They are mechanical turbine type meters that measure instantaneous and total volumetric flow.
In turbine meters, a tribune called rotor is rotated by using mechanical energy of the fluid. Fans on tribune are angled to convert energy from flow direction to rotational energy. Rotor shaft is rotated on bearings. Velocity of passing fluid is proportional to this rotation speed.
Rotation of the shaft is detected mechanically by the movement of the turbine. The rotation of the shaft is generally detected via NAMUR proximity sensor and the magnets placed on fans that generate pulses.
Turbine meters are used in fuel, water, gas and derivatives, chemicals and power plants, food, aviation and many similar industries.