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6. Resistance Thermometers – Measuring Circuit


     
Temperature measurement by means of resistance thermometers uses resistance detectors, in which the temperature dependence of the resistor is known and defined in standards. Predominantly platinum (and, to a much lesser extent, nickel) resistors, whose electrical resistance value increases with the rising temperature, due to the positive temperature coefficience, are used in industrial measurement technology. Unlike the voltage-generating thermocouple assembly, a resistor does not provide any electrical energy, meaning that (external) energy is supplied through an electrical measuring circuit. A constant current source, which generates a voltage drop proportional to the resistance at the resistance detector, is usually used to provide this.   


6.1 Schematic Diagrams    

 

Figure 4 shows the possible connection techniques for resistance thermometers   

 

     

2–wire circuit

 

2-wire circuit with loop

     

 


     

3-wire circuit

 

4-wire circuit

     

 

a. Resistance detector
b. Inner conductor (inner cable): Connection between resistance detector and connection point
c. Connection point: Junction of inner conductors and connection cable
d. Connection cable (copper cable): Connection between resistance thermometer and measuring equipment
e. Measuring equipment and constant current source: Measurement of the voltage drop (U, U1, U2) above the resistance detector through which there is a constant current flow.   

6.2 Connection Techniques: 

     
Figure 4 shows the possible connection techniques for resistance thermometers, making a division into two-wire, three-wire and four-wire circuits. The two-wire circuit connects the resistance detector to the measuring equipment through a two-wire cable. The existing lead resistances which are connected in series to the resistance detector cause a higher total resistance and hence some significant measurement errors. For this reason, (cable) compensation is also required here. As the lead resistances change additionally depends on temperature, the two-wire circuit is only suitable for simple measurements. The two-wire circuit with loop provides a variant to the simple two-wire circuit. Provided that all four leads have the same properties and are exposed to the same temperatures, the lead resistances can be measured under operational conditions via the loop and compensated accordingly. Instead of this connection variant, however, current industrial measurement technology makes almost exclusive use of the three-wire circuit described below. With the three-wire circuit, an additional cable is carried to the measuring equipment from a connection contact of the resistance detector, giving rise to two measuring circuits. Provided that the resistances of all three leads are the same, the influence of the cable resistance is compensated, in current standard measuring equipment with a three-wire input, by a corresponding internal connection technique. No cable compensation is required here. In the case of the four-wire circuit, if the cable resistances of the four leads differ, then they are also compensated. The voltage drop "U" above the resistance detector through which there is a constant current flow undergoes high-impedance tapping here via two measuring cables. Provided that the input/ inner resistance of the (voltage) measuring equipment is a multiple of the cable resistance, no significant measurement error occurs. For this reason, the four-wire circuit is used for temperature measurements that require particularly high measurement precision.

Go to Section 7   (Configuration of Resistance Thermometers)
   

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WT-Measuring Circuit

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