Detection of Breathing Gas Contamination

The accurate assessment of the concentration of contaminants is best left to specialists, such as air pollution analysts. The tests outlined give the user a reasonable assessment of air quality. Some tests should not be used for samples where a death or legal action may be involved because they require large amounts of air for an imprecise answer. In some countries, there is a requirement that air testing be conducted by an independent tester rather than by the supplier.

For most compressor operators, the purchase of an indicating tube gas analyzer system is a sound investment. These are made by Mine Safety Apparatus (Pittsburgh, USA), Auer (Berlin, Germany) and Dräger (Lubeck, Germany), among other companies. The devices operate by passing a metered volume of air through a glass tube filled with chemicals. These chemicals react with the contaminant and cause a colour change. A scale on the tube indicates the amount of contaminant present in the sample. Tubes from different manufacturers cannot be mixed because the tube systems use different flows and volumes of gas.

The oxygen concentration may be checked using an oxygen electrode, analyzer or indicator tube. This is particularly important for diving with gases other than air.

Carbon dioxide can be measured using an indicating tube or a variety of chemical and physical techniques. Infrared absorption is commonly used.

Oil and dust can be determined by filtering and weighing, with the increase in the dry weight indicating the weight of oil and dust. A solvent such as hexane may be used to dissolve the oil; the remaining weight is particulate matter.

This procedure requires an accurate balance and is not commonly used in the diving industry. An indication of the presence of oil can be obtained by directing a jet of air on to a clean sheet of white paper and then examining the paper under ultraviolet light. Some oils will fluoresce, although, despite some exceptions, synthetics generally do not. Indicating tubes can be used, as well as a newer dedicated device for detection of oil aerosols in compressed air.

Nitrogen oxides may be detected using indicating tubes. These tubes may also be used for detection of water vapour, but a method involving a measurement of the dew point is more suitable.

Combinations of gas chromatography and mass spectrometer systems have, until recently, been needed to obtain an accurate identification of trace contaminants in divers’ air. These expensive laboratory-based systems need a competent operator and a large stock of reference samples to give satisfactory service. Laboratories involved with air pollution measurement may be able to provide these facilities. Gas analysis technology is advancing rapidly, however, and there are now handheld photo-ionization detectors (PIDs) and modestly priced flame ionization detectors (FIDs) that can detect and/or quantify contamination at levels relevant to diving. Work is continuing on ‘gas detectors on a chip’ that promise to enable real-time monitoring of gases in the future, potentially at both the compressor output and within the breathing circuit, especially of rebreather equipment.

Treatment of Breathing Gas Contamination

For most of the conditions caused by contaminated air, the first step is to replace the contaminated air supply with an appropriate uncontaminated breathing gas. Underwater, this will mean reverting to an alternative source. On the surface, breathing fresh air may suffice in mild cases. However, rest, breathing high concentration oxygen and general first aid measures may be required. In more severe cases, resuscitation may be needed.

It is generally believed that patients with serious cases of carbon monoxide toxicity benefit from hyperbaric oxygen therapy, and this is widely used.

Treatment of nitrogen dioxide poisoning requires rest in all cases. This may prevent the condition from progressing. If the exposure was thought to be to a toxic concentration or if the patient develops further symptoms, then 100 per cent oxygen is indicated. If pulmonary oedema develops, it should be appropriately managed.

Prevention of Breathing Gas Contamination

Contamination should not occur if clean, dry air is pumped by a suitable, well-maintained compressor into clean, corrosion-free cylinders. Any deviation from this procedure will lead to the risk of contamination.

Prevention of contamination involves the use of suitable, well-maintained compressors, adequate filters, clean cylinders and regular analysis of the gas.

Filtering will be necessary to remove any contaminants introduced by compression. It will also be needed if the air compressed comes from a polluted area. Water removal will be needed in most situations. The choice of filtering agents and the frequency of replacement are the purview of a specialized field of engineering, and these issues should be considered with experts in the field. The following methods and agents are commonly used.

