The output air must usually be filtered to control purity to a level appropriate for breathing gas at the relevant diving depth.

Failure to comply with these specifications can lead to damage to the compressor due to excessive friction and overheating, and contamination of the breathing air by toxic breakdown products of the lubricants.

[2] The compression process helps remove water from the gas, making it dry, which is good for reducing corrosion in diving cylinders and freezing of diving regulators, but contributes towards dehydration, a factor in decompression sickness, in divers who breathe the gas.

[citation needed] The compressed air output by the compressor must be filtered to make it fit for use as a breathing gas.

The following impurities may be checked for:[citation needed] Filters remove:[5][6] The intake air for a high pressure compressor should be clean and have a low carbon dioxide content.

Removal of particulate contamination is usually by a paper type dust filter at the first stage intake.

Carbon dioxide scrubbing requires moisture for the absorbent material to work effectively, and moist air is undesirable for the other filter media, so carbon dioxide scrubbing is often removed by a pre-filter system before the air is compressed.

The air is also heated by compression, and when cooled between stages in the inter-cooler coils, the relative humidity increases, and when it exceeds 100% will tend to condense out onto the surface of the tubes and as droplets carried by the air-stream.

The air from the inter-cooler coils is led into the large diameter vertical axis tube of a separator, where it changes direction by about 90 degrees and is slowed down considerably.

[5][6] The final stage of air treatment is filtration of residual moisture, oil and hydrocarbons, and where necessary catalytic conversion of carbon monoxide.

Slow air flow is easily achieved by high compression, so filtration works best at or near the working output pressure of the compressor, and this is achieved by the back-pressure valve, which only allows air to flow out of the filter above the set pressure.

[6] Delivered air should have a dew point lower than the operational temperature of the cylinder, which is generally above 0°C when immersed, but can be colder during transport.

Desiccant media used in HP breathing air filters include: activated alumina, silica gel, sorbead, and molecular sieve.

[6] Activated carbon absorbs both condensable and gaseous hydrocarbons, and is effective in removing odours, organic compounds, and halogenated solvents.

[6] Firstly it ensures that after a short starting period, all of the compressor stages are operating at their designed discharge pressures, so that the loads on the pistons are steady and evenly distributed round the crankshaft.

During start-up the compressor first builds up pressure on the first stage, and is unbalanced, with a greater load on that cylinder's piston, and will vibrate more than normal, as there is no equivalent load on the other stage pistons, then pressure in the other stages builds up in sequence, until all cylinders are operating at their working pressures, the loads on all the pistons are similar, and the back-pressure valve starts to open to let the compressed gas flow to the distribution panel.

Divers, to maximise their dive time, generally want their cylinders filled to their safe capacity, the working pressure.

[9] Cylinders are often filled at a rate of less than 1 bar (100 kPa or 15 lbf/in²) per second to allow time for heat transfer to the surroundings to limit this increase in temperature.

[10] Compressors may be connected to a bank of large, high-pressure cylinders to store compressed gas, for use at peak times.

This allows a cheaper low-powered compressor, which is relatively slow at pumping gas, to fill the bank automatically during idle periods, storing a large volume of pressurized air so that a batch of cylinders can be filled more quickly at peak demand without being delayed by the slow-running compressor.

[2] Compressors may be linked to a gas blending panel to make nitrox, trimix, heliair or heliox mixes.

[11] The panel controls the decanting of oxygen and helium from cylinders bought from commercial gas suppliers.