Diving air compressor
Modern diving compressors are three- or four-stage-reciprocating air compressors that are lubricated with a very high-grade compressor oil (a few use ceramic-lined cylinders with O-rings, not piston rings, requiring no lubrication). Oil-lubricated compressor operators must only use lubricants specified by the compressor's manufacturer. Special filters are used to clean the air of any residual oil (see "Air purity").
The compression process helps remove water from the gas, making it dry, which is good for reducing rust in diving cylinders and freezing of diving regulators, but causes dehydration, a factor in decompression sickness, in divers who breathe the gas.
In addition, the compressed air output by the compressor must be filtered to make it fit for use as a breathing gas. The following filters remove:
- Water, using silica gel, activated alumina or a molecular sieve
- Oil, using activated carbon or a molecular sieve
Periodically, the gas produced by a compressor must be tested to ensure it meets air purity standards. The following impurities are checked for:
- Carbon dioxide
- Carbon monoxide
- Lubricating-oil vapour
- Nitrogen dioxide
- Odor and taste
- Solid particles
- Water vapor
Often compressors are connected to a bank of large, high-pressure cylinders to store compressed gas, for use at peak times. As cheap and low-powered compressors are relatively slow at pumping gas, the bank can be filled automatically during idle periods, storing a large volume of pressurized air so that a batch of cylinders can be filled quickly one after the other without being delayed by the slow-running compressor.
The "cascade system" is used to decant economically from banks of storage cylinders so that the maximum possible gas is removed from the bank. This involves filling a diving cylinder by decanting from the bank cylinder with the lowest pressure that is higher than the diving cylinder's pressure and then from the next higher-pressure bank cylinder in succession until the diving cylinder is full. The system maximizes the use of low-pressure bank gas and minimizes the use of high-pressure bank gas.
If diving cylinders are filled too quickly, the gas inside them becomes hot as a result of adiabatic heating, increasing in pressure, which results in a drop in pressure when the cylinder cools later. Cylinders are often filled at a rate of less than 1 bar (100 kPa or 15 lbf/in²) per second to reduce this increase in temperature. In an attempt to cool the cylinder when filling, some people “wet fill”, immersing their cylinders in a cool water bath. This increases the risk of internal cylinder corrosion by moisture from damp components entering the cylinder during filling.