• Scuba divers can use a variety of different air compressors to fill their tanks.scuba diving 01 image by Undy from Fotolia.com
  
  An air compressor is a device that takes in air at a normal, atmospheric-level of pressure, and then compresses it so that it can supply the air at a much higher level of pressure. Scuba divers utilize air compressors to refill their scuba tanks prior to dives. While some divers own compressors and refill their tanks at home, others use the compressors that local dive shops provide. According to Trails, there are several different types of air compressors that divers can use.
  
  

Rotary Screw

• The rotary screw compressor relies on positive displacement, or the process of forcing a substance into an enclosed, sealed-off cavity, and then squeezing it out of a small tube or opening. In this particular instance, that substance is air and the compressor uses two rotors to perform the squeezing. According to The Engineering Toolbox, one style of rotary screw compressor, known as a flooded compressor, has no valves, and instead uses oil to cool down its rotors as they complete the compression inside of the chamber. Another type, the oil-free compressor, uses a sliding valve on the side of the chamber to keep the device cool.
  
  

Rotary Sliding Vane

• Another type of air compressor that is suitable for diving, and relies on positive displacement, is the rotary sliding vane compressor. The World Intellectual Property Organization describes the compressor as, in addition to utilizing rotors like the basic rotary screw compressor, also featuring blades or vanes that extend out from the rotors to help increase compression.
  
  

Reciprocating

• Reciprocating air compressors are another variety of compressor that use positive displacement in order to refill tanks. However, unlike the above two compressors, which rely on components spinning within a chamber, this compressor relies on pistons moving back and forth, which compact the air inside of a chamber with each pump. The process is similar to a combustion engine operating in reverse: Instead of a pressurized substance forcing the piston out, like in the engine, the compressor pushes the piston in and creates the pressurized substance. Trails says that as of 2010 divers use reciprocating air compressors more often than any other variety.
  
  

Centrifugal

• Centrifugal air compressors rely on dynamic displacement, as opposed to positive displacement, in order to pressurize and pump out air. The process involves accelerating a substance in a circular motion until it condenses to due to centrifugal forces. In this particular instance, that substance is air, and the compressor gets it up to speed by using a rotating impeller. The inertia of the moving air will keep it spinning round and round, which allows for continuous compression.