Boyle's Law (Mariotte's Law)
The volume of an ideal gas vs. pressure.
Boyle's law (Mariotte's Law) states that in a perfect gas where mass and temperature are kept constant  the volume of the gas varies inversely with the absolute pressure.
Boyle's law can be expressed as
PV = constant (1)
or
P_{1}V_{1} = P_{2}V_{2} (1b)
or
P_{1 }/ P_{2} = V_{2 }/ V_{1} (1c)
where
P = absolute pressure (Pa, psi...)
V = volume (m^{3}, ft^{3}...)
Example  Final Pressure when Compressing an Air Volume
A gas with volume 1 m^{3} and initial pressure 2 MPa is compressed to final volume 0.5 m^{3}. With constant temperature the required final pressure can be calculated as
P_{2 }=_{ }P_{1}V_{1} / V_{2}
= (2 MPa) (1 m^{3}) / (0.5 m^{3})
= 4 MPa
Example  Final Volume when Compressed Air is Expanded to Standard Conditions
Air is compressed to 114.7 psia in a 4 inches schedule 40 steel pipe. Inside diameter of the pipe is 4.03 inches and the compressed air volume in one foot of the pipe can be calculated as:
V_{1} = π (((4.03 in) / (12 in/ft)) / 2)^{2} (1 ft)
= 0.09 ft^{3}
Expansion of the compressed air to a normal pressure of 14.7 psia can be calculated by rearranging Boyle's law:
V_{2} = P_{1} V_{1} / P_{2}
= (114.7 psia) (0.09 ft^{3}) / (14.7 psia)
= 0.7 ft^{3}
Related Topics

Gases and Compressed Air
Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves.
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