What does a resistance thermometer include?

A traditional resistance thermometer consists of one or more measuring resistors, electrical conductors, a thermowell, insulation material and suitable connections.

In essence, resistance thermometers can be split into line sensors and measuring inserts. While measuring inserts are installed in fittings with a thermowell, connection socket and connection head and can be replaced at any time, line sensors consist of a flexible line and a sensor element.

Along with measuring resistance, the accuracy of the resistance thermometer is ultimately determined by the selected circuit type.

Two-wire circuits are the most cost-effective yet most inaccurate measuring method. The two connection pins on the measuring resistor are extended by one conductor and connected to the measuring device. This requires minimal material as the intrinsic resistances of the conductor are fully incorporated in the measurement signal. As a result, the conductors need to be as short as possible or have a large cross-section, or any errors need to be arithmetically compensated for.

The four-wire circuit requires the most materials and evaluation, yet yields the most accurate measurements. A smart combination of a constant current across the resistance provided by a red-white conductor pair and the measurement of the voltage drop across the measuring resistance by the second conductor pair ensures the intrinsic resistances of the conductors are fully compensated. The measurement signal is measured without any distortion.

The compromise between these two circuits is the three-wire circuit.

A bridge circuit such as the Wheatstone bridge, is used to compensate for the intrinsic resistances in a circuit. The challenge lies in the fact that the three conductors must be completely identical in terms of structure, material properties and external conditions to ensure measurements can be taken without any distortion. However, the differences arising from the actual design of the conductors and the actual operating conditions are significantly smaller than with the two-wire circuit.