1. THERMOCOUPLE
INSTRUMENTS
PRINCIPLE OF OPERATION
CONSTRUCTION
THERMOELEMENTS
ADVANTAGES
DISADVANTAGES

2.  What is Thermocouple?
Thermocouple consists of two different metals which are placed in contact each other & a voltage is
generated at the junction which is nearly proportional to the temperature .This junction is called a
Thermocouple.

3.  Purpose Of Operation:
It is used to convert heat energy to electrical energy as shown in above fig.
Due to the heater element temp. will increase at the junction & this causes production of an EMF (DC voltage)
which is directly proportional to the root mean square value of electric current.
A PMMC instrument is connected to the second part to read the current passing through heater

4.  Principle of operation
Generated EMF,
E = a (Ta – Tb) + b (Ta – Tb)2
Where , Ta= temp. of heater element
Tb= temp. of cold metal
a & b are const. depends upon the type of metal we are using
The above eqn. shows Parabolic fn.
Value of b can be neglected if their air gap field of PMMC is uniform,
E=a(Ta-Tb) ,b=0

5.  Principle of operation
Loss through heater coil I2R
Assuming temp. of cold junction maintained at room temp.
Then,
(Ta-Tb) ∝ I2R
Or, (Ta-Tb)= kI2R
So the Deflection angle of PMMC ,
x=KE=K[a(Ta-Tb)]
hence we can write k.K.a.I2R = k1I2, where k1 is some constant.
Instrument shows the square law of response

6.  Precaution
• Measurement of current and voltage of high frequencies. So the instruments should be free from
skin effect.
• In order to reduce skin effect a very fine wire of non-magnetic material having a high resistivity
 Reason Behind using PMMC:
Greater accuracy & sensitivity towards the measurement of DC value.

7.  Contact Type
• Have a separate heater wire and thermocouple has one junction(hot) in contact with heater wire.

8.  Non Contact type
• There is no electric contact between the heating element & the thermocouple.
• This two are separated by electric insulation.
• This is make the instrument sluggish & less sensitive than contact type.
• This type of instrument basically use for measurement of currents in circuit operated at above the
ground potential

9.  Vacuum type
• In order to attain a temperature rise (80c to 100c)needed to develop the required output in
the thermo-couple instrument ,we must increase the efficiency & sensitivity of the
thermo-element.
• For that cause we enclose both the thermo-couple & heater in glass tube.
• Efficiency in increased cause there is no cooling of heater due of convection of air
current.
• For this case heater element & thermo-couple element also remain isolated from each
other by a bead of insulating material.
• This is done to minimize the effect of capacitance between the thermo-couple & the rest
of the circuit at high frequencies.

10. Vacuum type

11.  Bridge type
• For this case no heater element is used.
• The thermo-couples connected to form a bridge
• The current to be measured passes directly through the thermo-couple raises there temperature
proportional to I2R
• This thermo-couple gives greater output than is given by single thermo-couple used in vacuum
thereby giving greater sensitivity.
• It is much robust in withstanding mechanical shocks & overloads.

12.  Bridge Type

13. ADVANTAGES
1. THE THERMOCOUPLE INSTRUMENTS CORRECTLY INDICATES THE RMS VALUE OF
VOLTAGE OR CURRENT IRRESPECTIVE OF THE WAVE FORM.
2. THESE INSTRUMENTS ARE NOT AFFECTED BY STRAY MAGNETIC FIELDS.
3. THESE INSTRUMENTS ARE FREE FROM FREQUENCY ERRORS AND HENCE THEY
CAN BE USED OVER A VERY WIDE FREQUENCY RANGE.
4. THESE INSTRUMENTS HAVE HIGH SENSITIVITY.
5. THEY ARE VERY CAREFUL WHEN USED AS TRANSFER INSTRUMENT.

14.  DISADVANTAGES
1. AT NORMAL RATED CURRENT THE HEATER ATTAINS THE TEMPERATURE OF 300C.
2. AT NORMAL CURRENT THE HEATER WOULD GIVE A TEMPERATURE OF NEARLY 4
TIMES THE NORMAL TEMPERATURE,1200C.
3. IT IS OBVIOUS THAT SQUARE LAW RATE WILL BRING THE HEATER TO NEARLY ITS
BURN OUT TEMPERATURE.
4. THE FUSES DO NOT PROVIDE ANY PROTECTION AS DUE TO OVERLOAD THE
HEATER WIRE MAY BURN OUT BEFORE FUSE BLOWS OUT.

15. Prepared by-
• Suman Mahato
• Koustav Seth
• Sayak Mondal
• Amitava Ghosh
• Sayak Mukherjee
• Srinjoy Kar
Students of 2nd year Electrical Engineering Dept
UNIVERSITY OF ENGINEERING AND MANAGEMENT,JAIPUR

16. Thank You