thermocouple or thermocouple thermometer is a junction between two
different metals that produces a voltage related to a temperature
difference. Thermocouples are a widely used type of temperature
sensor for measurement and control and can also be used to convert
heat into electric power. They are inexpensive and interchangeable,
are supplied fitted with standard connectors, and can measure a wide
range of temperatures.
thermocouple circuit has at least two junctions: the measurement
junction and a reference junction. Typically, the reference junction
is created where the two wires connect to the measuring device. This
second junction it is really two junctions: one for each of the two
wires, but because they are assumed to be at the same temperature
(isothermal) they are considered as one (thermal) junction. It is
the point where the metals change - from the thermocouple metals to
what ever metals are used in the measuring device - typically
principle of thermocouples depends on the see beck effect.
See beck effect:
Seebeck effect states when two
dissimilar metal wire are connected with each other in a loop to
form two junctions, maintained at two different temperatures, a
voltage potential or electromotive force (E=emf)
will be generated and the current will flow through the loop
The current will
be proportional to the difference in temperature between the
junctions and the metals used. The higher the temperature
difference, the higher is the electromotive force (emf)
and the current flow in the loop.
types vary in the
combinations of metals used and the
'standard' thermocouple types are commonly used. Eight have been
given internationally recognized letter type designators. The letter
type designator refers to the emf table,
not the composition of the metals - so any thermocouple that matches
the emf table within the defined
tolerances may receive that table's letter designator.
The four most common calibrations are J,
K, T and E. Each calibration has a different temperature range and
environment, although the maximum temperature varies with the
diameter of the wire used in the thermocouple. 
four "classes" of thermocouples (based on the metals used):
The home body class (called base metal),
The upper crust class (called rare metal
or precious metal),
The rarified class (refractory metals)
The exotic class (standards and
Base metals - up
Type J, Type E, Type T, Type K
Noble metals –
up to 2000˚C
Type R, Type S,
metals – up to 2600˚C
Type C, Type D,
Selection of Thermocouple:
important question that has to be solved when installing a
thermocouple is which one to use? The following criteria are used in
selecting a thermocouple:
Temperature range (Industry generally prefers K and N types because
of their suitability to high temperatures).
Chemical resistance of the thermocouple or sheath
material (including corrosion resistance).
Abrasion and vibration resistance.
Installation requirements (may need to be compatible with existing
equipment; existing holes may determine probe diameter) i.e. the
ease of use is important.
Sensitivity of thermocouple (the T type is usually favored by
industries on this basis).
can be very rugged
are immune to shock and vibration
are useful over a wide temperature range
(Capable of being used to directly measure temperatures up
are simple to manufacture,
require no excitation power,
there is no self heating and
can be made very small.
The thermocouple junction may be grounded and
brought into direct contact with the material being measured.
Temperature measurement with a
thermocouple requires two temperatures be measured, the junction at
the work end (the hot junction) and the junction where wires meet
the instrumentation copper wires (cold junction). To avoid error the
cold junction temperature is in general compensated in the
electronic instruments by measuring the temperature at the terminal
block using with a semiconductor, thermistor, or RTD. This is called
The output signal produced is relatively
Thermocouple operation is relatively
complex with potential sources of error. The materials of which
thermocouple wires are made are not inert and the thermoelectric
voltage developed along the length of the thermocouple wire may be
influenced by corrosion etc.
The relationship between the process
temperature and the thermocouple signal (millivolt) is not linear.
The calibration of the thermocouple
should be carried out while it is in use by comparing it to a nearby
comparison thermocouple. If the thermocouple is removed and placed
in a calibration bath, the output integrated over the length is not
The main limitation is accuracy: system
errors of less than one kelvin (K) can be difficult to achieve. 
are widely used in science and industry; applications include
temperature measurement for:
Gas turbine exhaust.
Some other major
applications in industry includes
Type B, S, R and K
thermocouples are used extensively in the steel and iron industries
to monitor temperatures and chemistry throughout the steel making
process. Disposable, immersible, type S
thermocouples are regularly used in the electric arc furnace process
to accurately measure the temperature of steel before tapping. The
cooling curve of a small steel sample can be analyzed and used to
estimate the carbon content of molten steel.
Thermopile radiation sensors
used for measuring the intensity of incident radiation, typically
visible or infrared light, which heats the hot junctions, while the
cold junctions are on a heat sink. It is possible to measure
radiative intensities of only a few μW/cm2
with commercially available thermopile sensors. For example, some
laser power meters are based on such sensors.
can generally be used in the testing of prototype electrical and
mechanical apparatus. For example, switchgear under test for its
current carrying capacity may have thermocouples installed and
monitored during a heat run test, to confirm that the temperature
rise at rated current does not exceed designed limits.
