Thermal mass flow meters, also known as thermal dispersion or immersible mass flow meters comprise a family of instruments for the measurement of the total mass flow rate of a fluid, primarily gases, flowing through closed conduits.

Thermal flow meters are primarily used in air and gas flow measurement applications.

The meters consist of a transmitter and probe with temperature sensors (RTDs) located in the pins at the bottom of the probe.

One sensor measures the process temperature and the other sensor is heated to a specific temperature above this.

As the flow rate increases, heat gets taken away from the heated sensor in the form of convective heat transfer.