Principle of operation: The measuring system consists of a submerged element, a capillary wire and a tubular spring in the casing.

These elements are connected to a single device, which under pressure is filled with an inert gas. A change in temperature causes a change in volume or internal pressure in the submerged device. The pressure deforms the measuring spring, the deviation of which is transmitted by the arrow mechanism to the arrow.

The ambient temperature fluctuations may not be taken into account, since a bimetallic element is built in to compensate for the difference between the pointer and the measuring spring.

The ranges of readings lays in a range -200 ° C and + 700 ° C for measurements with accuracy class 1 according to DIN 16 203.