The sensor element consists of a Bi-metal combination having different thermal expansion coefficient. This difference in their thermal expansion coefficient produces a mechanical motion. The sensing element of the Controller is installed directly in the process vessel. The temperature changes inside the process will result in the following

• Incase of a rise in temperature, the inner metal rod will expand and will contract incase of a decrease in temperature. The change in length is proportional to the change in temperature.
• The outer metal tube has a low expansion coefficient and thus change in length is negligible. But since one end is attached to the inner metal tube, the inner rod moves as the outer tube changes length.
• The opposite end of inner rod pushes against a floating link. It is held concentric with a spring loaded inner valve. When the temperature decreases, the inner rod applies pressure on the floating link, it causes pivot to move around the adjustment fulcrum point which tilts the inner valve plug thus the output signal increases. The temperature can be adjusted by moving the knob on the fulcrum.