The probe consists of two separate sensor components, both RTDs. One RTD is the reference and measures the temperature of the fluid where the probe is immersed. The second RTD is self-heated at a constant power. This creates a temperature difference between the two RTDs. Varying the set point changes the power to the heated RTD which enables the user to adjust the setpoint for the specific application When used as a level switch convective heat is the mechanism of heat transfer.
Any liquid has a thermal conductivity greater than the corresponding vapor.
In the dry state there is a temperature difference between the two sensors. This reduces the temperature difference and provides a level indication. When the sensor goes dry, the temperature difference increases. The electronics detect the increase in temperature difference and provides a low level indication. Thermatel can also be used for interface detection.