Standard Fixed Point Temperatures
Temperature Measurement
Prof. Dr. M. Zahurul Haq
zahurul@me.buet.ac.bd
http://teacher.buet.ac.bd/zahurul/
Department of Mechanical Engineering
Bangladesh University of Engineering & Technology
ME 361: Instrumentation & Measurement
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Temperature Measurement
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Temperature Measurement by Mechanical Effects
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Temperature Measurement
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Temperature Measurement by Mechanical Effects
Liquid-in-glass Thermometer
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Inexpensive, simple, portable & no need for additional indicator.
High heat capacity =) time lag exists.
Not suitable for distant reading.
Not suitable for surface temperature measurement.
Alcohol is limited to low-temperature measurements. Its high
coefficient of expansion makes it more sensitive.
Mercury cannot be used below its freezing point of 37:8o C . Its
upper limit is usually 315o C , but it may be extended to 540o C by
filling gas filling above the mercury.
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Temperature Measurement
Temperature Measurement by Mechanical Effects
Temperature Measurement by Mechanical Effects
Reading in Liquid-in-Glass Thermometer
Bimetallic Strip Thermometer
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Temperature Measurement
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Temperature Measurement by Thermoelectric Effects
Thermocouple
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Low cost, negligible maintenance & longer stable operation.
Good resistance to mechanical shock.
Close linearity within temperature range ( 30 ! 550o C).
Compact, low thermal inertia and reduced lag.
Accuracty 1-2% of the scale range.
Widely used as on-off temperature-control device, thermostat.
Also used for measuring ambient and oven temperatures.
Prof. Dr. M. Zahurul Haq (BUET)
Temperature Measurement
ME 361
Seebeck EMF: is generated when heat is applied to the junction
of two dissimilar metals.
Peltier EMF: While measuring, Seebeck emf may be slightly
altered due to Peltier emf caused by the current flow.
Thomson EMF: results from a temperature gradients in metals.
Usually, Peltier and Thomson emfs are negligible in the circuit.
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Temperature Measurement by Thermoelectric Effects
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Law of Interior Temperature
Law of Inserted Material
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The thermocouple is unaffected by hot spots along the
thermoelement and the reading is only a function of T1 and T0
Prof. Dr. M. Zahurul Haq (BUET)
Temperature Measurement
ME 361
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Temperature Measurement by Thermoelectric Effects
The thermocouple is unaffected by the presence of the inserted
material and any local hot spot
Prof. Dr. M. Zahurul Haq (BUET)
Temperature Measurement
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Temperature Measurement by Thermoelectric Effects
Law of Intermediate Materials
Laws of Intermediate Temperatures
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As there is no thermal gradient across the new thermoelectric
junctions then the presence of the inserted material does not
contribute to the net emf produced by the thermocouple
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Temperature Measurement
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Junction pairs at T1 and T3 produce the same voltage as two sets
of junction pairs spanning the same temperature range (T1 to T2
and T2 to T3 ; therefore V13 = V12 + V23
Prof. Dr. M. Zahurul Haq (BUET)
Temperature Measurement
ME 361
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Temperature Measurement by Thermoelectric Effects
Temperature Measurement by Thermoelectric Effects
Thermocouple Temperature Measurement Circuits
Thermopile Arrangement
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Thermopile, a multiple-junction thermocouple circuit, is used to
amplify the output of the circuit.
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Prof. Dr. M. Zahurul Haq (BUET)
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Temperature Measurement by Thermoelectric Effects
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Thermocouples Arranged in Parallel
Thermocouple Based System
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When a spatially averaged temperature is desired, multiple
thermocouple junctions arranged in parallel are used.
Prof. Dr. M. Zahurul Haq (BUET)
Temperature Measurement
Temperature Measurement
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Prof. Dr. M. Zahurul Haq (BUET)
Temperature Measurement
Temperature Measurement by Thermoelectric Effects
Temperature Measurement by Thermoelectric Effects
Standard Thermocouple Compositions
Resistance Thermometry
The essential elements required in precise resistance thermometry are a
resistor properly mounted & protected, a means for measuring its
resistance, and a relation between resistance and temperature.
(
R
=
Ro (1 +haT + bT 2 )i
1
Ro exp T1
To
RTD
Thermistor
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Resistance Temperature Detector (RTD)
Thermistor Examples
Platinum RTD is used as an interpolation standard from oxygen point
( 182:96o C ) to the antimony point (630:74o C )
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(a) Cylindrical PRT with wire set in grooves. (b) Bifilar wound
coil. (c) Multi-bore alumina tubing support. (d) Flat pack. (e)
Thin-film PRT.
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Prof. Dr. M. Zahurul Haq (BUET)
Temperature Measurement
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IC Temperature Sensor
A recent innovation is the ic temperature transducer.
Available in both voltage & current sensitive configurations. Both
supply an output that is linearly proportional to the absolute
temperature.
Used widely in on/off or alarm point control.
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Typical semiconductor temperature sensor packaging. (a) TO99
can. (b) TO-92 plastic moulding. (c) DIP plug
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Miscellaneous Thermal Measurements
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Miscellaneous Thermal Measurements
Blackbody Radiation
Optical Pyrometer
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Miscellaneous Thermal Measurements
Heat Flux Measurement
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Slug-type
Foil-type
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Thin-film type
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