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Temperature measurement

RTD temperature probe/Resistance thermometers – The most important at a glance

RTDs or resistance thermometers are the first choice for electrical temperature measurement up to 600 °C. The required design depends on the respective application. Learn more about the different types, the functionality and the design of the clever temperature sensors!

How is an RTD constructed?

Resistance thermometers contain a temperature sensor which, in the simplest case, is connected to a connecting cable. Depending on the application, RTDs are also available with a connection head or with a connection plug or as a plug-in, screw-in or contact sensor. Below you will find some examples of resistance thermometers.

Are resistance thermometers interchangeable?

As far as the resistance thermometers have been equipped with a platinum sensor (like Pt100 or Pt1000), they are compatible. Thanks to the DIN EN 60751 standard, a replacement thermometer will have the same output signal.

How can RTDs be connected?

Resistance thermometers can be connected via a two-wire, three-wire or four-wire connection.

Two-wire connection

In the simplest case, an RTD can be connected via two wires. However, the two-wire connection results in a so-called line offset. For every 0.38 Ω of lead resistance, a Pt100 indicates a temperature that is 1 Kelvin too high. With the same ratios, an offset of 0.1 Kelvin occurs with a Pt1000. Even with relatively short cable lengths, the deviation in the display will be so great that a temperature adjustment must be made in the evaluation unit. This creates unnecessary effort, therefore RTDs should preferably be connected in three- or four-wire technology.

Do resistance thermometers have to be checked regularly?

The platinum sensors in the resistance thermometers have only a low drift behavior, i.e. their output signal changes only slightly over time. Nevertheless, calibration should be carried out regularly to check whether the temperature is still being measured with the required accuracy. Finally, regular calibration ensures high product quality. If a plant operator works according to guidelines and standards such as ISO 9001:2008, ISO 14001:2004 + Cor1:2009, he is even obliged to calibrate the measuring chains.

Should a Pt100 or a Pt1000 be used?

If the RTDs are operated via three or four conductors, it is irrelevant whether it is a Pt100 or Pt1000 sensor. Only when used in two-wire technology does the Pt1000 have an advantage, because with it the lead offset is only 1/10 of the Pt100. Example: The lead resistance of a supply line is 1.9 Ω. When using a Pt100, a line offset of approx. 5 Kelvin occurs - a line compensation must be carried out. If a Pt1000 is used, the offset is 0.5 Kelvin - in many applications, an adjustment is not necessary.

How does the four-wire technology work?

The thermometer is supplied with the measuring current via the supply lines. The voltage drop at the measuring resistor is tapped via the measuring leads. If the input resistance of the downstream electronics is many times higher than the lead resistance, this is negligible. The voltage drop determined in this way is then independent of the properties of the supply lines.

The connection instructions required for installation are shown in the following overview: