Abstract: A pneumatic timer has a cylinder (3) containing a piston (4), which is movable upwards and downwards to vary the volume of a chamber (11) below the piston by means of a screw-threaded spindle (7) and a screw-threaded sleeve (9) which is rotatable by a knob (10). Air is supplied to the chamber (11) via an inlet (26) and a long spiral groove (17, FIG. 2) formed in a disc (14), which is incorporated in the piston (4). When the air pressure in the chamber (11) reaches a predetermined value, the valve (5) is actuated by a diaphragm (12) and the time taken for this to happen from initiation of the air supply is varied by moving the piston (4) to alter the volume of the chamber (11). The air flow into the chamber (11) is accurately metered by the groove (17) and may be made much smaller than that through a variable orifice as is done in existing pneumatic timers. The timer is accordingly more accurate and provides longer time intervals than can existing timers of a similar size.