Air Compressors are driven by either an electric motor or small fuel powered engine that in most cases turns a single piston to create air pressure. The piston forces air into a storage tank that is often capable of holding hundreds of pounds of air pressure. Most consumer and contractor models hold up to 125psi. An air hose is connected to the compressor with the other end connecting to the air tool to be used.
Common air tools are, an air ratchet or air gun, paint spayer, grinding wheels, impact wrench, air chisel, air hammer, tire inflator and many other various tools for automotive and construction use. Each tool will have a specific air pressure rating that is intended to keep it within a designed operating range. Many tools such as a paint sprayer, air-ratchet wrench or low speed grinder may operate at a Pounds per Square Inch (PSI) rating of 40. Other tools require a higher PSI rating and may operate at up to 90 PSI or beyond. To keep tools that are designed for lower PSI than is stored in the tank from being damaged, a pressure regulator is installed just before the outlet valve.
The Tank will usually have two indicator valves attached. The first valve inline will read direct air pressure within the tank. As the motor pumps air into the tank you can watch the pressure gauge climb. Most air compressors with have an automatic cut off switch that will stop increasing pressure inside the tank once a desired PSI rating has been achieved inside the tank. For example, once the pressure inside reaches 125 Pounds per Square Inch on many tanks, the compressor will stop and the first gauge will display 125 PSI. The next gauge inline in most types of air-compressors is connected to the output side of an integrated Pressure Regulator. A dial can be turned on the regulator and the pressure at the output side will 'bleed off'. This will effectively lower the possible output pressure to any tool hooked up to the air output of the compressor. Even thought the pressure inside the tank may be as high as 100 PSI, the regulator can limit the output pressure to a specific value such as 40 PSI if that is the designed operating pressure for the tool being used. The same compressor can immediately be dialed up to a higher PSI rating such at 90 PSI if a new tool is installed that requires a higher operating pressure.