Thermocouples are temperature sensors consisting of two dissimilar metals joined together at two junctions. One junction, the measuring or hot junction, is connected to the body whose temperature is being measured. The other junction, the reference or cold junction, is connected to a body of known temperature. A temperature difference between the junctions produces an electric voltage due to the Seebeck effect. Thermocouples are widely used to measure temperature in industrial processes like furnaces and engines as well as in thermostats and fire alarms.

1. Thermocouple
sensor.
Shamanth.S

2. What is a thermocouple ?
• A thermocouple is a temperature-measuring device consisting of
two dissimilar conductors that contact each other at one or more
spots.
• A thermocouple comprises of at least two metals joined together
to form two junctions. One is connected to the body whose
temperature is to be measured-this is the hot or measuring
junction.
• The other junction is connected to a body of known temperature;
this is the cold or reference junction. Therefore the thermocouple
measures unknown temperature of the body with reference to the
known temperature of the other body.

4. Where are they used ?
• Thermocouples are a widely used as temperature sensors
for measurement, control and can also convert a
temperature into electricity.
• Thermocouples are widely used in research and industry;
applications include temperature measurement for
furnaces, gas turbine exhaust, diesel engines, and other
industrial processes.
• Thermocouples are also used in homes & offices as
temperature sensors in thermostats and also in flame
sensors for fire detection.

5. Working principle.
• The working principle of thermocouple is based on three
effects discovered by Seebeck, Peltier and Thomson. They
are as follows:
• 1) 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.
• 2) Peltier effect: when two dissimilar metals are joined
together to form two junctions, emf is generated within the
circuit due to the different temperatures of the two
junctions of the circuit.

6. • 3) Thomson effect: when two unlike metals are joined
together forming two junctions, the potential exists
within the circuit due to temperature gradient along
the entire length of the conductors within the circuit.
• In most of the cases the emf suggested by the
Thomson effect is very small and it can be neglected
by making proper selection of the metals. The Peltier
effect plays a prominent role in the working principle
of the thermocouple.

7. How does it work ?
• when any conductor is subjected to temperature, it will
generate a voltage. This is now known as the
thermoelectric effect or Seebeck effect.
• Any attempt to measure this voltage necessarily involves
connecting another conductor to the "hot" end. This
additional conductor experiences the same temperature
gradient and also develops a voltage.
• The magnitude of the effect depends on the metal in use,
and so a nonzero voltage will be measured if two dissimilar
metals are used. After carefully calibrating the
temperature-voltage dependence for a given pair of metals,
these metals can be used as a thermometer.

9. Different types of thermocouples :-