Thermocouple:- KPC Principal

1. Thermocouple
Diptarup Bandyopadhyay

2. Thermocouple : Content
Thermo-Electric Laws
What is Thermocouple?
Types of Thermocouple
Construction
Laws of Thermocouple
Cold Junction Compensation
Calibration and Troubleshooting

3. Thermo-Electric Laws
• Seebeck Effect:
If two dissimilar homogeneous metals are
formed two junctions and a temperature
difference is maintained between these two
junctions , a thermo-emf will be produced in the
circuit. This is Seebeck Effect.
The Thermo-emf E α Temp difference ( T1 – T2)

4. Thermo-Electric Laws
• Peltier Effect:
If two dissimilar homogeneous metals are
formed two junctions and if a current is passed
through this , the heat is evolved in one junction
and heat is absorbed in other junction. It is
opposite of Seebeck Effect.
Peltier cooler is formed on this principle.

5. Thermo-Electric Laws
• Thomson Effect :
• When electric current passes through a circuit
of non homogeneous single material , a
temperature gradient will exist along its
length.

6. Thermocouple
• What is ?
• A thermocouple is an electrical device consisting
of two dissimilar homogeneous electrical
conductors forming two junctions and
maintained at different temperatures.
• It has two junctions – the measured junction is
known is HOT junction ( THot ) while the cold
junction is known as REFERENCE junction (TRef ).
• Measurement of EMF : By Potentiometric Circuit.

7. Thermocouple

8. Types of Thermocouple
Type Positive Material Negative Material Range Sensitivity Application
T Copper Constantan -200 to 350o
C
43 µV/°C Oxidizing and Reducing environment. Good moisture resistant.
J Iron Constantan -40 to 750oC 50 µV/°C Reducing environment. Most economical.
K Chromel Alumel −200 °C to
+1350 °C
41 µV/°C Oxidizing environment and most linear. Commonly used.
E Chromel Constantan −50 °C to
+740 °C
68 µV/°C Oxidizing environment. Highest Sensitivity.
S Platinum-Rhodium
(90%Pt/10%Rh)
Platinum up to
1600 °C
.003–.007 mV/oF Oxidizing environment. Small size & Fast response.
R Platinum-Rhodium
(87%Pt/13%Rh)
Platinum 0 to 1600oC .003–.008 mV/oF Oxidizing environment. Small size & Fast response.
B Platinum-Rhodium
(70%Pt/30%Rh)
Platinum up to
1800 °C
.0003–.006 mV/oF Inert and Slow Oxidizing
C Tungsten- Rhenium
(95%W/5%Re)
Tungsten-Rhenium
(74%W/26%Re)
2329 ℃ Inert or vacuum . High Temp. But brittle.
G Tungsten Tungsten-Rhenium
(74%W/26%Re)
2315oC Inert or vacuum . High Temp. But brittle.
D Tungsten- Rhenium
(97%W/3%Re)
Tungsten-Rhenium
(75%W/25%Re)
2300oC Inert or vacuum . High Temp. But brittle.

9. Construction of Thermocouple
• T/C s are made from two wires , twisted or wielded at one
end. This is measuring junction or hot junction.
• To prevent the entangled of the wires among themselves to
produce other points of couple , single hole or dual hole
ceramic beads are used. The wires passes through it.
• To protect from mechanical vibration, these wires are
protected by a s.s. tube ( which is known as sheath). The
introduction of sheathing material introduces the time lag.
To overcome that , either the junction is flashed with SS tube
or bare junction T/C is used.
• The two thermocouple wires are connected to a porcelain
base terminal block with screw fitted. The connecting
terminal is marked with polarity ( i.e. positive and negative).
• A threaded metallic cover protects the connecting terminals.
A connecting lead gland in terminal head allows the lead
wire to connect T/C with measuring system.

10. Construction of Thermocouple

11. Thermocouple: EMF Calculation
A
T1 T2
B
Either Clockwise or Anti Clockwise for each node
For T1 , EMF = EB-A /T1 For T2 , EMF = E A-B /T2
Total EMF = E B-A /T1 +E A-B /T2
EB-A /T1 = - EA-B /T1

12. Measurements : Laws of T/C
Law of Homogeneous Material:
The e.m.f of a thermocouple depends only on the temperatures
of the junctions and is independent of the temperatures of the
wires connecting the junctions. This means that the leads
connecting the instrument can be exposed to temperature
fluctuations without affecting the measurement.

13. Measurements : Laws of T/C
Laws of Intermediate Materials:
• If any third metals are introduced into a couple, keeping the
junction temperature of the third materials to the couple is
same then no additional e.m.f is contributed for third
material.

14. Measurements : Laws of T/C
Laws of Intermediate Temperature:
The law of intermediate temperature states that if a couple
produces an output E1 for the temperature difference of T1 and
T3 , then it is equivalent to the summation of emf E2 ( for
temperature T1 and T2) and E3 ( for temperature T2 and T3),
where T1>T2>T3 .

15. Cold Junction Compensation (CJC)
The cold junction compensation compensate
this variation of ambient temperature by
adding an additional emf for ambient
temperature to 0oC ( Eamb-0) along with the emf
for hot junction to ambient junction (EHot-amb).
EHot-0 = EHot-amb + Eamb-0
EMF due to
Measured
Temp.
EMF due to
ambient temp.
Compensated emf

16. Cold Junction Compensation (CJC)

17. Calibration & Troubleshooting
• Sensor : The sensors are being checked
against some known temperature by
mv - Temp chart for particular couple.
• Measuring Instrument: The measuring
instrument is basically a voltmeter.
Calibrated against some standard voltage
source.

18. Thank You