553BTechnical specifications
A.7 Thermocouple and RTD signal modules (SMs)
S7-1200 Programmable controller
712
System Manual, 11/2011, A5E02486680-05
When you connect a thermocouple to the SM 1231 Thermocouple module, the two dissimilar
metal wires are attached to the module at the module signal connector. The place where the
two dissimilar wires are attached to each other forms the sensor thermocouple.
Two more thermocouples are formed where the two dissimilar wires are attached to the
signal connector. The connector temperature causes a voltage that adds to the voltage from
the sensor thermocouple. If this voltage is not corrected, then the temperature reported will
deviate from the sensor temperature.
Cold junction compensation is used to compensate for the connector thermocouple.
Thermocouple tables are based on a reference junction temperature, usually zero degrees
Celsius. The cold junction compensation compensates the connector to zero degrees
Celsius. The cold junction compensation restores the voltage added by the connector
thermocouples. The temperature of the module is measured internally, then converted to a
value to be added to the sensor conversion. The corrected sensor conversion is then
linearized using the thermocouple tables.
For optimum operation of the cold junction compensation, the thermocouple module must be
located in a thermally stable environment. Slow variation (less than 0.1° C/minute) in
ambient module temperature is correctly compensated within the module specifications. Air
movement across the module will also cause cold junction compensation errors.
If better cold junction error compensation is needed, an external iso-thermal terminal block
may be used. The thermocouple module provides for use of a 0° C referenced or 50° C
referenced terminal block.
A.7.1.2
Selection tables for the SM 1231 thermocouple
The ranges and accuracy for the different thermocouple types supported by the SM 1231
Thermocouple signal module are shown in the table below.
Table A- 97 SM 1231 Thermocouple selection table
Type
Under-range
minimum
1
Nominal range
low limit
Nominal range
high limit
Over-range
maximum
2
Normal range
3, 4
accuracy @ 25°C
Normal range
1, 2
accuracy
0°C to 55°C
J
-210.0°C
-150.0°C
1200.0°C
1450.0°C
±0.3°C
±0.6°C
K
-270.0°C
-200.0°C
1372.0°C
1622.0°C
±0.4°C
±1.0°C
T
-270.0°C
-200.0°C
400.0°C
540.0°C
±0.5°C
±1.0°C
E
-270.0°C
-200.0°C
1000.0°C
1200.0°C
±0.3°C
±0.6°C
R & S
-50.0°C
100.0°C
1768.0°C
2019.0°C
±1.0°C
±2.5°C
N
-270.0°C
-200.0°C
1300.0°C
1550.0°C
±1.0°C
±1.6°C
C
0.0°C
100.0°C
2315.0°C
2500.0°C
±0.7°C
±2.7°C
