This week here on ATS’s thermal blog, we kick off a two part series on thermocouples. Today, here’s part 1.
With their versatility and easy use, thermocouples are the most common means for doing thermal analysis and performing temperature measurements. Data from such measurements can then be used to develop the right thermal management solution, including heat sinks, for an electronic system.
Thermocouples date back to 1821, when Thomas Seebeck, an Estonian scientist, found that when two dissimilar metals were connected at both ends, and one end was heated, there was a resulting flow of current in this thermoelectric circuit.
Today, this voltage is known as electromotive force (emf), and is a function of the junction temperature and composition of the two metals. The repeatable nature of emf, along with standardized material types and calibration curves has made thermocouples the devices of choice for many temperature measuring applications.
Despite simplicity of thermocouples, their emf voltage cannot be measured directly. As shown in Figure 1, using a voltage measuring device will create two additional thermocouple junctions, designated as J2 and J3 in the thermoelectric circuit.
Figure 1 Measuring junction voltage with a DMV[1]
The voltage reading then becomes a function of three temperatures, two of which are of no interest to the experimenter. To solve this problem, another junction needs to be created in series in the circuit, as shown in Figure 2.
Figure 2 External Reference Junction [1]
This added junction is kept at a known temperature, 0°C, by submerging it in an ice bath, and is the reference temperature. Most commonly, a mixture of pure water and pure ice at 1 atmospheric pressure is used for the reference junction. Today, electronic measuring devices simulate an ice reference bath without the need for an actual ice bath. This is often referred to as cold junction compensation (CJC).
After adding the ice bath, there are still two junctions at the device terminals, but each of these consists of the same two materials. If held at the same temperature, their emf voltage will be the same and cancel each other out.
In part 2 of our two part series on thermocouples, we’ll cover what thermocouples can be made of and how to select the right thermocouple for your project.