temprature

1. Types of temperature sensors
 RTD (Resistance Temperature
Detector)
 Thermistor
 Thermocouple

2. RTD, the basics
 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
650°C.
Thin Film RTD

3. RTD Advantages and Disadvantages
 Advantages:
 Stable
 Very accurate
 Change in
resistance is linear
 Disadvantages:
 Expensive
 Current source
required
 Small change in
resistance
 Self heating
 Less rugged than
thermocouples.

4. Thermistor, the basics of
 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 - 300°C
Thermistor

5. Construction
 Beads
 Chips
 Deposition on substrate

6. Epoxy encapsulated bead
thermistors

7. Thermistor Advantages and Disadvantages
 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

8. Thermocouple, some more basics
 How it works:
 Made up of two
different metals joined
at one end to produce
a small voltage at a
given temperature.
 Make up:
 Made of up two
different metals. Ex: A
type J is made up of
Iron and Constantan.
 Temperature Range
 Type J: 0°C to 1400°C
A few Thermocouples

9. Thermocouples
 Two wires of different
metal alloys.
 Converts thermal energy
into electrical energy.
 Requires a temperature
difference between
measuring junction and
reference junction.
 Easy to use and obtain.

10. Thermocouple Advantages and Disadvantages
 Advantages:
 Self Powered (does
not require a
current or voltage
source)
 Rugged
 Inexpensive
 Simple
 Disadvantages:
 Extremely Low
Voltage output
(mV)
 Not very stable
 Needs a reference
point

12. Working Principle
 Seebeck effect
when any conductor subjected to thermal gradient
generates a voltage.

13. Thermoelectric effect (cont.)
 The Seebeck effect: an emf produced
across the junction of two dissimilar
conducting materials connected together.
 The first to be discovered and used (1821)
 The basis of all thermoelectric sensors
 Peltier effect is used in Thermoelectric
Generators (TEG) devices

14. The Seebeck effect
 If both ends of the two conductors are
connected and a temperature difference is
maintained between the two junctions, a
thermoelectric current will flow through the
closed circuit (generation mode)

16. Thermocouple (measuring
junction)

18. Thermo electric laws
1. The application of heat to a single
homogeneous metal in itself is not
capable of producing electric current
2. Thermo electric emf is produced when
when two junctions of 2 dissimilar
homogenous metal kept at different temp (
emf not affected by temp. gradient)

19. 3. Emf produced by 2 dissimilar metals having
jns. at diff. Temp., emf developed will not be
affected when a third metal is made a part
of ckt. – based on law of intermediate
metals
4. Law of intermediate temp.
Eac= Eab + Ebc

20. Law of intermediate metals :
states that net emf in a circuit remains
unaltered if a third metal is introduced
provided that the two junctions of third
metals are at same temp

21. Thermocouple Materials
The three most common thermocouple materials for moderate
temperatures are Iron-Constantan (Type J), Copper-
Constantan (Type T), and Chromel-Alumel (Type K).
1-) The first named element of the pair is the positive
element.
2-) The negative wire is color coded red.

22. Standard Thermocouple Types
Iron-Constantan: Iron-Constantan (Type J, color) generates about 50
µV/°C (28 µV/°F). The Iron wire is magnetic. coded white and red
Chromel Alumel: Chromel-Alumel (wire is magnetic. Junctions can be
made by welding or soldering, butType K, color coded yellow and
red) generates about 40 µV/°C (22 µV/°F).
Copper-Constantan: Copper-Constantan (Type T, color coded blue and
red) generates about 40 µV/°C (22 µV/°F).

23. Thermocouples - standard
types
Table 3.4. Thermocouples (standard types and others) and some of their properties
Materials Sensitivity
[V/C]
at 25C.
Standard
Type
designation
Temperature
range [C]
Notes
Copper/Constantan 40.9 T 270 to 600 Cu/60%Cu40%Ni
Iron/Constantan 51.7 J 270 to
1000
Fe/60%Cu40%Ni
Chromel/Alumel 40.6 K 270 to
1300
90%Ni10%Cr/55%Cu45%Ni
Chromel/Constantan 60.9 E 200 to
1000
90%Ni10%Cr/60%Cu40%Ni
Platinum(10%)/Rhodium-Platinum 6.0 S  to 1450 Pt/90%Pt10%Rh
Platinum(13%)/Rhodium-Platinum 6.0 R  to 1600 Pt/87%Pt13%Rh
Silver/Paladium 10 200 to 600
Constantan/Tungsten 42.1 0 to 800
Silicon/Aluminum 446 40 to 150
Carbon/SiliconCarbide 170 0 to 2000
Note: sensitivity is the relative Seebeck coefficient.

24. Thermocouple Color Codes
Thermocouple wiring is color coded by thermocouple
types. Different countries utilize different color coding.
Jacket coloring is sometimes a colored stripe instead of a
solid color as shown.

