Lecture 13 Temperature Sensors

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Lecture 13Temperature Sensors
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Temperature Transducers

• Thermometer
• Platinum resistance thermometer
• Bimetallic switch
• Thermocouple
• Thomson EMF
• Peltier EMF
• Thermister
• Others
• Solid-state temperature sensor
• Infrared temperature sensor

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Commonly Used Temperature Scale
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Platinum Resistance Thermometer

• Made of Platinum (noble metal).
• Can withstand high temperature (up to 800 oC),
  and harsh environment.
• Often used as temperature interpolation standard
• Resistance changes with environment temperature
  (oC)

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Bimetallic Switch
Temperature controlled deflection
Fixed end
Metal A
Metal B
Adjustable contact

• Metal A and B have different thermal-expansion
  coefficient
• Mechanical bending occurs when temperature
  changes
• Electrical contact closes or opens according to
  temperature
• Commonly used in room and oven temperature
  control

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Thompson EMF






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Metal /Semiconductor material in a uniform
temperature environment


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Hot End
Cold End
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Metal /Semiconductor material in non-uniform
temperature environment
Temperature difference induced potential
difference in material
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Peltier EMF
Material 2 with high electron mobility
Material 1 with low electron mobility












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Electron mobility difference in materials induces
potential difference
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Thermocouple
Material A
V0 -
Reference Temperature T0
Temperature T
Material B

• The voltage V0 is the collective contribution
  from
• Thompson EMF in material A
• Thompson EMF in material B
• Peltier EMF from material AB contact

Net result V0 ?T-T0 Depend on materials A and B
properties
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Thermocouple Voltage to Temperature Conversion
NBS (National Bureau of Standard) polynomials
Table 4.7 of handouts 5 shows a0,a1, for
common thermocouple material For example, for
Iron-Constantan (Copper-Nickel alloy),
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Thermister
Conduction band
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Energy gap E

• R ? 1/?
• ? Ne
• Ne ? e-E/kT

Valence band
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E energy gap between conduction and valence
band k Boltzman constant, 8.610-5 eV/K T
temperature(K)
R ? eE/kT
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Temperature and Resistance Relationship
Where T is temperature (K) R(T0) is the
thermister resistance at temperature T0 ? is
determined by the thermister material (E/k)
Determine T vs. R experimentally
1/T
Slope 1/?
1/T0
lnR0
lnR
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Practical Thermister

• Time constant response time
• several seconds without insulation coating
• 10 times more with insulation coating
• Dissipation constant power needed to raise T 1oC
  above environment
• 10 mW/oC in well-stirred liquid
• 1mW /oC in still liquid
• Need to limit the amount of current through a
  thermister

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Temperature Sensing Using Thermister
(1)
(2)
Recall
(3)
Perform simultaneous measurement of T and V0
Compute RT from V0, We can calibrate a thermister
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Optimization of Thermister-Bridge Sensitivity
Maximize
Optimization of measurement sensitivity
(4)
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Maximum Sensitivity
Maximum sensitivity occurs when
R2R3RT

• Experimental approach
• choose R2R3RT0
• adjust R1 so V00. (Balancing)
• measure V0 as T changes.
• compute RT base on V0 reading using (2)

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Homework and Reading

• Handout5
• Reading P230-246
• Homework P272, 4.4. Due Fri. 4/1