Pressure calibration devices

A device for calibrating fluid pressure-sensitive instruments, eg. pressure gauge 10, comprises a piston and cylinder pump 1 and a pressure centre 2, the pump outlet connected through a first non-return valve 6, to supply fluid under pressure to the inlet of the pressure centre 2 and the pump inlet 5 connected through a second non-return valve 7 to a source of fluid. Opposite sides of the first non-return valve 6 are connected by a passage in which is a bridging valve 13 which when closed retains fluid pressure in the pressure centre 2 and when opened permits fluid to flow between the pressure centre and the pump. When the bridging valve 13 is open the pump piston may be used to increase or reduce fluid pressure in the pressure head under precise control.