  • Silica gel, to remove water vapour.
  • Activated alumina, to remove water vapour.
  • Activated charcoal, to remove oil mist and volatile hydrocarbons.
  • Activated zeolites and molecular sieves, to remove oil and water.
  • Reverse flow or centrifugal filters, to remove solids and large liquid drops.
  • Hopcalite (a combination of manganese and copper oxide) acts as a catalyst to converts carbon monoxide to carbon dioxide.
  • Soda lime, to remove carbon dioxide.
  • Cryogenic cooling, to remove impurities with a higher boiling point (normally water and carbon dioxide).
  • Refrigerant dryer, to reduce final outlet temperature to a pressure dew point low enough to facilitate the condensation of most of the moisture (water and oil) and thus extend the life of the chemical reagents.

Some companies incorporate several filtering agents into a cartridge, thereby simplifying the servicing of the compressor.

The lifespan of some filters can be reduced by certain conditions. For example, activated charcoal is exhausted faster when it is used with nitrox rather than air.

Breathing Gas: Gas Purity Standards

Standards specify the composition and the maximum allowed concentration of contaminants in breathing air and for gases used in deep diving. Greater purity is demanded for gases used in deep diving because of the effect of higher inspired partial pressures for all gases in a mixture. Standards vary among organizations with respect to the acceptable level of contaminants, how these are to be detected and for which contaminants the gas should be tested.

Table 19.1 shows some of the available standards. Readers who may rely on these data should review the source documents for verification and more specific information. The standards are updated periodically, so it is important for users to monitor any changes and act accordingly.


Detailed consideration of the standards could lead to the opinion that most limits are rather conservative. Safety margins are incorporated for two reasons. First, the standards are generally based on extrapolation of the effects of the contaminants in isolation at 1 ATA (i.e. the surface equivalent value [SEV]). This may not be entirely valid for contaminants in combination at high pressures. Second, a safety margin will help to allow for any deterioration in the air quality among tests.

Standards are only lists of the maximum allowed concentration of some common impurities. Air may meet these specifications and still contain toxic substances. A greater variety of contamination problems occurs in caisson work where industrial equipment is being operated. Mineral dust from excavation and blasting is a common problem. Unusual contamination has also occurred in recompression chamber operations, especially if therapeutic or research equipment is used in the chamber. Because exposure times are generally greater in chambers, the toxic contamination has more time to exert its effect, particularly during saturation dives. Toxic substances that may be present include mercury from manometers, ammonia or Freon from leaking air conditioning plants, anaesthetic residues and other vapours from pharmaceutical preparations used during hyperbaric treatments. Potential hazards can be minimized by conducting an appropriate risk assessment.

In using gas mixtures prepared for deep diving, problems can result from the great pressure at which the mixtures are used. This will increase the risk of toxicity because of the higher partial pressures of contaminants. For example, and ignoring the dilution by water vapour in the lungs, at 1 ATA a carbon dioxide concentration of 2 per cent in inspired air means an inspired partial pressure (FICO2) of 15.2 mm Hg, and this is well-tolerated. At 5 ATA (40 msw), however, the same concentration means an FICO2 of 60.8 mm Hg – which will cause dyspnoea, increased work of breathing, distress and ultimately unconsciousness. The same problems will result from a concentration of only 0.2 per cent at 50 ATA. Therefore, standards need adjustment if they are used for depths and times greater than those assumed by their designers.

In the hyperbaric chamber environment (and elsewhere), toxicological problems can be introduced by the use of cleaning agents, among other sources. This is a growing problem with the rise in the prevalence of multi-resistant organisms that require extensive decontamination of therapeutic chambers between uses. The basic rule must be ‘if in doubt, leave it out’. Useful guidelines are available from reference to experience with long-term exposures in spacecraft and nuclear submarines. However, with the increase in hyperbaric treatment centres and greater reporting and communication among these centres, information on potentially problematic substances in a hyperbaric environment is more readily available.

The reasons for listing the components and the concentrations commonly specified are outlined in the following subsections:

Nitrogen and oxygen

The concentration of oxygen in compressed air standards is close to the level in clean, dry air. Any significant deviation is most unusual. If the nitrogen concentration was elevated, it could increase the risk of decompression sickness, narcosis or hypoxia. If the oxygen concentration was increased, the risk of oxygen toxicity, and fire hazards in hyperbaric chambers, would rise. The oxygen may be elevated by connecting to a bulk oxygen supply. This may be accidental, but it has been deliberate in the misguided belief that increasing oxygen concentration will increase the endurance available from a cylinder.

