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.