Post on 12-Apr-2020
Lecture 3
Dr. Ali Karimpour Mar 2015
Instrumentation
In The Name of Allah
Dr. Ali Karimpour
Associate Professor
Ferdowsi University of Mashhad
Lecture 3
Dr. Ali Karimpour Mar 2015
2
Temperature measurement
Topics to be covered include:
v Resistive Thermometers
v Thermistor Thermometers
v Thermocouple Thermometers
v Semiconductor Junction Thermometers
v Bimaterials Thermometers (Bimetal)
v Pyroelectric(Infrared) Thermometers
v Fiber-Optic Thermometers
v …….
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Resistive Thermometers
v PTC
Positive Temperature Coefficient.
v NTC
Negative Temperature Coefficient.
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Resistive Thermometers
Sensitivity is defined as the amount of resistance change of the sensor
per degree of temperature change.
Nickel is very good for sensitivity.
However, have a limited temperature
range,
Copper has a very linear resistance-to-
temperature relationship. However,
copper oxidizes at moderate
temperatures and cannot be used above
150°C.
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Resistive Thermometers
Platinum is the best metal for RTD
elements.
• It follows a very linear resistance-
to-temperature relationship.
• It follows its resistance-to-
temperature relationship in a highly
repeatable manner over its
temperature range.
• It has the widest temperature range
among the metals used to make RTDs.
• Platinum is not the most sensitive metal; however, it is the metal
that offers the best long-term stability.
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Resistive Thermometers
Wire-wound RTD.
Thin film RTD.
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Resistive Thermometers
Industrial Platinum Resistance Thermometers (IPRTs) are designed to
withstand industrial environments and are almost as durable as
thermocouples.
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Linearity (least square method)
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Resistive Thermometers
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Dr. Ali Karimpour Mar 2015
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Resistive Thermometers
Calibration
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Resistive Thermometers
Calibration
Exercise 3-1: Resistance of an RTD at 60°C is 106Ω. The resistance
Variation with time is:
a) Derive a first order model for system and find FSO%(Suppose
113.6 is full scale).
b) Derive a second order model for system and find FSO %(Suppose
113.6 is full scale).
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Resistive Thermometers
PTC100.
• It is often from platinum.
• Its resistance at 0°C is 100 Ω.
• Its sensitivity is 0.00385
Example 3-1: What is the resistive
of PTC100 at 300°C ?
215.5 Ω.
Platinum RTD usually used in -200°C800°C
Nickel RTD usually used in -100°C260°C
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Thermistor Thermometers
A thermistor is a thermally sensitive resistor whose primary function is to
exhibit a change in electric resistance with a change in body temperature.
Unlike a wire-wound or metal film resistance temperature detector
(RTD), a thermistor is a ceramic semiconductor.
Thermistors have a large temperature coefficient compare to RTD but
it is nonlinear.
Thermistors usually used in -90°C130°C
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Thermistor Thermometers
Calibration
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Calibration
T: Degrees Kelvin
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Thermistor Thermometers
Calibration
Calibration curve of Thermistor by
Steinhart-Hart equation:
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RCRBAT
T: Degrees Kelvin
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Thermistor Thermometers
Calibration
Exercise 3-2: TR91 Series 10KΩ@25°C Thermistor calibration table
is:
a) Derive β for TR91 and find maximum error in ohms.
LSM. use and ofln derive :Hint)
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b) Derive A, B and C for Steinhart-Hart equation of TR91 and find
maximum error in ohms.
LSM. use :Hint
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RTD and Thermistor Thermometers
Suitable circuits to convert R V
Voltage divider. Voltage divider with follower.
DD
SEN
OUTV
RR
RV
1
1
Advantages:
• Simplicity
• Detection of open sensor (failure)
Disadvantages:
• Poor noise rejection
• Voltage is non-linear function of resistance
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RTD and Thermistor Thermometers
Suitable circuits to convert R V
Voltage divider by current source.
SENSENOUTIRV
Advantages:
• Linearity of resistance to voltage conversion.
• Robust in noise.
Disadvantages:
• Cost.
Two
wireFour
wire
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RTD and Thermistor Thermometers
Suitable circuits to convert R V
Wheatstone bridge
Two
wire
Three
wire
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RTD
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RTD and Thermistor Thermometers
RTDs and Thermistors self heating
The ability of the sensor to dissipate heat is defined by its dissipation
factor, δ, which has units of mW/°C.
The definition for δ is the amount of power that it takes to the
raise the body temperature of the sensing element 1°C.
Example 3-2: An RTD with R=500Ω at 20°C and sensitivity
coefficient 0.005 1/°C with dissipation factor δ=30 mw /°C let
R1=R2=500 Ω. Derive R3 for balance of bridge in 0°C. Us=10V
R3=450 Ω. In the case of no dissipation.
R3=454 Ω. By considering of dissipation.