Radioisotope thermoelectric generators
Thermopiles can also be applied to
generate electricity in radioisotope thermoelectric generators.
and petroleum refineries will usually employ computers for logging
and limit testing the many temperatures associated with a process,
typically numbering in the hundreds. For such cases a number of
thermocouple leads will be brought to a common reference block (a
large block of copper) containing the second thermocouple of each
circuit. The temperature of the block is in turn measured by a
thermistor. Simple computations are used to determine the
temperature at each measured location. 
A calibration is a
matter of qualifying the sensor-under-test. Only by knowing the
limitations of the sensor it is possible to thrust the measurements
and optimize the process control.
calibration is the process of adjusting the instruments output
signal to match a known range of variables.
important because all instruments tend to drift from their last
setting. This is because spring stretch, electronic component
undergo slight changes on the atomic level, and other working parts
bend or lose their elasticity. 
Process instrument calibration:
consists of comparing the output of the process instrument being
calibrated against the output of a standard instrument of known
accuracy, when the same input is applied to both instruments. During
this calibration process the instrument is tested over its whole
range by repeating the comparison procedure for a range of inputs.
The instrument used
as a standard for this procedure must be one which is kept solely
for calibration duties. It must never be used for other purposes.
Most particularly, it must not be regarded as a spare instrument
which can be used for process measurements if the instrument
normally used for that purpose breaks down. Proper provision for
process instrument failures must be made by keeping a spare set of
process instruments. Standard calibration instruments must be
totally separate. 
Calibration of thermocouples
A thermocouple is
calibrated by comparing its response with a standard thermometer at
the same temperature. The standard thermometer may be another
thermocouple, a platinum resistance thermometer or a liquid in glass
are calibrated by one or more of three general methods, depending on
- the type of
- the temperature range,
In the first
method, thermocouples are calibrated by comparison with a reference
In the second
method, thermocouples are calibrated against a standard platinum
In the third
method, thermocouples are calibrated at four defining temperatures,
the freezing points of zinc, aluminum, silver, and gold.
areas are immediately apparent. There must be available:
- Means for measuring the
output of the temperature sensor
- Controlled temperature
Thermocouple calibration procedure:
The following information gives the detail of equipment requirements
and proper techniques needed to accurately calibrate thermocouples
and thermocouple materials.
The temperature source used in the
process of calibrating should be stable enough to provide a constant
temperature for a short length of time at any temperature at which
the temperature bath or other source is to be used. The temperature
source should have a zone of uniform temperature into which the
thermocouple measuring junction may be inserted. The length of the
temperature source must be adequate so that the measuring junction
temperature is not affected by a temperature gradient along the
A thermocouple's output is based on
the difference in temperature between the measuring junction (hot
junction) and the reference junction (cold junction) 
A controlled temperature must be
provided in which the reference junction is maintained at a constant
chosen temperature. The reference junction temperature should be
controlled to a better accuracy than that expected from the
thermocouple calibration. The most commonly used reference
temperature is 32 degrees F., but other temperatures may be used if
One of the most common reference
junctions is the ice bath. The ice bath is made up of a mixture of
melting shaved ice and water. The ice bath is a convenient and
inexpensive way to achieve an ice point,
it can be reproduced with ease and with exceptional accuracy.
Junctions formed between the thermocouple materials and instrument
leads can be simply immersed into the slush mixture, or
alternatively glass "U" tubes containing mercury into the slush
mixture. Quick electrical connection can then be made between
thermocouple and instrument leads through the mercury. 
This method employs a compensation
circuit containing a source of current and a combination of fixed
resistors and a temperature sensitive resistor. This device can be
designed to produce similar EMF to that of the thermocouple being
The choice of a specific instrument
for measuring the thermocouple output will depend on the accuracy
required of the calibration being performed.
The reference thermometer to be used
for the comparison calibration of a thermocouple will depend upon
the temperature range
the accuracy desired,
preference of the calibration laboratory.
The following are different examples
of reference thermometers.
A standard platinum resistance
thermometer is the most accurate standard available, however, it is
the most expensive standard, and other standards are acceptable
alternatives depending upon the temperature range covered.
