Thermocouple Theory

The thermocouple is one of the least complex of all sensors and as such it can be both rugged and low cost making it ideally suited for use in industrial applications.

It consists of two wires of dissimilar metals that are joined at the measurement point; referred to as the hot junction. The output from the sensor is a voltage proportional to the temperature difference between the hot junction and the cold end of the wires. The voltage generated is both very small and nonlinear, this presents design challenges for manufacturers of thermocouple instrumentation. However if the instrumentation is designed correctly the thermocouple provides a very low cost sensor that can be easily replaced if damaged.

Thermocouples are divided into two basic groups; those constructed from base metals and those from noble metals. Base metals are lower cost elements typically copper, iron or nickel alloys. The exact ratios of metals in the alloys are not defined, but the voltage/temperature response curve is. So, at a microscopic level, base metal thermocouple wires from two different suppliers could have different ratios of metals. Noble metal thermocouples are constructed using expensive elements such as platinum and rhodium. The exact purity levels and ratios are defined, as are the voltage/temperature curves.

Some commonly used thermocouple types and their use range are listed below.

Careful electronic and mechanical design of thermocouple instrumentation, such as dataloggers is required to ensure best possible accuracy. The mechanical design has to ensure that the cold junction sensor is isothermal with the point at which the thermocouple wires connect with the copper tracks/wires of the circuit board. If the cold junction sensor cannot accurately track the changes in temperature it will result in an error in the final measurement equal to the tracking difference. It is critical this area is free from thermal gradients.

With the voltage from the thermocouple being as low as 4uV/C amplification is required prior to digitization of the signal. Thermocouples are generally long parallel wires and are often used in electrically hostile environments where they will behave as an antenna picking up interfering signals; therefore careful electronic design is needed to filter out the unwanted signals.