Unit normally attached to back bar of anaesthetic machine, with rotameter-type flowmeter and two Boyle's bottle type vaporizers for Trilene, by British Oxygen Company, 1951-1970

Carbon dioxide, oxygen and nitrous oxide would have passed through the top of the housing and through the two Boyle's bottles for the vaporisation of liquid anaesthetic agents. Rotameters of this type had been used in industry to measure gas and liquid flows and were adapted for anaesthetic use in 1937. The rotameter became a standard fitting for anaesthetic machines from the 1940s onwards, being more accurate than flowmeters. Computerised patient and gas monitoiring began to be introduced in the mid 1980s.

Introduced in 1934, Trilene (trichloroethylene) was an anaesthetic inhaled through a face mask after being vapourised.