A flow sensor is a device for sensing the rate of fluid flow. Typically a flow sensor is the sensing element used in a flow meter, or flow logger, to record the flow of fluids. As is true for all sensors, absolute accuracy of a measurement requires a functionality for calibration. There are various kinds of flow sensors and flow meters, including some that have a vane that is pushed by the fluid, and can drive a potentiometer, or similar devices. Other flow sensors are based on sensors which measure the transfer of heat caused by the moving medium (Thermal mass flow meter). This principle is common for microsensors to measure flow.