Gas Laws

Air can be considered to be a reasonable approximation to an ideal gas for the range of pressures and temperatures occuring with pneumatics systems and thus obey the ideal gas laws. These are:

1. Boyle's Law

    In an ideal gas in which the mass and temperature remain constant, the volume V veries inversely as the absolute pressure p, i.e.:

                  pV = a constant

2.  Charles's Law

      In an ideal gas in which the mass and the pressure remain constant, the volume V veries directly as  the absolute temperature T, i.e.:

                  V / T = a constant

       Absolute temperatures are measured on the kelvin's scale. To convert from temperatures on the Celcius scale, a reasonable approximation is to add 273. Thus 0^o = 273 ^oK.

 

3.  The Pressure Law

       In an ideal gas in which the mass and volume remain constant, the pressure p veries directly as  the absolute temperature T, i.e.:

                   p / T = a constant

 

        The combination of the three gas laws results in the general gas equation:

                   pV / T = a constant

      
         The constant is for a particular mass of a particular gas and thus the equation can be written for a mass m of gas as:

                  pV = mRT 

           with R, termed the characteristic gas constant, being the value of the constant for 1 kg of the particular ideal gas.  

 

* Reference: Pneumatic and Hydraulic Systems, by W. Bolton