As previously stated, compressibility is an outstanding characteristic of gases. The English scientist, Robert Boyle, was among the first to study this characteristic that he called the “springiness of air.” By direct measurement he discovered that when the temperature of a combined sample of gas was kept constant and the absolute pressure doubled, the volume was reduced to half the former value. As the applied absolute pressure was decreased, the resulting volume increased. From these observations, he concluded that for a constant temperature the product of the volume and absolute pressure of an enclosed gas remains constant. Boyle’s law is normally stated: “The volume of an enclosed dry gas varies inversely with its absolute pressure, provided the temperature remains constant.” The following formula is used for Boyle’s law calculations. Remember, pressure needs to be in the absolute.
- Volume 1 × Pressure 1 = Volume 2 × Pressure 2
- V1P1 = V2P2
Example: 10 ft3 of nitrogen is under a pressure of 500 psia. If the volume is reduced to 7 ft3, what will the new pressure be? [Figure 3-37]
- V1P1 = V2P2
- 10 (500) = 7 (P2)
- 10 (500) ÷ 7 = P2
- P2 = 714.29 psia
The useful applications of Boyle’s law are many and varied. Some applications more common to aviation are: (1) the carbon dioxide (CO2) bottle used to inflate life rafts and life vests; (2) the compressed oxygen and the acetylene tanks used in welding; (3) the compressed air brakes and shock absorbers; and (4) the use of oxygen tanks for high altitude flying and emergency use.