ABSTRACT

The commonly used medical gases are oxygen, nitrous oxide, air, carbon dioxide, entonox and heliox. The most common method of manufacturing oxygen commercially is by the fractional distillation of liquefied air. This method produces oxygen which is over 99% pure. Oxygen on the anaesthetic machine is stored as a compressed gas in molybdenum steel cylinders with black bodies and white shoulders at a pressure of 137 bar. Cylinder technology is old and filling pressures of compressed gases were originally measured in pounds per square inch. The cylinders were filled to 2000 psi, which is equivalent to 137 bar. If a gas mixture is exposed to a temperature below its pseudocritical temperature the individual gas components can liquefy and separate out. This is known as the Poynting effect and is important in relation to entonox cylinders. In order to minimise the risks, entonox cylinders should be stored horizontally at temperatures above 5 °C.