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Thermocouple Thermometers
The Seebeck effect is the conversion of temperature differences
directly into electricity
(Thomas Johann Seebeck 1821)
For example, the Seebeck coefficients
of Cu is 1.5 μV/K, and of Al −1.5 μV/K.
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Thermocouple Thermometers
The Peltier effect is the presence of heating or cooling at an electrified
junction of two different conductors(Jean Charles Athanase Peltier
1834.)
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Thermocouple Thermometers
The Seebeck effect:
How to use?
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Thermocouple Thermometers
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Thermocouple Thermometers
Seebeck Coefficients for Some Metals
and Alloys, Compared to Platinum.
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Thermocouple Thermometers
Properties of several different thermocouple types
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Thermocouple Thermometers
Two additional thermocouple:
External ice-point reference junction:
Thermocouple refrigeration method:
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Thermocouple Thermometers
Using two ovens to simulate ice-point reference:
Using a thermometer to sense the temperature:
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Thermocouple Thermometers
Scanning many thermocouples.
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Thermocouple Thermometers
Electronic ice-point compensation.
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Thermocouple Thermometers
The sensitivity of RTD is very small.
The thermistor’s output is quite nonlinear.
Resistive, thermistor and thermocouple thermometer
The RTD and thermistor’s
output is …...
The thermocouple output is a very
small signal that is nonlinear
with temperature.
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Semiconductor Junction Thermometers
A silicon temperature sensor 730 mV at –40°C and 300 mV at 150°C
The narrowly specified Vbe ranges between 580 mV -620 mV at 25°C.
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Semiconductor Junction Thermometers
Linearity error (in mV) vs. temperature for a silicon temperature
sensor
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Semiconductor Junction Thermometers
LM335 provides 10 mV/°K (A zener diode)
LM34 provides 10 mV/°F
AD592 provides 0.001 mA /°K
Or LM335 provides 2.73+10 mV/°C .
Output:
+10 mV/°C .
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Semiconductor Junction Thermometers
LM34 provides 10 mV/°F
5 °F 50 mV
300 °F 3.00 V
-5 °F ??
-5 °F -50mV
300 °F 3.00 V
Sensors are not near to source??
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Semiconductor Junction Thermometers
Sensors are not near to source??
We use two wires not three!
We transmit current instead of voltage!
IQ is 70 μA
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Semiconductor Junction Thermometers
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Most available thermometers
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Bimaterials Thermometers (Bimetal)
Bimetallic thermometers make use
of change of volume with temperature.
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Pyroelectric(Infrared) Thermometers
An infrared (IR) thermometer is a noncontact radiant energy detector.
IR thermometers capture the invisible infrared energy that is naturally
emitted from all objects warmer than absolute zero (0 K).
IR thermometers are ideal for moving targets such as glass, plastic, and
steel. They do not interfere with the process.
The TC or RTD will not respond to these temperature changes
quickly enough to permit close control.
The IR thermometer will instantly measure the actual product
temperature, not the environment surrounding the product.
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Pyroelectric(Infrared) Thermometers
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Pyroelectric(Infrared) Thermometers
Distance does not affect the measurement. Models are available that
can measure from 0.3 to 91 m.
However, the farther away from the target the sensor is, the larger the
spot.
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Pyroelectric(Infrared) Thermometers
The infrared thermometer must be carefully aimed to avoid focusing on
reflected solar radiation.
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Fiber-Optic Thermometers
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Photon Thermometers
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Some more thermometers
Temperature sensing by expansion
Temperature sensing by displacement
Temperature sensing
by pressure.
Lecture 3
Dr. Ali Karimpour Mar 2015
System Design
46
Two-point control: (also called on–off control)
is the simplest type of closed-loop control strategy.
Inverting Amplifier
PVSPSPPVerror VVVR
RV
R
RV )(
2
3
2
3
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System Design
Hysteresis circuit
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System Design
Exercise 3-3: Consider following TR91 Series 10KΩ@25°C Thermistor
calibration table.
Design a circuit that turn on the fan of a car at 95°C and turn it off at
85°C.
Lecture 3
Dr. Ali Karimpour Mar 2015
References
v Industrial Control Electronics J.M. Jacob, Prentice-Hall,
1989
v مبانی اندازه گیری در سیستمهای ابزار دقیق، حمید رضا تقی راد و سید
1392علی سالمتی، انتشارات دانشگاه صنعتی خواجه نصیرالدین طوسی،
v Principles and Practice of Automatic Process Control,
Third edition, C. A. Smith, A. Corripio, Wiley 2006
v Instrument Control Engineers' Handbook Fourth
Edition, Béla G. Lipták, editor-in-chief. CRC Press 2003
49
Lecture 3
Dr. Ali Karimpour Mar 2015
Some Useful websites for the course
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v http://saba.kntu.ac.ir/eecd/ecourses/instrumentation.htm
v http://profsite.um.ac.ir/~shoraka/Instrumentation.htm
v http://karimpor.profcms.um.ac.ir/index.php/courses/10328