Thermocouple Basics; Thermocouple Module Ranges - Siemens SIMATIC S7-200 System Manual

S7-200 Specifications

Thermocouple Basics

Thermocouples are formed whenever two dissimilar metals are electrically bonded
to each other. A voltage is generated that is proportional to the junction
temperature. This voltage is small; one microvolt may represent many degrees.
Measuring the voltage from a thermocouple, compensating for extra junctions, and
then linearizing the result forms the basis of temperature measurement using
thermocouples.
When you connect a thermocouple to the EM 231 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 disimilar
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 module 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. This
temperature is converted to a value to be added to the sensor conversion. The
corrected sensor conversion is then linearized using the tables.

Thermocouple Module Ranges

Table A-28 and Table A-29 show temperature ranges and accuracy for each type
of thermocouple.
A-74
S7-200 Programmable Controller System Manual
A5E00066097-02