Thermocouples, resistance temperature detectors (RTDs), and thermistors are the three main types of temperature transducers. Thermocouples generate a voltage proportional to temperature difference by joining two dissimilar metals. RTDs measure temperature by relating the precise resistance change of materials like platinum to temperature. Thermistors are semiconductors whose high resistance changes significantly with temperature. Each transducer has advantages - thermocouples cover a wide range but with low accuracy, while RTDs and thermistors offer higher accuracy over smaller ranges. Applications include temperature monitoring and control in industries like food processing, automotive, solar energy, and healthcare.

1. 2. Temperature Transducers
▫ Thermocouples
▫ Resistance-Temperature Detectors (RTD)
▫ Thermistors
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2. 1. Thermocouple
• In 1821, T.J. Seebeck discovered that an electric potential
occurs when 2 different metals are joined into a loop and
the two junctions are held at different temperatures.
• Seebeck emf – a voltage difference between the two ends
of the conductor that depends on the temperature
difference of the ends and a material property.
• If the ends of the wire have the same temperature, no emf
occurs, even if the middle of the wire is hotter or colder.
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3. • Thermocouple - Principle
Twisting or welding of 2 wires
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4. In normal operation, cold junction is placed in an ice
bath
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5. 5

6. Types of Thermocouples
• Type K : Chromel-Alumel
• Type J : Iron-Constantan
• Type E : Chromel-Constantan
• Type N : Nicros-Nisil
• Type T : Copper-Constantan
• It is important to note that thermocouples measure the
temperature difference between two points, not
absolute temperature.
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7. • Corresponding Sensitivities
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8. • Magnitude of thermal EMF
where
 c and k = constants of the thermocouple materials
 T1 = the temperature of the ‘hot’ junction
 T2 = the temperature of the ‘cold’ or ‘reference’ junction
)
(
)
( 2
2
2
1
2
1 T
T
k
T
T
c
E 



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9. Example:
A thermocouple was found to have linear
calibration between 0⁰C and 400⁰C with emf at
maximum temperature (reference junction
temperature 0⁰C) equal to 20.68 mV.
a) Determine the correction which must be
made to the indicated emf if the cold junction
temperature is 25⁰C.
b) If the indicated emf is 8.82 mV in the
thermocouple circuit, determine the
temperature of the hot junction.
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10. Solution:
(a) Sensitivity of the thermocouple
= 20.68/(400-0)
= 0.0517 mV/⁰C
Since the thermocouple is calibrated at the reference
junction of 0⁰C and is being used at 25⁰C, then the
correction which must be made, Ecorr between 0⁰C
and 25⁰C
Ecorr = 0.0517 x 25
Ecorr = 1.293 mV
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11. (b) Indicated emf between the hot junction and
reference junction at 25⁰C = 8.92 mV.
Difference of temperature between hot and cold
junctions = 8.92/0.0517 = 172.53⁰C.
Since the reference junction temperature is 25⁰C,
hot junction temperature = 172.53 + 25 = 197.53⁰C.
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12. • Thermocouple - applications
• Thermocouples are most suitable for measuring over a
large temperature range, up to 1800 K.
• Example:
Type K : Chromel-Alumel (-190⁰C to 1260⁰C)
Type J : Iron-Constantan (-190⁰C to 760⁰C)
Type E : Chromel-Constantan (-100⁰C to 1260⁰C)
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13. • Thermocouples are most suitable for measuring
over a large temperature range, up to 1800 K.
• They are less suitable for applications where smaller
temperature differences need to be measured with high
accuracy, for example the range 0–100 °C with 0.1 °C
accuracy.
• For such applications, thermistors and RTDs are
more suitable.
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14. 2. Resistance temperature detector (RTD)
• Resistance temperature detectors (RTDs), also called
resistance thermometers, are temperature sensors that
exploit the predictable change in electrical resistance of
some materials with changing temperature.
• Temperature Metal Resistance
• The resistance ideally varies linearly with temperature.
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15. • RTD unlike thermocouples are passive sensors
requiring an excitation” current to be passed
through them.
• The RTD is normally manufactured through a
known resistance typically 100 ohms at ice
point. It has positive temperature of resistance.
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16. • How it works:
 Utilizes the fact that
resistance of a metal
changes with temperature.
• Make up:
 Traditionally made up of
platinum, nickel, iron or
copper wound around an
insulator.
• Temperature range:
 From about -196°C to
482°C.
Thin Film RTD
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17. Advantages
•High accuracy and fast response
•Narrow span and good reproducibility
•Remains stable and accurate for many years
•Temperature compensation not necessary
Disadvantages
•High cost as compared to the thermocouple
•Large bulb size in comparison to thermocouple
• Self heating can be a problem
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18. • Resistance vs Temperature Approximations
• A straight line has been drawn between the points of
the curve that represent temperature, T1 and T2, and T0
represent the midpoint temperature.
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19.  RTD -Linear or Straight line equation
approximation
R(T)= approximation of resistance at temperature T
R(T0)= resistance at temperature T0
αo = fractional change in resistance per degree of temperature
at T0
ΔT= T - T0
2
1
]
1
)[
(
)
( T
T
T
T
T
R
T
R o
o 



