The tachometer is a centrifugal pump for which the magnitude of the delivery head, measured by the height of the column of liquid in a glass tube, is directly proportional to the speed of spindle 1. Mounted on this spindle, which is driven by the shaft whose speed is to be measured, is rotor 2 with radial holes a. As the rotor rotates, the liquid is thrown outward from the centre to the periphery and its pressure, measured by manometer 3, indicates the speed (in rpm) of the shaft being tested.
$3715$SHP,M$