9.2.1 Thermocouple Temperature Sensing Application

Thermocouples are among the most commonly used sensors to measure temperature. Thermocouples work on the principle that two dissimilar metals placed in contact will generate an output voltage as a function of temperature as shown in Figure 75. This output voltage is proportional to the difference between the hot junction temperature and the cold junction temperature by a scaling factor (α) known as the Seeback coefficient. To ensure the measured output voltage accurately represents that generated by the hot junction the two junctions from where Vout is measured should remain at the same (cold junction) reference temperature. Therefore, in order to determine the temperature of the hot junction the thermocouple output voltage must be measured, the cold junction temperature known, and the voltage versus temperature characteristics (Seeback coefficient) for the type of thermocouple used be known.

Figure 75. Basic Thermocouple Configuration

The output voltage of the common type thermocouples is very repeatable and well documented by the American National Institute of Standards (ANSI). Figure 76 shows the thermocouple output voltage versus temperature for the most common types of thermocouples. As the graph illustrates, the output voltage is relatively small, less than 90 mV across all types of thermocouples.

Figure 76. Thermocouple Voltage vs Temperature vs TC Type