Connecting Thermocouples - Siemens SIMATIC ET 200S Manual

12.2.2

Connecting Thermocouples

Introduction
This section contains additional information on connecting thermocouples.
Compensation of the Reference Junction Temperature
There are several ways in which you can measure the reference junction
temperature to obtain an absolute temperature value from the difference in
temperature between the reference junction and the measurement junction.
Table 12-25 Compensation of the Reference Junction Temperature
Option
No compensation
Using a
compensation unit
in the input lines
of a single
thermocouple
Use of a
resistance
thermometer
Pt100 climate
range to measure
the reference
junction
temperature
(practical method)
Internal
compensation
with 2AI TC High
Feature
ET 200S Distributed I/O System
EWA-4NEB 780602402-12
Explanation
You obtain not only the temperature
of the measurement junction. The
temperature of the reference junction
(transition from copper line to
thermocouple line) influences the
thermal voltage. The measured
value is thus faulty.
They are compensated via a
compensation unit. The
compensation unit is the transition
point of Cu wire to the thermocouple
cable. No additional processing by
the 2AI TC ST is required.
You can measure the reference
junction temperature using a
resistance thermometer (Pt100
climate range). With suitable
parameter settings, this temperature
value is distributed to the 2AI TC
Standard modules in the ET 200S
and is used to correct the measured
temperature value of the
measurement junction in the
modules.
Number of reference junctions: 1
A temperature sensor is installed in
the TM-E15S24-AT and
TM-E15C24-AT terminal modules.
The temperature sensor reports the
terminal temperature to the 2AI TC
High Feature. This value is
computed with the measured value
of the channel of the electronic
module.
Analog Electronic Modules
Reference junction
parameter
No
No
The parameters of IM151-1
and of the 2AI TC must be
adapted:
•
2AI RTD Standard
configured to Pt100
climate range in the
correct slot;
•
2AI TC Standard:
Reference junction: RTD;
select reference junction
number 1
•
IM :
assignment of junction to a
slot with 2AI RTD
Standard; selection of a
channel;
•
2AI TC High Feature:
Reference junction: Yes
12-21