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.