英文文献翻译

University of Tasmania

School of Engineering

KNE222 Electronic Engineering

Temperature Measurement – Thermocouples

Introduction.

A thermocouple is formed when two dissimilar metals are joined. If a second joint is made from the same materials, and one joint is heated with respect to the other, then a current will flow round the loop. Alternatively, if the circuit is opened, then a small voltage will appear at these terminals, proportional to the difference in temperature between the junctions. This voltage is known as the Seebeck voltage, after Thomas Seebeck who discovered the phenomenon in 1821.

Figure 1. Conceptual Application of a Thermocouple

(Ref: ”Operational Amplifier Circuits Theory and Applications” E.J Kennedy. HRW Inc.)

In order to use a thermocouple to measure temperature with one junction, a reference temperature must be established for the other. This can be done as illustrated in Figure 1, where a 0 ºC ice bath is provided for the reference junction. However an ice bath is not really practical for everyday measurements. Fortunately it is possible to introduce a correction voltage into the circuit so that the reference junction appears to be at zero degrees C. An example of this will be presented later.

Thermocouple Characteristics.

Table 1 provides a list of commonly used thermocouple materials, together with the useful range of temperatures for each. While the Seebeck voltage increases with the junction temperature difference, the relationship is unfortunately not entirely linear. The proportionality constant is known as the Seebeck coefficient, α, and has the units μV/ºC. Table 1 provides typical α values at 25 ºC; it also gives some idea of how α varies over the useable temperature range for each thermocouple.

Table 1. Characteristics of common Thermocouples

(Ref: “Operational Amplifier Circuits Theory and Applications” E.J Kennedy. HRW Inc.)

Figure 2 shows the variation of α as a function of temperature for K and T Type thermocouples. The K type device offers almost constant α value s over much of its useful range, and therefore is useable there without linearization. In contrast, the Seebeck coefficient of the T type thermocouple varies considerably with temperature and as a result these devices must be corrected using a linearization algorithm in order to obtain accurate temperature measurements.

Figure 2. Variation of the Seebeck Coefficient with Temperature, T and K Type Thermocouples.

(Ref: ”Operational Amplifier Circuits Theory and Applications” E.J Kennedy. HRW Inc.)

Finally, figure 3 shows the Seebeck Voltage as a function of temperature difference for the thermocouples listed in Table 1. In each case the reference junction (sometimes called the cold junction), is held at zero ºC, thus all these curves pass through the origin. Because they are slightly non-linear, it is common to apply a linearization algorithm to the Seebeck Voltage (E), in order to obtain the correct temperature (T). In its simplest form this algorithm assumes that the relationship between Seebeck Voltage and temperature is linear and an equation of the form α

T= is used. This assumption results in errors, particularly over a wide range

E

of temperatures. For more accurate results, higher order polynomials are used.

Figure 3. Seebeck Voltage as a function of Temperature Difference.

(Note: The Reference Junction temperature is 0 ºC.)

(Ref: ”Operational Amplifier Circuits Theory and Applications” E.J Kennedy. HRW Inc.)