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II - DCE FEL ČVUT v Praze

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3.3 Connection of Thermocouples, Voltage Sensors and Resistance<br />

Sensors to Analog Input SM 331; AI 8 x TC/4 x RTD<br />

Overview<br />

I/O Modules with Intrinsically-Safe Signals<br />

C79000-G7076-C152-04<br />

The following descriptions refer to the operation of transducers with the<br />

analog input module SM 331; AI 8 x TC/4 x RTD.<br />

A description the design and operating principle of thermocouplesand the<br />

use of compensation boxes<br />

A description of how you connect thermocouples to analog inputs<br />

A description of how you connect voltage sensors to analog inputs<br />

A description of how you connect resistance thermometers and resistance<br />

sensors to analog inputs<br />

3.3.1 Use and Connection of Thermocouples<br />

Introduction<br />

Design of<br />

thermocouples<br />

The design of thermocouples and what you must bear in mind when<br />

connecting thermocouples are described in this section.<br />

A thermocouple consists of<br />

the actual thermocouple (measuring sensor) and<br />

the necessary installation and connection parts.<br />

The thermocouple is made up of two wires which are made of different<br />

metals or metal alloys and whose ends are soldered or welded together. The<br />

different material compositions produce different types of thermocouples,<br />

e.g. K, J, N. Irrespective of the type of thermocouple, the measuring principle<br />

is the same for all.<br />

° C<br />

Measuring junction<br />

Thermocouple with<br />

positive and negative limbs<br />

Connection point<br />

Compensation line(material with same<br />

thermal e.m.f. as<br />

thermocouple)<br />

Reference junction<br />

Copper conductor<br />

Fig. 3-3 Measuring circuit with thermocouple<br />

SIMATIC S7 Ex Analog Modules<br />

Thermal e.m.f. acquisition point<br />

3-25

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