Absolute Pressure: is the existing gauge pressure plus atmospheric pressure. At sea level the gauge pressure in pounds per square inch (PSI) plus 14.7 gives the absolute pressure in pounds per square inch (PSIA).

Actual Capacity: The actual capacity of an air or gas compressor is the quantity of air or gas compressed and delivered. It is usually expressed in cubic feet per minute (CFM) at intake pressure and temperature. Standard cubic feet per minute (SCFM) is delivered air at specified laboratory conditions

Aftercoolers: Are devices for removing the heat of compression of the air or gas after compression is completed. They are one of the most effective means of removing moisture from compressed air.

Air Receivers: Are tanks into which the compressed air or gas is discharged from the compressor. Receivers help to eliminate pulsations in the discharge line and also act as storage capacity during intervals when the demand exceeds the capacity of the compressor. Use proper size tank as furnished and/or recommended by manufacturer. Do not reduce tank size or restrict discharge air line from compressor tanks.

Booster Compressors: Booster compressors are machines for compressing air or gas from an initial pressure, which is above atmospheric pressure, to a still higher pressure.

Compressed Air: is free air that has been pressed into a volume smaller than it normally occupies. As compressed air exerts pressure, it performs work when released and allowed to expand to its normal free state.

Compression Efficiency: is the ratio of the theoretical horsepower to the actual indicated horsepower required to compress a definite amount of gas.

Compressors: are machines designed for compressing air or gas from an initial intake pressure to a higher discharge pressure

Displacement of a Compressor Cylinder: is the volume swept through by the piston. This is usually expressed in cubic feet per minute.

Displacement of a Multi-Stage Compressor: is that of the first stage only, since the same gas passes through all stages in series.

Double-Acting Compressors: are those in which compression takes place on both strokes per revolution in each compressing element.

Enthalpy: (Heat Content) The sum of the internal and external energies.

Entropy: A measure of the unavailability of energy in a substance.

Free Air: Free air is defined as air at atmospheric conditions at any specific location. Because the altitude, barometer, and temperature may vary at different localities and at different times, it follows that this term does not mean air under identical or standard conditions.

Gas: While from a physical point of view a gas is one of the three basic phases of matter, and thus air is a gas, a special meaning is assigned in pneumatics practice. The term gas refers to any gas other than air.

Gauge Pressure: The pressure on the gauge measured above atmospheric pressure (PSIG); that is, that measured on a given gauge, not effected by altitude or atmospheric pressure.