Two-, three- and four-wire connection
Resistance thermometers can be connected to temperature measuring instruments, data loggers or measuring bridges using three different methods:
- Two-wire connection
- Three-wire connection
- Four-wire connection
Part 1: Two-wire connection
The following diagram shows the connection of a temperature sensor (shown on the left in the diagram) in two-wire connection to a measuring instrument.
The problem with using the two-wire connection is the following:
The resistance of the connecting cable is added to the sensor resistance!
The diagram shows how the measurement result is obtained. The measured resistance is the sum of the resistance of line 1, the measuring resistance (i.e. the actual temperature sensor) and line 2. The measurement result is therefore too high. To avoid measuring errors, the result must be corrected.
A calculation example shows how large the measurement error can be in a practical application. Let us assume that a temperature sensor is to be connected with a copper cable. The following basic conditions apply:
Specific resistance of the copper cable at room temperature: 0.017
Cable cross-section: 0.5 mm2
Length of the cable: approx. 50 m
The following equation can be used to estimate how large the measurement error caused by the connecting cable is:
Area of the cross section
In our example this results is:
The sensitivity E of a PT 100 temperature sensor is approximately 0.385 Ohm / Kelvin. Therefore the measurement error is
The measurement error in our setup is 8.8 Kelvin. It is important to note that the length of the connecting cable of approx. 50 m must be considered twice. Once for the “outward way” (line 1) and a second time for the “return way” (line 2).
It was traditionally quite common to connect thermometers in two-wire technology to production plants. Before digital technology, with which the systematic error can be corrected relatively easily, correction tables were used to obtain correct measured values. The following example shows correction values for a 10 m long connecting cable as a function of the cable cross-section:
For the connecting cable used in the example with a wire cross-section of 0.5 mm2 , the resistance for a 10-meter-long two-wire circuit is 0.6 ohms. This means that the measured values must be corrected by this contribution. With a Pt100, this corresponds to approx. 1.6 °C.
The following diagram shows the correction values for a Pt100 as a function of the cross-section of a 10-meter-long copper cable in two-wire connection.