II - DCE FEL ČVUT v Praze

3.3 Connection of Thermocouples, Voltage Sensors and Resistance
Sensors to Analog Input SM 331; AI 8 x TC/4 x RTD
Overview
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04
The following descriptions refer to the operation of transducers with the
analog input module SM 331; AI 8 x TC/4 x RTD.
A description the design and operating principle of thermocouplesand the
use of compensation boxes
A description of how you connect thermocouples to analog inputs
A description of how you connect voltage sensors to analog inputs
A description of how you connect resistance thermometers and resistance
sensors to analog inputs
3.3.1 Use and Connection of Thermocouples
Introduction
Design of
thermocouples
The design of thermocouples and what you must bear in mind when
connecting thermocouples are described in this section.
A thermocouple consists of
the actual thermocouple (measuring sensor) and
the necessary installation and connection parts.
The thermocouple is made up of two wires which are made of different
metals or metal alloys and whose ends are soldered or welded together. The
different material compositions produce different types of thermocouples,
e.g. K, J, N. Irrespective of the type of thermocouple, the measuring principle
is the same for all.
° C
Measuring junction
Thermocouple with
positive and negative limbs
Connection point
Compensation line(material with same
thermal e.m.f. as
thermocouple)
Reference junction
Copper conductor
Fig. 3-3 Measuring circuit with thermocouple
SIMATIC S7 Ex Analog Modules
Thermal e.m.f. acquisition point
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