Diving air compressor

Diving air compressor

A small stationary high pressure diving air compressor installation
Other names	Breathing air compressor
Uses	Filling dive cylinders (high pressure) Provision of surface supplied breathing air (low pressure)

A diving air compressor is a gas compressor that can provide breathing air directly to a surface-supplied diver, or fill diving cylinders with high-pressure air pure enough to be used as a breathing gas.

A low pressure diving air compressor usually has a delivery pressure of up to 30 bar, which is regulated to suit the depth of the dive. A high pressure diving compressor has a delivery pressure which is usually over 150 bar, and is commonly between 200 and 300 bar. The pressure is limited by an overpressure valve which may be adjustable.

High pressure diving compressors are generally three- or four-stage-reciprocating air compressors that are lubricated with a high-grade mineral or synthetic compressor oil free of toxic additives (a few use ceramic-lined cylinders with O-rings, not piston rings, requiring no lubrication). Oil-lubricated compressors must only use lubricants specified by the compressor's manufacturer as suitable for use with breathing air. Special filters are used to clean the air of most residual oil and water (see "Air purity").

Smaller compressors are often splash lubricated - the oil is splashed around in the crankcase by the impact of the crankshaft and connecting rods - but larger compressors are likely to have pressurized lubrication using an oil pump which supplies the oil to critical areas through pipes and passages in the castings. Most oil lubricated compressors will have a wet sump at the bottom of the crankcase, and require the oil level to be within limits indicated by a sight glass or dipstick for proper lubrication. The compressor should also be level within the manufacturer's specification while operating. These constraints ensure that the lubricant is in the right place for either the moving parts to contact it for splash lubrication, or for reliable suction to the oil pump. 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.

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.