Commercial mixed gas diving usually involves helium-oxygen gas mixtures prepared by commercial gas supply companies or mixed on the dive site by highly qualified life support technicians and dive supervisors. The large supply cylinders used are generally prepared by mixing gases from a high-purity supply, and the final mix is analyzed multiple times to check that the composition is correct. This process is costly in time and equipment.
The gas that fills the cylinders used in military and recreational technical mixed gas diving is generally sourced from large cylinders and then transferred into the smaller cylinders used by the diver. The mixture is often blended on site as each cylinder is filled.
In the recreational diving industry where nitrox is commonly used, oxygen is generally sourced in large cylinders from a commercial supplier. The oxygen is decanted into the cylinders, where the pressure is measured, and the cylinder is ‘topped-up’ with air to a pre-determined pressure to produce the mixture required. The mixture is then analyzed for its oxygen content. This procedure is known as ‘partial pressure blending’. If the gases supplied were of sufficient purity, significant contamination should be prevented. (Purity of supplied gas can vary, especially in some countries with lower standards of oversight). It is essential that the oxygen content is measured using two analyzers to verify the final mixture. The diver should also analyze the gas to ensure there has been no error. Failure to do so has sometimes been associated with serious consequences.
Another method of mixing (known as ‘continuous flow blending’) involves the air being first passed through a membrane system that removes some nitrogen and so creates a higher-oxygen mixture. This is then compressed. To reduce the risk of fire or explosion, the mixture must have an oxygen concentration of 40 per cent or lower. An alternative method of continuous flow blending for nitrox is carried out by introducing metered amounts of oxygen into the intake airflow stream of the compressor via mixing coils. It is important that the gas is mixed thoroughly before entering the compression stage to ensure that the oxygen concentration is lower than 40 per cent. The output is sampled and adjusted until the desired mixtures are obtained. This process can be used only on suitable compressors and to a maximum of 40 per cent oxygen concentration.
Trimix can be produced by combining helium with oxygen and air or a suitable nitrox mixture. Accurate measuring equipment is required, as are suitable valves to control the flow of gases. Once the trimix is produced, it is rolled or agitated for an hour or so to allow the gases to homogenize. To confirm the final mixture the oxygen content needs to be measured, although, ideally, the helium content should also be analyzed.
In any of these systems where air is one of the source gases used, or where a mixture is blended and then compressed by a high- or low-pressure compressor, the same potential sources of contamination exist as for air. Potential problems are multiplied by the complexity of the system and the fact that lubrication oil life can be much reduced through oxidation by oxygen-rich mixtures.
As an alternative to compressors, oil-free reciprocating ‘booster pumps’ are sometimes used to avoid these risks and to avoid the risk of oxygen fire and explosion when filling cylinders with oxygen-rich mixtures.