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Compressed Air Basics


Relative Humidity: measures how “full” the air is with water

  • The warmer the air, the more water it can hold and vice versa
  • For every 20ºF reduction in temperature, the amount of what that air can hold is cut in half

Dew point: is the temperature at which the air reaches 100% relative humidity

  • When the dew point is reached, water vapor becomes condensate
  • As compressed air cools, it forms condensate and diminishes tool life and performance and will freeze in winter


There is a direct relationship between pressure and temperature

  • As air is compressed, its temperature increases. As air expands, it cools.
  • Air compressed to 60psi in a single stage rises to a temperature of 375ºF
  • Air compressed to 147psi in a single stage rises to a temperature of 590ºF

Air Flow

CFM measures air flow (CFM, ACFM, and SCFM are generally interchangeable.)

  • CFM (cubic feet per minute)
  • ACFM (actual cubic feet per minute): a more specific way to define CFM
  • SCFM (standard cubic feet per minute)
  • CFM displacement: the stroke volume X pump RPM

PSI & Stored Air

  • PSI (pounds per square inch): measurement of pressure resulting from force
  • PSIG (PSI at the gauge)
  • The atmosphere at sea level weighs about 14.7psi
  • Think of PSI and volume as stored energy
  • Tools use air measured in cubic feet
  • Tanks are measured in gallons
  • There is 0.133681 cubic feet in one gallon or 7.48 gallons in one cubic foot
  • The key factor in the relationship between PSI, volume, and stored air is atmospheric pressure (for every atmospheric pressure (14.7psi) in the tank, multiply that times the volume of the tank).

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