Carbon dioxide

A typical specified carbon dioxide level of 0.05 per cent (500 parts per million [ppm]) means that at 10 ATA the partial pressure of carbon dioxide would still be well below that required to cause any physiological effect. It has been argued that the British and Australian/New Zealand standards may be too strict because some other standards set a maximum limit of 0.1 per cent. This would not be toxic to the depth limits of compressed air diving and would be easier for compressor operators to meet.

The carbon dioxide level, even if it is within the specification used, should be considered in relation to the level in the ambient air. Global increases in carbon dioxide levels, further increased in cities and industrial areas, can cause compressor intake air to have carbon dioxide levels in excess of standards, even though they are physiologically safe. Some compressors use intake scrubbing of carbon dioxide from ambient air with absorbent canisters.

Carbon monoxide

This toxic gas binds tightly to the oxygen binding sites of haemoglobin to form carboxyhaemoglobin – preventing the carriage of oxygen. If sufficient haemoglobin binds with carbon monoxide, the diver will become hypoxic. The formation of carboxyhaemoglobin will also interfere with the transport of carbon dioxide away from the tissues by preventing its combination with haemoglobin. Carbon monoxide also causes oxidative stress and direct cellular toxicity.

The sometimes described cherry red colour of these victims is an unreliable clinical sign, especially in patients with cardiorespiratory impairment. Exertion and increased ventilation will hasten the development of symptoms. Subjects with a low haemoglobin level are more susceptible to carbon monoxide poisoning.

The concentrations required for poisoning are considerably greater than the maximum carbon monoxide level of 3 to 10 ppm specified in most standards. Exposure to ambient air with carbon monoxide levels higher that 100 ppm is considered dangerous to human health. A limit of 25 to 50 ppm is a suggested maximum level for occupational workers exposed for up to 8 hours a day.

For divers breathing air, the higher partial pressure of oxygen tends to protect against the effects of increased carbon monoxide partial pressure while at depth. The toxic limits of carbon monoxide at depth and how they are modified by varying ambient and oxygen partial pressures have not been established. Divers are probably at greatest risk of unconsciousness as they surface and lose the protection offered by the increased transport of oxygen in plasma that occurs at depth when the partial pressure of oxygen in inspired air is elevated.

A lower maximum carbon monoxide concentration is needed for deep and saturation divers. This is because the exposure times are longer. In addition, the oxygen partial pressure is usually limited to about 0.4 ATA, so the protection from an elevated oxygen pressure is reduced.


Oil occurring as a mist or vapour can cause compressed air to have an unpleasant odour and taste. Its direct, toxic effects in normal people are not known except that in high concentrations oil vapour can cause lipoid pneumonia. In some people, low concentrations of oil vapour can trigger asthma. Condensed oil, especially if combined with solid residues, can cause malfunctions of equipment. The other problem with oil is that it can decompose if overheating occurs and can generate hydrocarbons and toxic compounds of carbon, nitrogen and sulphur, depending on the oil composition.

Some compressed air standards distinguish oil from other hydrocarbons and specify maximum limits for each. Most hydrocarbons in high-pressure systems can be serious fire hazards. Some have other undesirable effects, such as being carcinogenic.


Control of water vapour is needed to reduce corrosion and oxidation damage to equipment. A low water concentration may also prevent ice formation and supply blockage or a free-flowing regulator when diving in cold water as a result of adiabatic cooling during pressure reduction. Some investigators think that this problem has been overstated because the areas susceptible to blockage are at lower pressure than the cylinder. In these areas, the air will not be saturated because the gas has expanded. Water condensation can also impair the efficiency of the filters used to remove other contaminants. This is more common with some of the molecular sieve filter systems.

Deaths have been reported from diving with steel cylinders containing water. Rusting occurs if these cylinders are left unused for long periods. Rusting consumes oxygen and leaves a mixture that caused death from hypoxia. The other problem is that the rusting process weakens the cylinder and may cause it to become an ‘unguided missile’ if the gas rapidly discharges. Severe injuries and deaths have been caused by exploding cylinders.

Solid particles

These particles must be controlled by filters to protect the diver and the equipment. The effect of the particles depends on their size and composition. Particles such as pollens can cause hay fever and asthma in susceptible divers. Pollens have been found inside scuba cylinders. Other particles have various undesirable physiological effects depending on their size and composition. Any dust that causes coughing could be particularly hazardous, especially for a novice diver.

