TYPES OF THERMPCOUPLE'S
Thermocouple is the one type of sensor which used to measure
the temperature. Thermocouple is one type of thermoelectric device.
Thermocouple is made up of two different dissimilar metals joined together to
form a junction.
The basic principle
of thermocouple is to develops an EMF which is a function of the difference in
temperature of its measuring junctions. One junction is placed on surface or in
air that is being measured and this junction is called hot junction in other
hand the other junction is called cold junction and at cold junction the
temperature is remains constant. These two points also known as measuring
junction and reference junction. If the temperature of measuring junction is
known then we can determine temperature at measuring junction by measuring the
EMF generated in the circuit.
For
reference point of view the diagrammatic view of thermocouple shown below,
Pic.01
Material Requirements
of Thermocouple,
I) High coefficient of thermal EMF.
II)
Resistance to oxidation, resistance & corrosion.
III) Continuously increasing relation of EMF to temperature over a long
range.
Types of
Thermocouples,
Basically
two types of thermocouples as,
1) Base
Metal Thermocouples.
2) Nobel
Metal Thermocouples.
In
Base Metal Thermocouples there are five types as
1) J
Type
2) K
Type
3) T
Type
4) E
Type
5) N
Type
In Nobel Metal
Thermocouples there are three types as,
1) B
Type
2) R
Type
3) S
Type
1) J Types
Thermocouple :
This type
of thermocouples is commonly used thermocouples. The
temperature measuring range of J Type thermocouple is in between 0 to 750
Degree C. Out of two legs for positive leg Iron material is used while for
negative leg constantan material is used. Accuracy of such type of thermocouple
is +/- 2.2 Degree C.
# Sensitivity:
50 µV/Degree C
J Type thermocouple is find there application in vacuum
and most commonly used for general purpose of applications.
2) K Type
Thermocouple :
This type of
thermocouple is most commonly used thermocouple. The J Type
thermocouple is more accurate with wide range the range in between
-200 Degree C to +1250 Degree C. Accuracy of such type of thermocouple is +/-
2.2 Degree C.
# Sensitivity: 41 µV/Degree C
Wire is made up of Comel and Alumel material and composition as,
Comel : 90% Nickel + 10% Chromium
Alumel
: 95% Nickel + 2%
Manganese
+ 2% Aluminium + 1% Silicon.
On positive leg cromel material used and on negative leg alumel
material is used.
K Types Thermocouple
is find their applications where tempreture range 500
Degree C, in nuclear applications
3) E Type
Thermocouple:
Type
E thermocouples is generating Highest EMF per
degree as compared to other type of thermocouples. Type E
Thermocouple gives stronger accuracy as compared to K & J type
thermocouples. Temperature range in between -50 Degree C to +740 Degree C.
Accuracy is about +/- 1.7 Degree C.
#Sensitivity: 68 µV/Degree C
In E type
Thermocouples at +ve leg Chromel material used while at -ve leg Constantan
material is used. The E Type thermocouple find their applications in aerospace
Industries.
4)T type
Thermocouple :
For
requirement of low temperature the T Type thermocouple is used because T type
thermocouple stable at extremely low temperature. Temperature range in between
-250 Degree C to +350 Degree C. In such type of thermocouple at +ve leg copper
material is used while at -ve leg constantan material is used. The composition
of material constantan is about 55% Copper & 45% Nickel. Accuracy for the T
type thermocouple is +/- 1.0 Degree C.
#Sensitivity:
43 µV/Degree C
T Type thermocouple is find there
application in Bllod Banks and Laboratories.
5) N Type thermocouple:
Type
N thermocouple is slightly expensive as compared to K Typethermocouple. N Type
thermocouple gives same accuracy and tempreture range as in K Type thermocouple
type thermocouple is made up of Nicrosil and Nisil material. Temperature range
as in between -270 Degree C to 1300 Degree C. Accuracy is about +/- 2.2 Degree
C .
#Sensitivity: 39 µV/Degree C
6) B Type Thermocouple:
This
type of thermocouples extremely used for
high temperature applications. They give good accuracy at very
high temperature. Temperature range in between 0 to 1800 Degree
C. B Type thermocouple made of of 1 leg 70% Platinum and 30% rhodium while
second leg 94% platinum and 6% Rhodium. Such type of thermocouple gives
accuracy about +/- 0.5%.The main advantage of B type of thermocouple is high
accuracy as compared to other thermocouples.
7) S Type Thermocouple:
The
S type of thermocouple is used at very high temperature applications.
Temperature range of 0 to 1600 Degree C. The first leg made up of 90% Platinum
and 10% Rhodium while second leg platinum only. Accuracy is about +/- 1.5
Degree C.
8) R Type Thermocouple :
Such
type of thermocouples expensive because consisting high % of rhodium.
Temperature range in between 0 to 1482 Degree C. The first leg made up of 87%
of platinum an 13% of rhodium while second leg made up of platinum only.
Accuracy is about +/- 1.5 Degree C.
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