Resistance thermometers are manufactured and used today in a countless variety of designs. The construction of a resistance thermometer is basically characterized by a measuring resistor, a corresponding housing and the required connection possibility. The DIN 43772 standard defines only a very small number of designs, in which the components and dimensions are specified. The standardized resistance thermometers always consist of a connection head, a protection tube and an exchangeable measuring insert and can be assembled in a modular way.

The measuring resistor is connected to a constant measuring current. The resistor generates a voltage drop which is measured. This voltage drop can be derived directly from Ohm’s law:

U = R – I U = voltage in V, R = resistance in Ω, I = current in A

The measuring voltage must be transmitted as unaltered as possible to the place of evaluation or display. Especially due to the transmission in 2-wire circuit, the additional line resistance leads to an increase of the total resistance and thus inevitably to measurement errors. To prevent the measuring resistor from heating up due to the measuring current, it should be selected as small as possible. It can be assumed that a measuring current of ≤ 1 mA causes negligible self-heating in the measuring resistor.