Before attaching a full cylinder to the machine briefly open and close the cylinder valve to clear any dirt from the port. Anaesthetic machines operate at 4 bar pressure and therefore the compressed medical gases in cylinders pass through reducing valves to bring the gas pressure to a constant 4 bar. Non-return valves prevent empty cylinders still attached to the machine from refilling from fresh cylinders. Until recently cylinders of cyclopropane and carbon dioxide were also commonly found on anaesthetic machines. Cyclopropane is now rarely used and carbon dioxide should only be attached if the anaesthetist wants to use it for a specific reason.
Larger hospitals supply medical gases from a central store via pipelines in the floor and walls.
The central store may be a bank of large cylinders, or in the case of oxygen, a large insulated tank of liquid oxygen maintained at approximately minus 165 degrees centigrade. The piped medical gases are delivered to specific ports located in the wall of the operating theatre and anaesthetic room. The valves are specific for each gas and the tubing is colour coded and permanently bonded to the individual valve. All anaesthetic machines, including those on pipeline gases, should have reserve gas cylinders attached, but turned off. The flow rate of the individual gas is shown by a float or bobbin in a vertical glass tube.