# Boyle's Law (Mariotte's Law)

## Perfect gas and Boyle's law

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

P1V1 = P2V2                                   (1b)

where

P = absolute pressure (Pa, psi...)

V = volume (m3, ft3...)

### Example - Final Pressure when Compressing an Air Volume

A gas with volume 1 m3 at 2 MPa is compressed to a final volume of 0.5 m3. If the temperature is kept constant the final pressure can be calculated as:

P2 = P1V1 / V2

= (2 MPa) (1 m3) / (0.5 m3)

= 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:

V1 = π (((4.03 in) / (12 in/ft)) / 2)2 (1 ft)

= 0.09 ft3

Expansion of the compressed air to a normal pressure of 14.7 psia can be calculated by rearranging Boyle's law:

V2 = P1 V1 / P2

= (114.7 psia) (0.09 ft3) /  (14.7 psia)

= 0.7 ft3

## Search the Engineering ToolBox

- "the most efficient way to navigate!"

## Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your SketchUp model with the Engineering ToolBox - SketchUp Extension/Plugin - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!