Measurement and Control Basics Fourth Edition

Chapter 7 - Temperature Measurement: Thermocouples

Thermoelectric Circuit

When two wires composed of dissimilar metals are joined at both ends
and one of the ends is heated, a continuous current flows in the "thermoelectric"
circuit. Thomas Seebeck made this discovery in 1821. This thermoelectric
circuit is shown in Figure 7-4(a). If this circuit is broken at the
center, as shown in Figure 7-4(b), the new open-circuit voltage (known as
"the Seebeck voltage") is a function of the junction temperature and the
compositions of the two metals.

All dissimilar metals exhibit this effect, and this configuration of two dissimilar
metals joined together is called a thermocouple, which is abbreviated
TC. The most common TCs and their normal temperature ranges are
listed in Table 7-3.

For small changes in temperature, the Seebeck voltage is linearly proportional
to temperature:

 Vab = a(T1 - T2)
(7-8)

where a, the Seebeck coefficient, is the constant of proportionality.

Figure 7-4. The thermoelectric circuit

Table 7-3. Standard Thermocouple Types and Ranges

Example 7-5 shows how to calculate the Seebeck voltage for a thermocouple.

EXAMPLE 7-5

Problem: Find the Seebeck voltage for a thermocouple with a= 40 µv/°C if
the junction temperatures are 40°C and 80°C.

Solution: Using Equation 7-8, the Seebeck voltage can be found as follows:

 

 

 

 

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