In diving equipment, abrasive particles such as mineral dust would accelerate wear on the equipment by abrasive erosion. Soluble particles such as salt crystals can accelerate corrosion by promoting electrolysis. Organic dust can also contribute to a fire hazard. There have been cases of filters breaking down, letting material through and contributing particles of filter material to the air supply. Large concentrations of particulates can become a fire hazard.

Nitrogen dioxide and nitrous oxide

Some of the oxides of nitrogen, and nitrogen dioxide in particular, are intensely irritating, especially to the lungs, eyes and throat. These symptoms can occur when an individual is exposed to gas with a concentration of nitrogen dioxide greater than 10 ppm. At lower concentrations, the initial symptoms are slight and may not be noticed, or they may disappear. After a latent period of 2 to 20 hours, further signs that may be precipitated by exertion appear. Coughing, difficulty in breathing, cyanosis and haemoptysis accompany the development of pulmonary oedema. Unconsciousness usually follows.

The typical maximum level of no higher than 2 ppm is also the maximum allowed level for 24-hour exposure in some standards. If the effect is increased with pressure, then 0.2 ppm may be a more appropriate limit. In industrial cities, 2 ppm is often exceeded.

Nitrous oxide is an anaesthetic agent, but only at high concentrations. A low concentration of it is specified because, if nitrous oxide is generated within the compressor a precursor, nitric oxide must have been formed. Nitric oxide can also be converted to nitrogen dioxide at higher pressures and temperatures. Therefore, a compressor that adds nitrous oxide to the air being compressed can also form nitrogen dioxide.

Odour and taste

Odour and taste are controlled to avoid air that is unpleasant to breathe. They also provide a back-up for the other standards because if the air has an odour, it contains an impurity.

Volatile hydrocarbons

Volatile hydrocarbons such as benzene, toluene, xylene and ethane, among others, exist as gases at temperatures usual to diving situations and, as such, can be absorbed and distributed throughout the body in a similar manner to volatile anaesthetic agents. However, their side effects, such as impaired consciousness and increased cardiac irritability, present additional dangers to the diver. These gases may also be carcinogenic and present a fire hazard, so they need to be limited to a maximum of 5 ppm.

Most air purity standards do not require testing for these hydrocarbons, with the exception of the Canadian CSA 275.2 2004 standard (5 ppm of volatile non-methane hydrocarbons and 5 ppm of halogenated hydrocarbons); and the US CGA G-7.1-2011 standard (25 ppm of volatile hydrocarbons).


An experienced diver was diving in an area subject to tidal currents. He planned to dive at ‘slack water’ and anchored his boat a short time before the low tide. The hookah compressor was correctly arranged with the inlet upwind of the exhaust and the dive commenced. After an hour at 10 metres, the diver felt dizzy and lost consciousness but was fortunately pulled aboard by his attendant and revived.
Diagnosis: carbon monoxide poisoning, confirmed by blood analysis.

Explanation: As the tide turned, so did the boat. This put the compressor inlet downwind of the motor exhaust. The carbon monoxide from the exhaust was drawn into the compressor inlet and was breathed under pressure by the diver.


A 35-year-old man, with 20 years of diving experience and no relevant medical history, undertook a solo crayfish dive. He told the boat operator that he would be 15 minutes, but he failed to surface. A search by police divers found him the following day at a depth of 9 msw. The autopsy was limited because of destruction of the body by sea lice. The police investigation suggested that he was diving over-weighted with 17.5 kg on the weight belt, which was not released. All his equipment was intact and working correctly and the cylinder pressure was 194 bar, so he died very early in the dive.

Analysis of the cylinder contents revealed an extremely high carbon monoxide level, 13,600 ± 300 ppm (NZ standard <10 ppm), as well as increased levels of carbon dioxide and methane. A second cylinder owned by the diver returned similar analysis. Both cylinders were filled at the same time at the same dive shop.

The coroner’s finding was that ‘death [was] due to asphyxia due to his cylinder gas being contaminated with carbon monoxide, brought about by an idiosyncratic malfunction of the air compressing equipment’. There was no evidence of any other cylinders filled on that day reported as contaminated, so this was an isolated finding, the cause of which was unknown. (This case is from the New Zealand diving fatality data and was reported in Millar IL, Mouldey PG. Compressed breathing air: the potential for evil from within. Diving and Hyperbaric Medicine 2008;38(3):151.)

