Thermistor sensor

1. Unit-V
Thermistor &
Thermocouple

2. Thermistor
• Resistive temperature sensor
• Made up of semiconductor material
• Most thermistors have –ve temperature coefficient of
resistance.
• The temperature coefficient of resistance of thermistor is
found to be as large as several percent/ 0C.
• Large value of temperature coefficient of resistance allows
thermistor to detect very small changes in temperature which
may not be possible with RTD.
• Thermistor exhibits a highly non-linear R-T characteristics.

3. Thermistor R-T Characteristics
Where, β is material constant, temperature are in
Kelvin.

4. Thermistor …
Temperature coefficient of resistance of Thermistor;

5. Thermistor …

6. Thermistor …

7. Thermistor …

8. Thermocouple

9. Thermocouple
Seebeck Effect:
QA and QB are thermal transport constts of two metals.
QA and QB are nearly independent of temperature.

10. Thermocouple as Temperature Sensor
• Thermocouple is active temperature sensor.
• Response is voltage, which is proportional to the temperature
difference.
• When thermocouple is used as temperature sensor then one of
the junction is called measurement junction which is exposed
to the unknown temperature.
• The other junction is called reference junction. The
temperature of the reference junction must be accurately
known.
• Reference junction must be maintained at constant
temperature. It is better, if reference junction is maintained at 0
0C, but it is not always possible.

11. • Generated voltage is proportional to temperature difference
• Since Tref is constt., therefore,
Thermocouple as Temperature Sensor

14. • In standard thermocouple table reference junction is
maintained at 0 0C.
• The standard thermocouple table is used to find the
temperature of measurement junction against generated
thermo-emf for given reference junction temperature.
• Typical thermocouple table readings;
Vj(210 0C) = 11.34 mV (type J, 0 0C)
T(4.768 mV) = 555 0C (type S, 0 0C)
• When exact value is not available in table then linear
interpolation is used;
Calibration of Thermocouple

15. Empirical Laws of Thermocouple
• The thermal emf of a thermocouple with junctions at T1 and
T2 is totally un-effected by temperature elsewhere in the
circuit. It states that the lead wires connecting two junctions
may be safely exposed to unknown and/or varying temperature
environment without effecting the voltage produce and hence
the measurement.

16. Laws of Thermocouple …
• If a third homogeneous metal C is inserted into either A or B,
as long as two new resultant thermo-junctions are at same
temperature, the generated net thermo-emf is unchanged
irrespective of the insertion of new metal and also temperature
of C away from the junctions.

17. Significance of Law2

18. • If homogeneous metal C is inserted between A and B at one of
the junctions, the temperature of C at any point away from AC
and BC junctions is immaterial. As long as the junctions AC
and BC are both at same temperature T1, the net generated
thermo-emf is same as if C is not inserted.
Laws of Thermocouple …

19. Significance of Law3

20. • If the thermal emf with metals A and C is Eac and that of with
metals B and C is Ecb, then the thermal emf of metals A and B
is Eac + Ecb.
Laws of Thermocouple …
Significance: All possible combination of metals need not be
calibrated since individual metals can each be paired with
one standard metal (Pt is used) and calibrated. Any other
combination then can be calculated.

21. • If a thermocouple produces E1 when its junctions are
maintained at temperatures T1 and T2 and produces E2 when
at T2 and T3, then the thermocouple will produce E1+E2 when
junctions of the thermocouple is maintained at T1 and T3.
Laws of Thermocouple …

22. • Reference junction compensation: When reference junction is
maintained at temperature other than 0 0C, the standard
thermocouple table can not be used directly for calibration. It
requires compensation. Law 5 is used for the compensation as
follows;
Significance of Law5

23. Thermocouple: Signal Conditioning ICs

24. Thermocouple: Signal Conditioning ICs

25. Thank you