Typical Number in Hospital: 500 Cost Bands: 1 References: 2

Medical gases for therapy or anaesthesia are supplied by pipeline or in bottles (cylinders). The cylinders are steel, very heavy and strong enough to contain the enormous pressures required to get enough gas into a small volume. At the pressures used, some gases liquefy, and therefore behave differently during storage and delivery. Oxygen (1980 psi) and Entonox (1980 psi) remain gases, while nitrous oxide (639 psi), carbon dioxide (723 psi), and cyclopropane (64 psi) liquefy. The liquids will cool considerably during expansion and this may cause problems, although this drawback is put to good use in cryosurgery where nitrous oxide evaporation and expansion is used as the energy source. In the case of Entonox, it should not be stored below freezing point (0[d]C) since the mixture (50/50 nitrous oxide and oxygen) may separate.

Medical gas cylinders, when empty, are exchanged for full ones by a commercial medical gas company. The fullness of the cylinder is determined by the pressure (in the case of gases), or by weight (in the case of the liquefied gases).

The top of the cylinder has a tapered thread into which is fitted a valve, usually turned by a special key. The gas outlet from this valve is connected to a pressure reducing valve and pressure gauge and other devices depending on the application. Cylinders for use with anaesthetic apparatus usually have a pin- index system on the valve block to ensure that only the correct gas can be connected to each port (yoke) on the machine. Medical gas cylinders are also colour coded but unfortunately there is no international agreement as yet.

The UK standards for cylinder coding are set out in BS 1319 (1976).

The common codes are listed in Table 3.

Table 3. Colour codes for medical gas cylinders

GAS Colour - valve end Cylinder body


Oxygen White Black

Nitrous oxide Blue Blue

Cyclopropane Orange Orange

Carbon dioxide Grey Grey

Nitrogen Black Grey

Medical air White and black Grey

Entonox White and blue Blue

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details