Breathing Gas: Sources of Contaminants

A contaminant in compressed gas may have been in the air before compression, added during compression because of some fault in the compressor system or have been present or generated within the storage system.

There are many potential contaminants in the atmospheric air taken in by compressors, particularly if the compressors are located in an industrial area or near any running internal combustion engines (e.g. near a boat jetty). Carbon monoxide and nitrogen oxides are components of polluted city air in levels that may be toxic, particularly when the inspired partial pressure is raised by breathing at depth. They may also enter the compressed air if the compressor is driven by, or operated near, an internal combustion engine that produces these compounds. Volatile hydrocarbons and organic compounds, such as methane, may also be present in environmental air.

Compressor lubricating oil may also contaminate the air if excessive oil vapour or even liquid oil passes around the piston rings to enter the supply and overload filters. This is most likely when the rings are damaged or if the air intake is restricted. In reality, some oil is present in the air delivered by even the best-maintained oil-lubricated compressor, and this may increase at higher temperatures. Hence, the drainage from the interstage and final aftercooler separators will always be oily. A well-maintained, modern compressor will have lower levels present in the compressed air produced, but an older, less well-maintained compressor will have higher levels.

More complex forms of contamination can also arise from within the compressor. High temperatures (‘hot spots’) and high pressures within the compressor produce an ideal environment for contaminants to form. Both these factors promote chemical reactions and hence the production of contaminants. If an unsuitable lubricating oil is used in the compressor, it may produce oil vapour, which may contaminate the air as oil, break down to produce volatile hydrocarbons or burn and form carbon monoxide. The same trouble can also result if the compressor overheats, causing ‘cracking’ (oil breakdown) or ‘flashing’ (oil combustion). The air becomes contaminated with volatile hydrocarbons or combustion products such as carbon monoxide and nitrogen oxides. Many low molecular weight volatile contaminants can cause some level of anaesthesia, and the effect is magnified by increased pressure and inert gas narcosis. This may lead to impaired cognition, a reduced seizure threshold and a greater potential for cardiac arrhythmias while diving. It has been suggested that these contaminants could be contributory to some morbidity and mortality in divers.

Overheating may also be caused by poor design or maintenance of the compressor – a restriction in the compressor intake, a dirty filter, excessive length of intake or a kinked intake hose can all cause problems such as overheating and reduced output.

Therefore, a compressor that may produce satisfactory air when tested running under normal circumstances may deliver contaminated air if it temporarily overheats. This may lead to part of a batch of cylinders being filled with contaminated gas, whereas others are not.

Other problems include leaks between the compressor stages, via piping, loose fittings or head gaskets or around the pistons.

Overall, the most common dangerous contaminant remains carbon monoxide, whether from excessive intake levels or from within the compressor. In some ways this is most dangerous when the air is otherwise clean because carbon monoxide is odourless and tasteless and will not be detected by the diver. Conversely, many potential contaminants have readily noticeable and generally unpleasant smell and taste and are obvious from the first breath.

Contaminants in compressed gas may have been present in the gas before compression, added during compression or resident in the storage system.

Some divers believe that using an electric compressor prevents carbon monoxide contamination. It removes one common cause of carbon monoxide – that of the driving motor exhaust. However, it does not reduce the other external and internal sources of carbon monoxide described earlier.

With more and more divers visiting dive destinations in developing countries, especially in the tropics, gas contamination is becoming an increasing problem. Problems are most likely when small, low-cost, compressors designed for filling a few cylinders are overused to fill cylinders for hours at a time in hot and humid environments, thus leading to overheating and ineffective filtration through water-saturated filters. These risks are compounded where there is lack of knowledge of appropriate purity standards, poor regulatory oversight or just lack of a proper air quality testing program.

Contamination from other sources

Air contamination from residues within the storage cylinder is not common. Residues of cleaning and scouring materials and scale formed by rusting can contribute vapours or dust if the cleaning operation is not conducted properly or if the cylinder or storage vessel is allowed to deteriorate. Water may be introduced into storage cylinders if there has been a failure of the drying system. There have been problems with paint systems that have been used to protect the interior of steel storage cylinders, where pressure cycling can cause the release of solvent vapour.

