LESSON 1
THE THERMOCOUPLE
TASK
Describe and identify the theory of a thermocouple used in the applications of DCLF
thermal measurements.
CONDITIONS
Given the information and illustrations about thermocouple theory relating to DCLF
thermal measurements.
STANDARDS
Demonstrate competency of the task skills and knowledge by correctly responding to
at least 75 percent of the multiple-choice test covering theory and application of
a thermocouple in DCLF thermal measurements.
REFERENCES
None
Learning Event 1:
DEFINE AND IDENTIFY THE THEORY OF A THERMOCOUPLE
1. The thermocouple is perhaps the simplest of all primary measuring elements. In
its simplest form it consists of two dissimilar electrical conductors joined at
each end to form a continuous circuit, and electrically insulated from each other
along their length between these two junctions.
This thermocouple develops an
electromotive force (EMF/voltage) when there is a temperature difference between
the two junctions and a current will flow in the closed circuit. In practice, one
of these, junctions is called the measuring (hot) junction and the other is called
the reference (cold) junction.
The power or current that is available from a
thermocouple to drive a measuring device is quite small; however, this drawback has
been largely overcome by the development of high-gain amplifiers, millivoltmeters,
and galvanometers of improved sensitivity.
2. It has been stated that the power or current available from a thermocouple
is quite small; but what type of current is it? The voltage available
from a thermocouple is direct current (DC).
This DC output to the
measuring instrument is determined by the amount of current applied to the
measuring junction regardless of whether it is DC or alternating current (AC). The
thermocouple output is determined by the heat difference between the measuring and
reference junctions.
This heat difference is determined by the root mean square
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