 
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20. R2 = resistance at T2
R1 = resistance at T1
)
(
)
(
1
1
2
1
2
0 T
T
R
R
T
R
o




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21. • Example:
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22. RTD – quadratic approximation
• More accurate representation of R-T curve over some span
of temperatures.
R(T) = quadratic approximation of resistance at temperature T
R(T0)= resistance at temperature T0
α1 = linear fractional change in resistance with temperature
α2 = quadratic fractional change in resistance with temperature
ΔT = T - T0
2
1
2
2
1 ]
)
(
1
)[
(
)
( T
T
T
T
T
T
R
T
R o 





 

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23. Example
Solution
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24. RTD - Sensitivity
• Sensitivity is shown by the value αo
Platinum – 0.004/ °C
Nickel – 0.005/ °C
• Thus, for a 100Ω platinum RTD, a change of 0.4
Ω would be expected if the temperature is
changed by 1°C.
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25. Platinum
Copper
Tungsten
Nickel
Platinum: very repeatable, less sensitive,
expensive
Nickel: not quite repeatable, more sensitive,
less expensive
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26. RTD – response time
• Generally 0.5 to 5 seconds or more
• The slowness of response is due to the slowness of
thermal conductivity in bringing the device into
thermal equilibrium with its environment.
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27. Construction of a platinum resistance thermometer
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28. Wire is in a coil to achieve small size and improve thermal conductivity
to decrease response time.
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29. Protect from the environment
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30. 3.Thermistors
• Semiconductor resistance sensors
• Unlike metals, Thermistors respond negatively to
temperature and their coefficient of resistance is of the
order of 10 times higher than that of platinum or copper.
• Temperature semiconductor resistance
• Symbol
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31. 31
• How it works:
Like the RTD a thermistor uses the fact that
resistance of a metal changes with temperature.
• Make up:
Generally made up of semiconductor materials
• Temperature Range:
About -45°C - 150°C
Thermistor

32. 32
• Advantages:
– Very sensitive (has
the largest output
change from input
temperature)
– Quick response
– More accurate than
RTD and
Thermocouples
• Disadvantages:
– Output is a non-
linear function
– Limited temperature
range.
– Require a current
source
– Self heating
– Fragile

33. Advantage of Thermistors:
• Has extremely high sensitivity. for example, a 2252 w
thermistor has a sensitivity of -100 w/°c at room
temperature.
• Higher resistance Thermistors can exhibit temperature
coefficients of -10 kw/°c or more. In comparison, a 100 w
platinum RTD has a sensitivity of only 0.4 w/°c.
• Physically small size which yields a very fast response to
temperature changes.
• The thermistor has been used primarily for high-resolution
measurements over limited temperature ranges. the classic
example of this type of application is motor winding
temperature and in medical thermometry
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34. • Has relatively high resistance. Thermistors are
available with base resistances (at 25° c) ranging
from hundreds to millions of ohms.
• This high resistance diminishes the effect of inherent
resistances in the lead wires, which can cause
significant errors with low resistance devices such as
RTD
• The major tradeoff for the high resistance and
sensitivity of the thermistor is its highly nonlinear
output and relatively limited operating range.
• Depending on the type of Thermistors, upper ranges
are typically limited to max 300° c.
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35. Thermistors: resistance vs temperature
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36. Example :
The circuit in figure below is used for temperature
measurement. The Thermistor is a 4kΩ type identified from
the standard. The meter is a 50mA ammeter with a
resistance of 3Ω, Rc is set to 17Ω and the supply voltage
VT=15V.What will be the meter readings at 77oF and at
150oF.
• Solution : From the standard graph of a 4kΩ thermistor,
the resistance at 77oF is 4kΩ. Thus the current at 77oF is
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37. • At 150oF, the graph shows that the thermistor resistance is
950 Ω. The meter reading at this temperature is
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38. Applications of temperature sensing
• Food industry
 Monitor temperature-time cycles to ensure high food quality
• Automotive industry
 Combustion and exhaust temperature
• Solar Energy conversion
 Accurate temperature measurement to achieve optimal heat
flow
• Energy efficiency in the home and industry
 Measurement of temperature
• Hospital infant incubator
 Temperature must be kept in the proper range
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