Lubricants used on regulators and reducers can sometimes cause fears of contamination. A diver may taste the lubricant in the regulator and think that the air is contaminated, and, if the wrong lubricants have been used, these fears may be justified. With enriched oxygen mixtures, only lubricants approved for use with oxygen should be used, to reduce the risk of fire.

The most difficult source of contamination to isolate is intermittent inlet contamination. For example, one report details a company that had an air compressor inlet on the roof (a not uncommon position). On rare occasions, its air was contaminated with organic chemicals. Only after much work and customer dissatisfaction was the source identified. A nearby factory sometimes used spray painting equipment with an exhaust fan. With a particular wind direction, these fumes would blow across to the compressor inlet.

There is a low risk of contamination in mixture diving when the source gases are produced by liquefaction. This reduces the risk of contamination because most of the potential impurities are separated by their higher boiling points. For mixtures prepared by mixing compressed air with other components, there are potential problems with the air and in the mixing process.

There is a possibility of increasing the concentration of trace contaminants when using recycled and reclaimed gases for deep diving operations and gases compressed in submarines. The compounds that are not removed in any purification process can accumulate with recycling until they are at level that causes a problem. For this reason, a thorough risk analysis should be done to determine likely contaminants, including those that could be infrequent (e.g. methane from animals or from sewerage). A suitable screening gas analysis program for the identified contaminants is highly desirable.

In rebreather diving, contamination with expired carbon dioxide as a result of failure of the ‘scrubber’ is a potential problem that periodically leads to fatalities.

With an increase in depth, and hence par-tial pressure of contaminants, toxicity will increase.

Gases for Mixed Gas and Technical Diving

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.

Air Compressors: High-Pressure/Low-Pressure

Breathing Gas Preparation and Contamination:  Contamination of a diver’s breathing gas can cause a variety of effects. These range from relatively mild discomfort resulting from an oily taste or odour to a mild headache, respiratory changes or respiratory distress, impaired cognition and consciousness, cardiac arrhythmias and death through toxicity or drowning. In earlier editions of this text, this chapter began by stating that the death of a diver as a result of breathing a contaminated gas mixture is an uncommon event. Unfortunately, this statement has become less true with the increased use of various forms of mixture diving and rebreathers. Readers need be aware of the causes of contamination and the methods of prevention and treatment.

Suitable technologies to compress breathing gas are essential to the conduct of underwater diving. Early ‘standard dress’ divers used simple hand pumps to provide a constant flow of compressed air to their helmets. Modern demand regulators used in both scuba diving and commercial surface-supply breathing apparatus (SSBA) require a supply of compressed gas at around 5 to 10 bar pressure that can be delivered either from the first-stage regulator on a dive cylinder or from the surface via a diving umbilical hose. Compressors are therefore necessary either to supply breathing gas directly to the diver or to fill the dive cylinders.

High-pressure air compressors

Compressors used to fill scuba cylinders take in air at atmospheric pressure and compress it to 200 to 300 bar. The high-pressure air is then filtered, dried and often stored in storage cylinders until it is decanted into scuba cylinders. Alternatively, small compressor units may be used to fill the diver’s cylinder directly.

If the air was compressed to high pressure in one step, there would be two major problems. First, the gas would become very hot, and subsequently a large volume of condensation would form when the gas cooled. Second, it would be difficult to design seals to prevent air leakage around the large pistons required. Because of these problems, most systems compress the air in three or four stages.

The basic design of a three-stage system is shown in Figure 19.1. At each stage the gas is compressed by a piston that moves up and down in a cylinder with inlet and outlet valves controlling the airflow. These compressors are oil lubricated, and small amounts of oil mist mix with the air as it is compressed. This must be removed, along with the water vapour in the air.

Diagramme of a three-stage compressor.
Figure 19.1 Diagramme of a three-stage compressor.

