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Temperature
URVISH SONI
Brief Overview
Types of Sensors and how they work
Sensor Applications
Advantages and Disadvantages
Sensors that will work with project
How is heat transferred?
Conduction

Metal coffee cup
Convection
Radiation
003
Temperature scales
Types of Temperature Sensors
Bi Metallic
Thermocouples
Resistance
Temperature
Detectors (RTDs)
Thermistors
Infrared Sensors
Expansion thermometer
• Solid Expansion Thermometer
Bimetallic thermometer
Spiral Bimetal element
Helix Bimetal element
• Liquid Expansion Thermometer- Mercury in
Glass type.
Bimetal thermometer
Two dissimilar Forces due to
metals, tightly
bonded
0
thermal
expansion
Result
100 200 300
400
Thermal Expansion Co efficient
Of Metal
Spiral and Helical
Liquid thermometer
Liquid-filled thermometer
100
0






Accurate over a small range
Accuracy & resolution= f(length)
Range limited by liquid
Fragile
Large thermal mass
Slow
 Mercury
Class I-Liquid Filled Systems
Class II- Vapour Systems
Class III- Gas Filled Systems
Class V- Mercury Filled Systems
Seeback effect
Peltier
• an effect whereby heat is given out or
absorbed when an electric current passes
across a junction between two materials.
Thomson effect
• Thomson effect is related to the emf that
develops between two parts of the single
metal when they are at different temperature
• Thus thomson effect is the absorption or
evolution of heat along a conductor when
current passes through it when one end of the
conductor is hot and another is cold
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.
Cold junction
• Maintaining an ice water slurry and actual
cold junction is rarely feasible. Typically, the
cold junction is omitted, and the potential is
measured directly across the two terminal
ends of the thermocouple wires at ambient
temperature.
• Simulate the potential effects that would result for a thermocouple
wire pair between the terminals, at their measured temperature, and
another junction at a reference temperature of 0 degrees. Measure
the potential across the thermocouple wire pair in series with the
simulated potential. Apply the linearizing curve to the sum, thus
obtaining an estimated absolute temperature directly. This is known
as cold junction compensation. Usually, the simulation is done
electronically with specialized integrated circuit devices.
• electronic cold junction compensation
• Independently measure the temperature of the cold
junction. Measure the thermocouple potential and
apply conversion curves to determine the
temperature difference across the thermocouple.
Then add the known cold junction temperature to
the measured temperature difference to determine
the absolute temperature measurement.
• independent cold junction measurement
Thermowell
Thermocouple extension wires
Thermocouples selection criteria
Thermocouple Applications
 Plastic injection molding
machinery
 Food processing equipment
 Deicing
 Semiconductor processing
 Heat treating
 Medical equipment
 Industrial heat treating
 Packaging equipment
Thermocouples
Advantages




Simple, Rugged
High temperature operation
Low cost
No resistance lead wire
problems
 Point temperature sensing
 Fastest response to
temperature changes
Disadvantages
 Least stable, least repeatable
 Low sensitivity to small
temperature changes
 Extension wire must be of
the same thermocouple type
 Wire may pick up radiated
electrical noise if not
shielded
 Lowest accuracy
Common Thermocouples
Type
J
K
T
S
E
N
Metals
Seebeck
Coeff: uV/C
Fe-Con
Ni-Cr
Cu-Con
Pt/Rh-Pt
Ni/Cr-Con
Ni/Cr/Si-Ni/Si
50
40
38
10
59
39
Microvolt output is a
tough measurement
Type "N" is fairly new..
more rugged and higher
temp. than type K, but
still cheap
037
Resistance Temperature Detectors
(RTDs)
 Wire wound and thin film
devices.
 Nearly linear over a wide
range of temperatures.
 Can be made small
enough to have response
times of a fraction of a
second.
 Require an electrical
current to produce a
voltage drop across the
sensor
Measuring an RTD: 2-wire method
Rx
100d
Pt
Rlead
Rlead
+
V
-
I ref= 5 mA
R= Iref*(Rx + 2* Rlead)

Error= 2d/.385= more than 5 degrees C for 1 ohm
Rlead!
Self-heating:
 For 0.5 V signal, I= 5mA;
P=.5*.005=2.5 mwatts
 @ 1 mW/deg C,
Error = 2.5 deg C!
Moral: Minimize Iref; Use 4-wire method
 If you must use 2-wire, NULL out the lead resistance
018
3-Wire bridge
100d
1000d
Rlead 1
V
1000
d
Sense wire
3-Wire
PRTD
Rlead 2
Keeps bridge away from heat source
Break DMM lead (dashed line); connect to
100d
RTD through 3rd "sense" wire
If Rlead 1= Rlead 2, sense wire makes
error small
Series resistance of sense wire causes no
error
022
The 4-Wire technique
Rx
100d
+
Rlead=1 d
-
V
I ref= 5 mA
 R= Iref * Rx
 Error not a function of R in source or sense leads
 No error due to changes in lead R



Twice as much wire
Twice as many scanner channels
Usually slower than 2-wire
019
Bridge method
100 d
d
1000
V
d
1000
100d
High resolution (DMM
stays on most sensitive
range)
Nonlinear output
Bridge resistors too
close to heat source
021
RTDs
Advantages
• Most stable over time
• Most accurate
• Most repeatable
temperature measurement
• Very resistant to
contamination/
• corrosion of the RTD element
Disadvantages
• High cost
• Slowest response time
• Low sensitivity to small
temperature changes
• Sensitive to vibration (strains
the platinum element wire)
• Decalibration if used beyond
sensor’s temperature ratings
• Somewhat fragile
Thermistors
• A semiconductor used as a temperature sensor.
• Mixture of metal oxides pressed into a bead, wafer or other
shape.
• Beads can be very small, less than 1 mm in some cases.
• The resistance decreases as temperature increases, negative
temperature coefficient (NTC) thermistor.
Thermistors
• Most are seen in medical
equipment markets.
• Thermistors are also
used are for engine
coolant, oil, and air
temperature
measurement in the
transportation industry.
Advantages
Thermistors
• High sensitivity to
small temperature
changes
• Temperature
measurements
become more stable
with use
• Copper or nickel
extension wires can
be used
Disadvantages
• Limited temperature
range
• Fragile
• Some initial accuracy
“drift”
• Decalibration if used
beyond the sensor’s
temperature ratings
• Lack of standards for
replacement
emissivity
• The emissivity of the surface of a material is
its effectiveness in emitting energy as thermal
radiation.
• Thermal radiation is electromagnetic radiation
and it may include both visible radiation (light)
and infrared radiation,
Stefan–Boltzmann law
• Stefan–Boltzmann law, statement that the
total radiant heat energy emitted from a
surface is proportional to the fourth power of
its absolute temperature.
• 5.6704 × 10−8 watt per metre2∙K4
• E = σT4
Black body
• A black body is an idealized physical body that
absorbs all incident electromagnetic radiation,
regardless of frequency or angle of incidence.
• A white body is one with a rough surface
[that] reflects all incident rays completely and
uniformly in all directions
List sources of error in Non-contact
type thermometry
•
•
•
•
•
Radiation pyrometer
Optical pyrometer
Optical Fiber Thermometry
Ultrasonic thermometry
Laser thermometry
Optical pyrometer
Optical Fibre Thermometry
Ultrasonic thermometry
Temperature switches
Thermostats
Actual Image