In order to understand compressors, or scuba diving for that matter, one has to know some general physics. Air compressors operate on the basis of reciprocating pistons, like a car engine. The air is sucked into the first stage, compressed, and routed to the next stage. Most small compressors have three cylinders or stages of compression. Air contains moisture called humidity. Humidity is defined in terms of saturation levels called "relative" humidity. When relative humidity is 100% condensation takes place. The ability of air to hold water vapor is related to the temperature and pressure. The conditions inside the compressor cylinders cause water to condense into droplets. These droplets emulsify with the small amounts of oil which are also present in the cylinder. It is this emulsion along with remaining vapors and gases which must be cleared from the breathing air. The devices which separate out this mix are called condensators and filters. Most compressors not only have a condensator but also a final filter. The condensator uses a mechanical device to separate emulsion. The filter contains dessicant and a purifier. The purifier is activated charcoal. These devices only work efficiently under conditions of high pressure and reasonably low temperature. The high pressure condition is maintained by a special valve called a backpressure valve. More recently, some have been calling these valves "priority" valves. They are the same thing. The temperature is controlled by cooling coils and a fan. Water and oil accumulate in the condensator (separator) and filter canisters. These are equipped with drain valves for clearing the condensate. The filter cartridge with the dryer and charcoal must be changed out every 20-30 hours. This is because the chemicals gradually lose their efficiency. Normally, the compressor operator needs only to read the manual, hook up the fill whip, fire up the machine, close the drains and open the tank valve. Drain the condensate from both canisters every 15-30 minutes while running the machine. When the tank pressure is correct, close the tank valve, open the drain valves and shut down the compressor. Most of this is intuitive once the operation and physics of compressed air is understood. Since high pressure air is potentially dangerous read the manual twice. However, there is little chance of blowing something up because the compressor is equipped with blow off valves (pressure relief valve) and the tank is equipped with burst disks. Nevertheless. the filling pressure should be monitored so as not to over charge the scuba tank.
