Thermal Mass Flow Measurement
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.