25. Features
 O/p in mv
 Need ampr
 Compesion ckts.
 Less accurate
 Kept inside protective wells
 Cheaper when compared to RTD

26. Thermopile
 n thermocouples in
series electrically
 In parallel
thermally
 Output is n times
the output of a
single
thermocouple
 Used to increase
output

27. Lets Experiment!
 In lab a RTD, thermistor, and
thermocouple were placed in a beaker
of 750mL of water and readings were
taken from 19°C to 80°C.
 The next two slides show the results.

28. The Data (some of it)
Temperature Thermocouple RTD Thermistor
(degrees Celsius) (mille-Volts) (ohms) (kilo-ohms)
19 -0.10 108.00 105.60
20 -0.10 108.40 99.80
21 0.00 108.70 94.20
22 0.00 109.00 88.20
23 0.00 109.50 83.80
24 0.10 110.00 79.70
25 0.10 110.40 75.90
26 0.10 110.90 73.30
27 0.20 111.30 70.00
28 0.20 111.50 68.40
29 0.30 112.00 63.40
30 0.40 112.90 60.50
32 0.50 113.20 54.80
34 0.70 114.10 49.20
36 0.70 114.80 45.50

29. characteristics
Thermocouple
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0 10 20 30 40 50 60 70 80 90
Temperature (∘C)
Voltage(mV)
Thermistor
0.00
20.00
40.00
60.00
80.00
100.00
120.00
0 10 20 30 40 50 60 70 80 90
Temperature (∘C)
Resistance(KΩ)
RTD
100.00
105.00
110.00
115.00
120.00
125.00
130.00
135.00
0 10 20 30 40 50 60 70 80 90
Temperature (∘C)
Resistance(Ω)

30. Thermocouple Materials
The three most common thermocouple materials for moderate
temperatures are Iron-Constantan (Type J), Copper-
Constantan (Type T), and Chromel-Alumel (Type K).
1-) The first named element of the pair is the positive
element.
2-) The negative wire is color coded red.
All three types (J, K, and T) are available as insulated
duplexed pairs from 0.001-inch diameter on up. For accuracy,
and minimum system disturbance, the smaller the wire the
better, but wire smaller than 0.003-inch diameter is very
fragile.

31. Standard Thermocouple Types
Iron-Constantan: Iron-Constantan (Type J, color) generates about 50
µV/°C (28 µV/°F). The Iron wire is magnetic. coded white and red
Junctions can be made by welding or soldering, using commonly
available solders and fluxes.Iron-Constantan thermocouples can
generate a galvanic EMF between the two wires and should not be
used in applications where they might get wet.
Chromel Alumel: Chromel-Alumel (wire is magnetic. Junctions can be
made by welding or soldering, butType K, color coded yellow and
red) generates about 40 µV/°C (22 µV/°F). The Alumel high
temperature silver-solders and special fluxes must be used.Chromel-
Alumel thermocouples generate electrical signals, while the wires are
being bent, and should not be used on vibrating systems, unless
strain relief loops can be provided.
Copper-Constantan: Copper-Constantan (Type T, color coded blue and
red) generates about 40 µV/°C (22 µV/°F). Neither wire is magnetic.
Junctions can be made by welding or soldering with commonly
available solders and fluxes.Copper-Constantan thermocouples are
very susceptible to conduction error, due to the high thermal
conductivity of the copper, and should not be used unless long runs
of wire (100 to 200 wire diameters) can be laid along an isotherm.

32. Thermocouple Color Codes
Thermocouple wiring is color coded by thermocouple types.
Different countries utilize different color coding. Jacket coloring is
sometimes a colored stripe instead of a solid color as shown.

33. Comparisons
Stability (Drift)
Reasonable for
limited lifetime Good Good
Repeatability Reasonable Good Good
Hysteresis Excellent Good Good
Vibration Very Resistant Less Resistant Good
Measurement Area Single Point Whole RT Element
Whole
bead
(Small)
Diameter
Small Sizes (to
0.25mm) Larger (3.0mm min)
Small
(0.5mm
min)
Linearity Not Linear Reasonably Linear Not Linear
Reference Junction Required Not Required
Not
Required
Lead Wire Resistance No Problem Must be Considered No Problem
Contact Required Yes Yes Yes
Response Fast Slower Medium
parameter Thermocouple
Resistance
Thermometer (RTD) Thermistor

34. Bimetal sensors
 Two metal strips welded together
 Each metal strip has different coefficient of
expansion
 As they expand, the two strips bend. This
motion can then be used to:
 move a dial
 actuate a sensor (pressure sensor for example)
 rotate a potentiometer
 close a switch

35. Bimetal sensors (cont.)
 To extend motion, the bimetal strip is bent
into a coil. The dial rotates as the coil
expands/contracts

36. Bimetal coil thermometer