This presentation consists of description about thermocouple its types and its applications.

1. THERMOCOUPLE
WORKING AND APPLICATIONS
KESHAVAKRISHNAN S
312213114045
MECHANICAL
SSNCE

2. SYNOPSIS
 DEFINITION
 GOVERNING EFFECTS
 WORKING PRINCIPLE
 TYPES OF THERMOCOUPLES
 COMPARISON B/W VARIOUS TYPES
 APPLICATION AND USES

3. DEFINITION
A Thermocouple is a device used for the measurement of temperature.
It can be even considered as a sensor for the measurement of temperature. The
device consists of two dissimilar metals or semiconductors that contact each
other in one are more points. The junction in most of the cases are welded.
Temperature is measured mostly at welded region. One end is connected to the
region where the temperature is to be measured. This end considered as the hot
region or the measuring region. The end is connected to the body of known
temperature. This end is considered as cold region or the reference junction.
Thus the thermocouple measures the temperature of the known region with
respect to the known junction.

4. GOVERNING EFFECTS
The three main principles that governs the operation of thermocouple are
 SEEBECK EFFECT
 PELTIER EFFECT
 THOMSON EFFECT

5. SEEBECK EFECT
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.

6. PELTIER EFFECT
As per the 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.

7. THOMSON EFFECT
As per the 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.

8. WORKING PRINCIPLE
A Thermocouple comprises of two dissimilar metals. These are joined
together to form two junctions which are maintained at the temperatures. 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. 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 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.

9. PICTORIAL REPRESENTATION

10. PICTORIAL REPRESENTATION CONTD…

11. PICTORIAL REPRESENTATION CONTD…

12. TYPES OF THERMOCOUPLES
 TYPE E
 TYPE J
 TYPE K
 TYPE M
 TYPE N
 TYPE T
 TYPE B
 TYPE R
 TYPE S

13. TYPE E
Type E (chromel – constantan) has a high output (68 µV/°C)
which makes it well suited to cryogenic use. Additionally, it is non-
magnetic. Wide range is −50 °C to +740 °C and Narrow range is −110 °C
to +140 °C. This Thermocouple has the highest EMF output per degree of
all recognized thermocouples. If the temperature is between 316 to
593°C, (600 to 1100°F) type J or N is recommend because of aging
which can cause drift of 1 to 2°C, (2 to 4°F) in a few hours time.

14. TYPE J
Type J (iron – constantan) has a more restricted range (−40 °C to
+750 °C) than type K, but higher sensitivity of about 50 µV/°C. The Curie
point of the iron (770 °C) causes a smooth change in the characteristic,
which determines the upper temperature limit. It is not susceptible to
aging in the 371 to 538°C, (700 to 1000°F) temperature range.

15. TYPE K
Type K (chromel – alumel) is the most common general purpose
thermocouple with a sensitivity of approximately 41 µV/°C (chromel positive
relative to alumel when the junction temperature is higher than the reference
temperature). It is inexpensive, and a wide variety of probes are available in its
−200 °C to +1350 °C / -330 °F to +2460 °F range. Type K thermocouples usually
work in most applications as they are nickel based and exhibit good corrosion
resistance. It is the most common sensor calibration type providing the widest
operating temperature range. This type of thermocouple should be protected
with a suitable metal or ceramic protection tube, especially in reducing
atmospheres. In oxidizing atmospheres, such as electric furnaces, tube protection
is not always necessary when other conditions are suitable; however, it is
recommended for cleanliness and general mechanical protection.

16. TYPE K CONTD…

17. TYPE M
Type M (Ni/Mo 82%/18% – Ni/Co 99.2%/0.8%, by weight) are used in
vacuum furnaces. Upper temperature is limited to 1400 °C. It is less commonly
used than other types.

18. TYPE N
Type N (Nicrosil – Nisil) thermocouples are suitable for use between
−270 °C and +1300 °C owing to its stability and oxidation resistance. Sensitivity
is about 39 µV/°C at 900 °C, slightly lower compared to type K. The Nicrosil
and Nisil thermocouple alloys show greatly enhanced thermoelectric stability
relative to the other standard base-metal thermocouple alloys because their
compositions substantially reduce the thermoelectric instabilities.

19. TYPE T
Type T (copper – constantan) thermocouples are suited for
measurements in the −200 to 350 °C range. Often used as a
differential measurement since only copper wire touches the probes.
Since both conductors are non-magnetic, there is no Curie point and
thus no abrupt change in characteristics. Type T thermocouples have
a sensitivity of about 43 µV/°C. Note that copper has a much higher
thermal conductivity than the alloys generally used in thermocouple
constructions, and so it is necessary to exercise extra care with
thermally anchoring type T thermocouples. Type T is very stable and is
used in a wide variety of cryogenic and low temperature
applications.

20. TYPE T CONTD…

21. TYPE B
Type B thermocouples (Pt/Rh 70%/30% – Pt/Rh 94%/6%, by weight)
are suited for use at up to 1800 °C. Type B thermocouples produce the same
output at 0 °C and 42 °C, limiting their use below about 50 °C. The emf
function has a minimum around 21 °C, meaning that cold junction
compensation is easily performed since the compensation voltage is
essentially a constant for a reference at typical room temperatures. It is easily
contaminated, and damaged by reducing atmospheres.

22. TYPE S
Type S thermocouples (Pt/Rh 90%/10% – Pt, by weight), similar to type R, are
used up to 1600 °C. Before the introduction of the International Temperature Scale of
1990 (ITS-90), precision type S thermocouples were used as the practical standard
thermometers for the range of 630 °C to 1064 °C, based on an interpolation between
the freezing points of antimony, silver, and gold. Starting with ITS-90, platinum
resistance thermometers have taken over this range as standard thermometers.

23. TYPE R
Type R thermocouples (Pt/Rh 87%/13% – Pt, by weight) are used up
to 1600 °C. When protected by compacted mineral insulation and
appropriate outer sheath, Type R is usable from 0 to 1482°C, ( 32 to
2700°F).Type R has a higher EMF output than type S. Also easily
contaminated, and damaged by reducing atmospheres.

24. OTHER TYPES OF THERMOCOUPLES
 Platinum/rhodium alloy thermocouples
 Tungsten/rhenium alloy thermocouples
 Chromel – gold/iron alloy thermocouples
 Type P (noble metal alloy)
 Platinum/molybdenum alloy thermocouples
 Iridium/rhodium alloy thermocouples
 Pure noble metal thermocouples Au–Pt, Pt–Pd

25. COMPARISON B/W VARIOUS TYPES

26. COMPARISON B/W VARIOUS TYPES CONTD…

27. COMPARISON B/W VARIOUS TYPES CONTD…

28. COMPARISON B/W VARIOUS TYPES CONTD…

29. COMPARISON B/W VARIOUS TYPES CONTD…

30. APPLICATIONS AND USES
o Steel industry
o Gas appliance safety
o Thermopile radiation sensors
o Power production
o Thermocouple as vacuum gauge
o Thermistor
o Other applications include temperature measurement
for kilns, gas turbine exhaust, diesel engines, other
industrial processes and fog machines etc..

31. THERMOCOUPLE AS VACUUM GAUGE