxygen from high pressure O2 cylinders or pressure swing absorption systems discussed below into an oxygen injection system. Here, a series of precalibrated valves determine the amount of oxygen injected into the normal air intake flow. The mixture then passes through gas mixing coils into an oil-free high-pressure compressor system.

When the blend exits the compressor, it flows into a distribution and analysis system. During this phase, a flowmeter is opened so gases in the coils and compressor can be tested with an oxygen analyzer. This allows the mixing technician to make any adjustments in the O2 injection system. Once the blend is correct, It is routed through the distribution panel to storage cylinders or into a scuba tank properly cleaned for oxygen service.

In this system the operator must take extreme care to ensure that the facility’s air system is oil free and cleaned for use with oxygen. If equipment and the compressor system aren’t continually monitored and cleaned, a low-level buildup of contaminants can create residues that cause oxygen ignition problems.

The Pressure Swing Absorption machine replaces the oxygen storage flasks used in the tank. Oxygen exits through a pressure regulator/flowmeter mechanism that controls the oxygen ratio delivered. This is adjusted with a flowmeter knob until a ball inside a sight glass is centered on the required flowline. The oxygen is then fed into the compressor’s air intake. The PSA machine eliminates safety hazards such as leakage, storage and transport associated with bottled oxygen. It does require that oxygen be blended with air before it travels into a compressor. This makes an oxygen clean system tank.

The PSA machine eliminates safety hazards such as leakage, storage and transport associated with bottled oxygen. It does require that oxygen be blended with air before it travels into a compressor. This makes an oxygen clean system absolutely essential.

NOTE: The manufacturers of PSA systems state in the user’s manual that the oxygen produced meets industrial applications and not used for "respiratory" medical applications.

Permeable membrane technology is the only EANx system that doesn’t require oxygen cleaned equipment or technically trained personnel. The design selectively separates slow gasses such as nitrogen from fast gasses like oxygen. Instead of enriching air by adding oxygen, the system enhances the air by removing the nitrogen. The resulting Denitrogenated Air Nitrox (DNAx) is achieved in the following manner:

Air from a feed compressor or air storage banks flows into an inline filter canister system that removes hydrocarbons and other contaminants. It then passes into a membrane canister containing thousands of hairlike, hollow membrane fibers. Oxygen, which is more transferable than nitrogen, permeates across the membranes at a highly manageable rate.

The oxygen to nitrogen ratio is determined by a needle valve installed after the membrane. This allows the operator to dial out the nitrogen. Increasing the flow increases the oxygen ratio and slowing it decreases the oxygen percentage of the blend flowing out of the canister.

Once the operator determines the desired ratio and confirms the content with an oxygen analyzer, the EANx goes into the compressor and directly into a storage bank or cylinder. There are no air and oxygen mixing coils and no six hour wait for the gases to mix before use.

Unlike oxygen mixing and blending setups where high oxygen concentrations require oxygen clean equipment, this system never has more than 40% concentration, reducing oxygen fire hazard concerns. In fact the nitrogen by-product is delivered into the crankcase of the compressor to eliminate oxidation and extend lubricant and compressor life.

EANx can increase continuing education profits, increase sales, and enhance the safety of the instructional staff. However, a dive operator should consider the following before offering EANx

Enrichment systems producing EANx gasses for diving require that pure O2 flow through at least a portion of the system. While this seems like a minor consideration, the US Food and Drug Administration (FDA) regards oxygen intended for humans as a drug that cannot be purchased without a prescription. The FDA also requires that high-pressure systems containing oxygen for human use be clearly labeled as such. While the FDA has not enforced regulations concerning oxygen supplies, this situation may change as more divers in the US seek EANx certification.

Other enrichment systems considerations that must be addressed by a facility owner include:

It is mandatory that enriched air be tested with an oxygen analyzer by both the technician and the end user. No matter what blending system is used, the diver must log the mix by name, date, initial percentage of oxygen, maximum depth for the mix, tank psi, and tank number.

Analyzing needs and selecting an appropriate EANx gas supply system allows recreational dive operators to meet the demands of dive professionals and the evolving diver who wants the advantage of enriched air.