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Thermocouples sense temperature as a result of a voltage generated between the joined
end of two dissimilar metal wires and the open end when the temperature at the joined end,
the "hot" junction, is different from the temperature at the open end, the
"cold" junction. The voltage, known as the Seeback voltage after Thomas Seeback
the discoverer of this effect, is proportional to the difference in temperature. Seeback
voltage is given by the product of the change in temperature times the Seeback coefficient
which represents the rate of change of thermal EMF (electromotive force) with respect to
temperature. This is usually stated as microvolts per degree C, and is dependent on the
temperature difference and the metals or alloys used in the thermocouple wire.
The combination of metals used in each thermocouple gives each type its name. For
example, the wires used in a "Type J" thermocouple are iron (a relatively pure
metal) and constantan (copper-nickel alloy).
Thermocouples are generally more rugged and less expensive than other types of sensors.
Where thermocouple leads are connected to a voltmeter and the metals in each pair of
wires are dissimilar, additional thermocouple-like junctions are created. For example, if
the voltmeter connections are copper, connecting a Type T (copper/constantan) thermocouple
creates two additional junctions: copper/copper and constantan/copper. No thermal EMF is
created at the copper/copper junction. At the constantan/copper junction, however, an EMF
is generated which opposes the EMF generated at the hot junction of the thermocouple.
If the temperature at the additional dissimilar junction is known, it is a simple
matter to calculate the voltage at the hot junction, which yields the correct temperature
of the process being measured. One method of compensating for this effect, is to create a
"reference" junction through the use of "isothermal blocks'. Because the
exact temperature of these blocks is known, usually a precise correction can be made,
which yields an accurate value of the temperature at the "hot" or measured
junction. These calculations are usually made by the microprocessor that is present in
most modern digital controls.
Compensation can also be made with hardware. A battery may be used to offset the
voltage generated at the reference junction. In addition, electronic circuits are
commercially available which provide the electrical equivalent of 0° C at the reference
junction.
Both the computer and hardware methods have their advantages and disadvantages, but
both can be used to provide accurate temperature measurements.
Thermocouples can also be classified by the type of junction. Exposed junction
thermocouples do not have their junctions protected by a sheath. Response time is fastest
with this type of thermocouple but the maximum allowable temperature is less than that of
a sheathed thermocouple.
A grounded thermocouple has its measuring junction in contact with a metal surface.
This is the most common type. Where electrical isolation is necessary, an ungrounded
junction is used. In this type, the junction is electrically isolated from its protective
sheath. To provide a path for noise, the sheath can be grounded.
In selecting thermocouples, it is necessary to know the temperature range to be
measured, the accuracy required, the materials to which the thermocouple will be exposed
(e.g., acids), and the environment in which the thermocouple will be used (e.g., abrasive,
vibratory, etc.).
SensorPulse manufactures a wide variety of thermocouples. All are manufactured to industry
standards and meet stringent ANSI standards. This assures interchangeability with other
standard thermocouples without requiring additional instrument recalibration.
SensorPulse thermocouples can be used in all types of applications, can measure wide
temperature ranges, and are offered in a large variety of standard configurations.
Service temperature ranges are limited by the materials used in construction. See the
application chart for wire temperature limits and service temperatures for the various
thermocouples.
See Applications Guide for additional information
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