Temperature Sensors - Thermocouple

As the other groups mentioned, resistive temperature detectors (RTD), simple mercury thermometers, and more technologically advanced infrared sensors are all used to sense temperature. I would like to discuss a different type of temperature measurement – thermocouples. As Elda mentioned, thermocouples tend to be less accurate than RTDs, but they are still widely used in industry. They are simple and easy to understand. They have some advantages too which include a wide temperature range, robustness, rapid responsiveness, and lack of self heating.

Like most scientific inventions, the thermocouple was invented by accident. An Estonian physician accidentally discovered the ability to sense temperature by the effect of joining two different metals together. When two different metal wires are joined together and a temperature difference exists along them, they generate voltage, which is indicative of the temperature difference. Figure 1 presents a simple thermocouple diagram: the junction where the metals meet is called the measurement junction, or the hot junction. That point should be exposed to the temperature we would like to measure. The wires should then be placed in what’s referred to as the reference junction, or the cold junction. At that point the wires are generally inserted in a bath of ice water to maintain a constant 0 degrees Celsius. Thermocouples measure the relative temperature between the two junctions, and therefore the reference junction must be known, and is usually kept at 0 degree Celsius.

Figure 1: Thermocouple Diagram

The metals used are indicative of the sensitivity, temperature range, and voltage range measured by the thermocouple. Table 1 has this information about the common types of thermocouple. The types also indicate the error in measurements. As mentioned before, the error in measurement can be significant when using thermocouples. Figure 2 shows the possible error for four different thermocouples for the temperature range of 0 to 400 degree Celsius.

Table 1: Types of Thermocouples

Figure 2: Error in Thermocouple Measurements

What is actually measured when using a thermocouple is the voltage created by the difference in temperature. In order to interpret this date, one needs to know how to convert the voltage data to meaningful temperature data, which is, again, dependent on the type of thermocouple. The seedback coefficient is the voltage change per degree Celsius in μV per degree Celsius, and it is represented in Table 2 for the different thermocouple types at 25 degree Celsius. The seedback coefficient is not constant, though, which makes the fitted graphs used to interpret the temperature nonlinear. Software needs to collect the voltage data and have a function imbedded within it used to convert these measurements to useful temperature data.

Table 2: Seedback Coefficient at 25 Degree Celsius

References:

http://cds.linear.com/docs/Application%20Note/an28f.pdf

http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.pdf