A respirator comprises a pneumatic module and a control module 6. The pneumatic module comprises manifolds 2-4, connected to supplies of compressed gases, and to a pneumatically operated pressure equilibrator 21 which is connected to a mixing chamber 22, controlled by a dual pressure switch 26, which supplies batteries of valves 7, 16 having stepped series-connected resistances signalling the control module 7. Battery 7 forms an inspiration valve assembly and incorporates a flow transducer 15, while the other controls the expiration valve 7. The control module 6 comprises a microprocessor 35 incorporating a keyboard 38 and a VDU40 simultaneously showing alpha- numeric and graphic information. The information to the control module 6 from the pneumatic module passes through amplifying blocks 42, 43, 44 and A/D converters which are connected to buses of the microprocessor 35. The data buses are also connected to a plurality of optical couplers and amplifiers 46 to 49 which produce signals for controlling the various valves and members of the pneumatic module.
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