About Thermal Mass Flow Measurement

Thermal Mass Flow Meter

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.  Some manufacturers use a variable power operation to keep the temperature difference constant, while others keep the power constant and measure the temperature difference.  The Magnetrol® Model TA2 measures the power it takes to maintain a constant temperature difference between the sensors.  This relationship between power and mass flow rate is established during calibration.

Thermal Dispersion Flow/Level/Interface Switch

Similar to thermal flow meters the switches are based on heat transfer.  One sensor is at the process temperature and the other is being heated by a constant power.  As the flow rate increases, the temperature difference between the sensors decreases.

A set point is established so when that specific temperature difference is reached the relay changes state.  This can be on either increasing or decreasing flow or flow/no flow.  When used in a level or interface application it is primarily the thermal conductivity of the fluid that will provide the difference in heat transfer.