Liquid-in-glass thermometers are
available to cover the range from -300 to 950 degrees Fahrenheit.
with an accuracy of from .1 to 3
Fahrenheit depending on the type of thermometer and the width of the
range covered. They are relatively inexpensive but they are fragile,
and if the highest degree of accuracy of which they are capable is
to be achieved, an individual thermometer must cover a very narrow
temperature range so that the graduation intervals can be as large
as possible. A further disadvantage of the liquid-in-glass
thermometer is that because of their fine graduations reading errors
are a distinct possibility.
Depth of immersion is the most
important consideration if accurate calibration results are to be
obtained. The depth of immersion must be sufficient to eliminate the
effects of heat transfer away from the junction. It is impossible to
establish a minimum depth of immersion that would be useable under
all circumstances since heat transfer characteristics are dependent
on the mass of material being put into the temperature source.
connection from test assembly to read out instrument:
The actual wiring necessary to
connect the test assembly, reference junction and readout instrument
will depend on the quantity of thermo elements in the test assembly,
the type of reference junction used and whether or not a switching
device is used. Thermocouple extension wire is used to connect the
thermo elements to the reference junction. Copper wires are used
between the reference junction and readout instrument.
wire, wiring procedure:
Ideally, the samples of the
thermocouple material to be calibrated and the standard thermocouple
element should be cut long enough so that they reach directly from
the temperature source to the reference junction without the need
for extension wires. If this is not possible extension wires may be
used, but they must be securely connected directly to the test
assembly conductors. If extension wires must be used, remove any
oxide layer that may be on the surface of the test assembly
conductors and attach an extension wire of the same material to each
conductor by laying the extension wire alongside the conductors and
joining them securely by means of an alligator clip.
calibration wiring procedure:
When calibrating thermocouples, it is
faster and more convenient to use a thermocouple switching box. The
extension wires from the thermocouples are placed into one side of
the reference junction. Multiple pairs of copper lead wire will exit
the reference junction and will be connected to the switch box. One
pair of copper lead wires will run from the readout instrument to
the thermocouple switch box.
One of the primary advantages of
calibrating thermocouple materials against a base-metal standard of
similar EMF output is that the sample to be calibrated are welded to
the base-metal standard forming a common junction thus achieving
good isothermal conditions between the test thermo element and the
standard. Furthermore, because the test thermo element and the
standard produce nominally the same EMF vs. platinum the EMF output
changes little over a fairly broad temperature range, thereby
reducing the need for precise temperature source control.
Set your controlled temperature
source to the specified temperature and allow it to adequately
stabilize. Immerse the test assembly into the test temperature
medium and provide sufficient time for the test assembly to
stabilize. Once the test assembly is stable the EMF generated
between the test specimen and the reference standard can be
recorded. Avoid soaking the test assembly at temperature for a
prolonged period of time, as it can cause permanent changes to occur
in the thermo elements.
Once the reading is taken, raise the
test temperature to the next higher temperature, first removing the
test assembly from the temperature source, or advance the test
assembly to the next temperature source. Allow the temperature
source and the test assembly to stabilize as before, and take a
second set of readings at the new temperature. In all cases take the
reading in sequence from the lowest to the highest temperature. A
base metal reference standard shall be used for one series of
temperature changes only.
The Test thermocouple junction should
be located so that it is in intimate contact with the junction of
the standard. Without making a radiograph of the thermocouple it is
impossible to know exactly where the junction is located. A few
generalizations can be made which enables junctions to be located
quite closely. First, the cap weld on a metal sheathed thermocouple
is normally about as thick as one-half the sheathed diameter.
Second, a "U" junction is normally about one-half the sheathed
diameter. The thermocouple standard should be tied to the
thermocouple with a fine gauge wire. The junction of the standard
should be bent so that it is in contact or at least very close to
the point where it has been calculated that the junction is located.
The calibration curve for a
thermocouple is often constructed by comparing thermocouple output
to relatively precise thermometer data. Then, when a new temperature
is measured with the thermocouple, the voltage is converted to
temperature terms by plugging the observed voltage into the
regression equation and solving for temperature. 
[retrieved on November 9, 2010]
[retrieved on November 9, 2010]
[retrieved on November 9, 2010]
Singh;”Industrial Instrumentation and Control” McGraw Hill
publishing company limited, New Delhi; page no (139-140)
 Alan S.
Moris; “Principles of Measurement and
Instrumentation”; second edition page no (70-71)
 Fundamentals of
Temperature, pressure and Flow measurements; “Robert P. Benedict”;
John Wiley and Sons publisher; third edition page no (146)
[retrieved on November 9, 2010]
[retrieved on November 9, 2010]