In the first stage, the air is compressed to about 8 bar. It then passes through cooling coils, and the resultant drop in temperatures causes some reduction in the pressure. Some compressors have an interstage filter between the first and second stages, although this is not standard in the diving industry. The second stage of compression raises the pressure further before the air is again cooled. In conjunction with an interstage filter, water and oil are condensed and filtered out. The final stage compresses the air to the pressure required for the storage tanks or scuba cylinders. A final filter system should be incorporated, which may include a fine filter (to remove further moisture), activated carbon (for odour) and molecular sieve material (to remove moisture and oil particles). In an environment where there is likely to be high levels of carbon dioxide, it may advisable to add an extra filter with carbon dioxide absorbent (e.g. soda lime). If carbon monoxide is of particular concern a catalytic converter element such as Hopcalite medium may also be added. A four-stage compressor differs from the three-stage device shown only in that there are smaller pressure increase steps over each stage.

The cylinder, piston and valves in each stage of the compressor are smaller than the preceding because the volume of gas decreases in accordance with Boyle’s Law. In the previous example, if the volume of the first stage cylinder was 1000 ml, to match the pressures given, the second stage would need to be less than 200 ml and the third stage about 25 ml.

The moisture that condenses is the water vapour in the air that was compressed. If the relative humidity of the air entering the compressor was 50 per cent, condensation would occur when the air pressure is increased to more than 2 bar (if the temperature remains constant). It is difficult and expensive to remove water from compressed air by chemical means only, so this is achieved throughout the process primarily by means of condensation as the air passes through pipes cooled by either fans (air) or water. Interstage separators, fitted with drains, then remove this excess (condensed) water. The process is optimized at the final filter stage by the fitting of a pressure maintaining valve (PMV). The PMV maintains pressure within the final filter (to around 160 to 190 bar, depending on the final pressure) and so increases the efficiency of (chemical) filtration.

A refrigeration system is sometimes used to increase condensation (Figure 19.2). This is fitted after the final (after) cooler and is usually sold separately from the compressor. Both a high outlet valve pressure and refrigeration will also increase the removal of any contaminants that condense with, or in, the water.

A portable four-cylinder Bauer compressor (Bauer AG, Schrobenhausen, Germany) in which some of the components have been cut away to show the internal design.
Figure 19.2 A portable four-cylinder Bauer compressor (Bauer AG, Schrobenhausen, Germany) in which some of the components have been cut away to show the internal design. To reduce the size of the compressor, the cylinders are arranged in a radial pattern. The spiral coils are to cool the air between stages of compression; cooling is aided by a fan attached to the far side of the unit. Contaminants are removed from the compressed air by chemicals in prepared packs that fit into two cut-away vertical cylinders in front of the compressor.

There is a divergence of opinion on how best to lubricate compressors. Some advocate synthetic oils, and others recommend mineral oils. The manufacturers’ instructions are the best guide to selecting the lubricant. Standard motor oils should not be used because they may break down under high pressures and heat.

Attempts have been made to design compressors that do not need lubrication. Another option is to use water as coolant and lubricant. Neither approach has won general acceptance. The reality is that all high-pressure compressors require oil, at least in the crank case.

Compressors range in size from small units that can be carried by one person to large units that weigh several tonnes. In output terms, these range from as little as 28 to 1500 litres per minute and even larger.

Low-pressure air compressors

SSBA involves much more complex equipment than scuba to provide safety for deeper occupational diving operations. Compressed air may be supplied from large cylinders filled before the dive or from a low-pressure compressor that runs throughout the dive. The air passes through a diver’s control panel and then via an umbilical hose to the diver’s helmet or demand mask. When a low-pressure compressor is used, a large-volume storage vessel and/or high-pressure cylinders may be incorporated into the system as a back-up. ‘Hookah’ is a colloquial term sometimes used to refer to minimalist versions of SSBA in which a simple low-pressure compressor supplies a mouth-held regulator via a floating hose. Hookah is commonly used in the seafood harvesting sector and by some recreational divers.

There is a larger variety of compressor types in use for SSBA and hookah diving, with currently used options including reciprocating piston and rotary screw-type compressors. Low pressure air compressors are either lubricated and sealed with oil, or of an oil-free “dry” design and there are advantages and disadvantages to each of the multiple available configurations. Because the compressor is running at the dive site and is usually powered by a diesel or petrol internal combustion engine, there is probably a greater risk of unexpected intake of contaminants compared with a properly installed high-pressure compressor at a fixed location. The quality of compressors used in practice varies dramatically, from well-filtered and well-monitored systems to inexpensive compressors designed for workshop compressed air and sometimes equipped with only makeshift filtration.