Thermocouples are a popular type of temperature sensor due to their ruggedness, relatively low price, wide temperature range, and long-term stability. The Seebeck effect discussed in the previous article is the underlying principle that governs the thermocouple operation. The Seebeck effect describes how a temperature difference (ΔT) between the two ends of a metal wire can produce a voltage difference (ΔV) across the length of the wire. This effect is characterized by the following equation:
Where S denotes the Seebeck effect of the material. This equation can be also expressed as:
Here, S(T) emphasizes that the Seebeck effect is a function of temperature. Note that the Seebeck effect is also observed in metal alloys and semiconductors. Let’s see how this effect can be used to measure temperature.
Seebeck Effect of an Individual Material: Copper Wire Example
Equation 1 suggests that by having the Seebeck coefficient of a material, the voltage difference across a conductor can be used to determine the temperature difference between the two ends. Although this is theoretically correct, the direct measurement of an individual material’s Seebeck voltage is impossible. As an example, consider the setup shown in Figure 1.
The ends of the copper wire are at T1 = 25 °C and T2 = 100 °C. Assume that, over this temperature range, the absolute Seebeck coefficient of copper is constant and equal to +1.5 μV/°C. Using Equation 1, we can find the voltage difference across the wire as:
The voltage measured by the multimeter will be different because the path consisting of the multimeter leads and the input circuitry of the multimeter also experiences a temperature difference of 75 °C. Unwanted Seebeck voltage across the test leads and the input circuitry of the multimeter introduces errors.
Avoiding Seebeck Voltage—Keeping the Multimeter at Uniform Temperature
To avoid creating a Seebeck voltage in the test leads and the multimeter, we should keep these parts at a constant temperature. For example, we can keep the measurement system at 25 °C as shown in Figure 2.