A thermocouple is a device that uses the Seebeck effect to convert temperature differences into electrical signals. It consists of two dissimilar metals joined together at two points called thermocouple junctions. When the junctions are at different temperatures, it generates a voltage that can be measured to determine the unknown temperature. Common types of thermocouples use combinations like copper-constantan, iron-constantan, and chromel-alumel wires. Thermocouples have a wide temperature range but lower accuracy compared to resistance temperature detectors. They are cheaper, respond faster, and are well suited for applications that require temperature measurements over a broad range.

2.  A transducer is a device that converts one form of energy to
other form. It converts the measurand to a usable electrical
signal.
 In other word it is a device that is capable of converting the
physical quantity into a proportional electrical quantity such
as voltage or current

3.  Thermocouple is a couple of two wires which measures
unknown temperature of the body with reference to the
known temperature of the other body.

5.  A thermocouple can be formed by joining the two dissimilar
metals at their ends forming two junctions called the
thermojunctions. There are various methods of joining the
two dissimilar metals to form the thermojunction. One is to
weld the wires together. This produce a brittle joint.

6.  Another method of forming thermojunction is by soldering
the wires together.

7.  Seebeck effect
The Seebeck effect states that when two different or unlike
metals are joined together at two junctions, an electromotive
force (emf) is generated at the two junctions. The amount of
emf generated is different for different combinations of the
metals.
Eemf = - S T
where, S is the Seebeck coefficient
= T1 – T2

8.  Peltier effect
When a current is made to flow through a junction between
two conductors, A and B, heat may be generated or removed
at the junction. The Peltier heat generated at the junction per
unit time is equal to
where is the Peltier coefficient of conductor A (B) and
I is the electric current (from A to B)

9.  It comprises of two dissimilar metals, A and B. These are
joined together to form two junctions, p and q, which are
maintained at the temperatures T1 and T2 respectively.
Remember that the thermocouple cannot be formed if there
are not two junctions. Since the two junctions are maintained
at different temperatures the Peltier emf is generated within
the circuit and it is the function of the temperatures of two
junctions.

10. If the temperature of both the junctions is same, equal and
opposite emf will be generated at both junctions and the net
current flowing through the junction is zero. If the junctions
are maintained at different temperatures, the emf will not
become zero and there will be a net current flowing through
the circuit. The total emf flowing through this circuit depends
on the metals used within the circuit as well as the
temperature of the two junctions. The total emf or the current
flowing through the circuit can be measured easily by the
suitable device.

11. The device for measuring the current or emf is connected
within the circuit of the thermocouple. It measures the amount
of emf flowing through the circuit due to the two junctions of
the two dissimilar metals maintained at different
temperatures.
Now, the temperature of the reference junctions is already
known, while the temperature of measuring junction is
unknown. The output obtained from the thermocouple circuit
is calibrated directly against the unknown temperature. Thus
the voltage or current output obtained from thermocouple
circuit gives the value of unknown temperature directly.

12. i. Type T : Copper - Constantan (270 / 400)
ii. Type J : Iron - Constantan (0 / 816)
iii. Type E : Chromel - Constantan (0 / 900)
iv. Type K : Chromel - Alumel (-270 / 1260)
v. Type R : Platinum - Rhodium (0 / 1600)
vi. Type S : Platinum - Rhodium (0 / 1750)
vii. Type C : Copper – Rhodium (0 / 2200)

14.  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
 Gas-fed heating appliances such as ovens & water heaters
 In the testing of prototype electrical and mechanical apparatus
 Thermopile radiation sensors
 Power production
 Process plants

15. Parameter Thermocouple RTD
Typical measurement
range
-267°C to 2316°C -240°C to 649°C
Sensitivity Medium Medium
Speed of response High High
Accuracy Moderate High
Size Small Large
Cost Low High
Material Used Copper-constantan, Iron-
constantan etc.
Nickel, Copper, Platinum
etc.
Applications Suitable for applications
which require wide
temperature range
Suitable where speed of
response and accuracy
are more important

16.  Thermocouple have a better response.
 They have a higher range of temperature measurements.
 The sensing element of a thermocouple easily installed.
 Cheaper than resistance thermometers.
 Very convenient for measurement of temperature at one
particular point in a piece of apparatus.

17.  Low accuracy and as such they can’t be use for precision
work.
 The milli-voltmeters used with thermoelements must be more
sensitive and therefore careful handling.
 A rise in temperature causes a change in the resistance of
heater.