Medical gases supplied in cylinders are compressed to a high pressure in order to store a big mass in a relatively small volume (it follows from Boyle's law that the higher the pressure inside the cylinder, the bigger the final volume of decompressed gas). The human respiratory system operates at low pressures; high pressure can produce barotrauma. Gas supply pressure has to be reduced in stages and safety features are incorporated. The wide range of pressures in the medical gas supply and in the breathing system has led to the establishment of various units suiting each particular purpose: bars for high pressures, lb/psi for moderate pressures, mmHg for atmospheric and lower pressures, and cmH2O for very low pressures measured as a column of water. (See more on gas pressure in the chapter on real gas compression, and on units in simple mechanics.)

Figure 44 shows the pressures inside the gas supply system on a logarithmic scale; in this way, it is possible to include a wide range of pressures in one graph. In particular, notice that the gas supply operates in terms of hundreds of kPa (bars) but the breathing system pressures are kPa units or their fractions. The logarithmic scale of the ordinate also demonstrates that each stage of decompression reduces the pressure very approximately by a factor of 10.

The volume shown on the abscissa is that which would be obtained by decompression of 1 litre gas at the pressure indicated to atmospheric pressure.