Fuel flowmeters measure the amount of fuel being delivered to the engine. The system consists of two elements: a transmitter and an indicator.

The transmitter can be either volumetric or gravimetric. In the volumetric type transmitter, fuel flows through a rotor spinning at a speed determined by the fuel flow rate. This rate is sensed by an electrical pick-off that transmits the signal to an indicator in the flight deck. In the gravimetric type transmitter, the fuel flow is measured by a vane restrained by a calibrated spring, which deflects at a certain angle proportional to the mass flow. The gravimetric transmitter is compensated for density variations with temperature, by a bleed outlet.

As you can imagine, the indicator of the mass flowmeter displays a mass flow straight away. However, the fuel density needs to be manually inserted in the volumetric type system to indicate a mass flow rate.