Mounted on shaft A of the tachometer, driven by the shaft whose speed is to be measured, is impeller 1 with radial vanes. Liquid is drawn from tank 2 and is pumped by the impeller into tank 3 whose free space is filled with air. This air is compressed and flows through tube a to a pressure gauge whose hand indicates the angular velocity of the shaft being tested.
$3718$SHP,M$