Measurement and Control Basics Fourth Edition

Chapter 7 - Temperature Measurement: Thermocouple Tables

To take advantage of the voltage that thermocouples produce, scientists
have developed comprehensive tables of voltage versus temperature for
many types of thermocouples. These tables (presented in Appendix B)
give the voltage that results for a particular type of thermocouple when
the reference junctions are at 0°C and the measurement junction is at a
given temperature. For example, according to Table B-4 in Appendix B, for
a type K thermocouple at 200°C with a 0°C reference, the voltage produced
is as follows:

 V(200°C) = 8.13 mV(type K, 0°C)

Conversely, if a voltage of 29.14 mV is measured with a type K thermocouple
with a 0°C reference, we find from Table B-4 that

 T(29.14 mV) = 700°C(type K, 0°C)

However, in most cases the TC voltage does not fall exactly on a table
value. When this happens, you must interpolate between the table values
that bracket the desired value. You can find an approximate value of temperature
by using the following interpolation equation:

 (7-9)

In this equation, the measured voltage Vm lays between a higher voltage
Vh and lower voltage Vl given in the tables. The temperatures that correspond
to these voltages are Th and Tl. Example 7-6 illustrates this concept.

EXAMPLE 7-6

Problem: A voltage of 6.22 mV is measured with a type J thermocouple at a
0°C reference. Find the temperature of the measurement junction.

Solution: From Table B-1 in Appendix B we see that Vm = 6.22 mv lies
between Vl = 6.08 mv and Vh = 6.36 mv, with corresponding temperatures of
Tl = 115°C and Th = 120°C, respectively. Therefore, using Equation 7-10, we
can find the junction temperature as follows:

 

 

 

 

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