Resistance thermometers change their electrical resistance in dependence on temperature. In order to record the output signal, the line drop created by a constant measuring circuit is measured.
Acc. to the Ohm ́s law the following is valid for this line drop: U = R x l
In order to avoid the heating of the sensor, a small measuring circuit shall be chosen. A measuring circuit of 1 mA does not have any considerable impact. This current creates a line drop of 0,1 V with a PT 100 at 0°C. This measuring voltage has tobe transferred to the display for evaluation as accurately as possible. We distinguish between four connection techniques:
2 - Wire Circuit
Pt100 2-wire connection
The connection between evaluation unit and thermometer is made by a 2 conductor cable. Like any other electrical conductor, such a cable has a resistance itself in serial mounting with the resistance thermometer. Thus, the two resistances are added that is interpreted as a higher temperature by the processing unit. In case of far distances the cable resistance can amount to several ohms and in this way falsify the measuring result.
Example:
cable section: 0,35 mm2
spec. resistance: 0,0175 Ω mm² m-1
cable length: 50 m
cable material: copper (E-CU)
R = 0,0175 Ω mms² m-1 x 2 x 50 m/ 0,35 mm²= 5,0 Ω
Configuration example: Pt100 sensor with 2-wire circuit
3 - Wire Circuit
Pt100 3-wire connection
In order to minimize the influences of the cable resistance and its temperature dependant fluctuations, the 3-wire circuit is frequently used instead of the above mentioned 2-wire circuit. Therefore, an additional cable is led to a contact of the RTD. Thus, 2 measuring circuits are created, one of them being used as reference.
Due to the 3-wire circuit, the cable resistance is compensated with regard to its amount as well as with regard to its temperature dependence provided that the 3 conductors have the same characteristics and are exposed to the same temperature. Therefore, a compensation of the cable resistance is no longer necessary.
Conficuration example: Pt100 sensor with 3-wire circuit
4 - Wire Circuit
Pt100 4-wire connection
The best connection type for resistance thermometers is the 4-wire circuit. The measuring result is neither influenced by the cable resistance nor by their temperature dependent fluctuations. A compensation of the cable resistance is no longer necessary. The thermometer is fed with the measuring circuit via cable.
Is the incoming resistance of the topped electronics a multiple of the cable resistance, it is to be neglected. Thus the voltage drop is independent from the characteristics of the line. For the 3-wire as well as for the 4-wire circuit, it has to be considered that the circuit is not always led to the measuring element. The connection of the sensor to the connection head in the armature, the so called inner circuit is often done in a 2-wire circuit. This results in the problems of a 2-wire circuit – even to a smaller extent.
Conficuration example: Pt100 sensor with 4-wire circuit
Pt100 inner conductor colors
Pt100 Connection and circuit of the conductors (inner conductor) | ||||
Measuring element | Pt100 2-wire connection colors | Pt100 3-wire connection colors | Pt100 4-wire connection colors | Pt100 2-wire with loop |
Pt100 | red/white | red, red/white | red, red/white, white | red, red/blue, blue |
2 x Pt100 inner conductor colors
Circuit of the inner conductors | |||
Measuring element | 2-wire circuit colors | 3-wire circuit colors | 4-wire circuit colors |
2 x Pt100 | yellow, yellow / white, white | red, red / black, black / white | red, red / white, white / black, black / white, white |
3 x Pt100 inner conductor colors
Circuit of the inner conductors | |
Measuring element | 2-wire circuit colors |
3 x Pt100 | red, red / white, white / yellow, yellow |
More information about Pt100 sensors.