Industrial Instrumentation Lecture 1
Industrial Instrumentation
Lecture 1
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Definitions and standards calibration Thermal expansion methods, Bimetallic,
Liquid-in-glass, Pressure thermometers. Thermocouples-Law of
thermocouple–Common thermocouples–Reference junction considerations–
measuring circuits, thermopiles. RTDs – RTD materials– 3 lead and 4 lead
RTDs – Junction semiconductor sensors – Digital thermometers. Radiation
thermometry– radiation fundamentals – total radiation type– optical
pyrometer – Ratiometric type. Quartz crystal thermometer.
Measurement of TemperatureModule-1
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� Temperature is a measure of stored or
potential energy in a mass of matter. It
is the state of agitation, both lateral and
rotational oscillation, of the molecules of
the medium
Definitions and Standards
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Definitions and Standards
� Heat is thermal energy. The quantity of heat in a body is proportional to the temperature
� The heat capacity multiplied by its absolute temperature.
� Heat is measured in joules. One calorie is approximately 4.2 joules
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Definitions and Standards
� Heat capacity, or more usually the specific heat, of a
substance is the amount of heat which, when
absorbed by 1 kg of that substance, will raise its
temperature by 1◦C
� Pressure ,flow ,level etc are calibrated in terms of
Mass ,Length and Time
� Temperature is not independently definable
� PV=nRT
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Definitions and Standards
� Accuracy The difference between a
measurement reading and the true value of that
measurement
� Calibration The comparison of a device with
unknown accuracy, to a device with a known,
accurate standard to eliminate any variation in
the device being checked
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Definitions and Standards
� Precision is the resolution with which a
measurement may be made
� Repeatability is the difference between
successive measurements of the same quantity
� Sensitivity The smallest change in a
measurement that an instrument is capable of
detecting
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Techniques
� Longitudinal change – solids
� Volume change – liquids at constant P
� Pressure change – gas at constant V
� Resistance change – Conductors, Semiconductors
� Thermocouple ( dissimilar metals)
� Change in magnetic properties
� Frequency change – Crystal Oscillators
� Velocity change – SAW
� Optical radiations at high temperatures
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Performance specifications
Nonlinear700 to 3000Optical/Radiation
Approx. linear-150 to 1500Thermocouple
Nonlinear-50 to 300Thermistor
Approx. linear-200 to 700Resistance
Nonlinear-120 to 450Gas and Vapour pressure
Linear-125 to 500Liquid Expansion
Nonlinear-150 to 500Solid Expansion
ScaleRange (◦C)Technique
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Solid expansion type
� Solids tend to expand when heated
� Depends on size, material and
Temperature
re Temperatuin ChangeT
expansionlinear of tCoefficien
length Originall
heatingafter lengthl
)T1(ll
0
0
=∆
=α
=
=
∆α+=
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Coefficient of linear expansion αααα
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Bi Metallic Strip
� Coefficient of linear expansion α is too
small
� To amplify, bond two dissimilar metals
together
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Bi Metallic Strip
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Bi Metallic Strip
� Usually radius ρ is large and the
deflection of the tip is small
� If the difference of the coefficients are
large, the deflection can be increased
� Materials with large differences in
coefficients are chosen
� The deflection of the free tip is
proportional to temperature for small
deflections
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Bi Metallic Strip
� The linearity region is too limited
and depends on materials
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Bi Metallic Strip
� Spiral or helix shape gives larger
deflection
� Reduction in thickness increases the
deflection further
� Accuracy is fairly good
� Repeatability is low
� Poor frequency response
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Bi Metallic Strip
� Used mostly as
thermostats
� Relays and cutouts
� Temperature
compensation
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Filled in Systems
Mercury (Volume)Class 5
N.AClass 4
Gas (Pressure)Class 3
Vapour (Pressure)Class 2
Liquid (Volume)Class 1
Filled inTypes � Volumetric expansion
� Liquid and gas
thermometers
� Rugged, cheap and
very less
maintenance
� Good response
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Liquid expansion type
� Molecules in a fluid exerts large pressure on the walls of the enclosing vessel, when heated
� By measuring this pressure, and/or by allowing the fluid to expand under constant pressure, the temperature of the fluid can be measured
� Self contained and self operated
� Clinical thermometer
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Liquid expansion type
� Accuracy and sensitivity
lower , limited range
� Remote indication with
longer capillaries
(industrial type)
[ ])TT(1VV 1212 −β+=
re TemperatuInitialT1
re TemperatuFinalT2
expansion
ricof volumet tCoefficien
VolumeInitialV1
VolumeFinal2V
=
=
=β
=
=
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Liquid expansion type
� Pressure spring,
capillary and bulb
� Change in volume
due to expansion
� Pressure spring
expands
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Liquid expansion type
γ+= ∑
=
n
1j
j
jit t1VV
difference Tempt
expansion
ricof volumet tCoefficien
temp final at VolumeV
temp initial at VolumeV
j
j
t
i
=
=γ
=
=
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Liquid expansion type
non linear-80 to 250Toluene5
non linear20 to 90Ether4
non linear-40 to 400Xylene3
non linear-45 to 150Ethyl Alcohol2
Linear-40 to 650Mercury1
ResponseRange (◦C)LiquidSl.No
� Mercury range is almost linear ( class V )
� Bulb size, coefficients of expansions determines the
range
� High Pressure filling improves the range
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Gas expansion type
� Constant volume
thermometer
� Pressure developed
expands the
bourdon/bellows
� Volume of the tube connecting the sensing bulb to the indicating element must be minimal
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Gas expansion type
� Since, bourdon tube expands substantially,
bulb volume is chosen 100 times larger than
capillary and bourdon element
� Nitrogen gas is commonly used with a range
from -120 to 450 ◦C
� At high temperatures, the gas may leak
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Vapour pressure thermometer
� Vapour pressure of the
liquid (Class II)
� Vapour pressure is a
function of surface
temperature of the
liquid
� Bulb must hold the
entire liquid at low
temp
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Vapour pressure thermometer
� At higher temperature, volume of the liquid
completely fills the entire system
� Free surface of the liquid should be available, at
the lowest as well as highest temperature
� Vapour pressure must be large through out the
range to operate the bourdon/bellows
� Range is limited by the critical point of the lquid
( -25 to 250 ◦C )
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Vapour pressure thermometer
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Vapour pressure thermometer
� The scale of the vapour pressure thermometer is
nonlinear and cramped at lower sides
� Different liquids are used for different ranges
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Vapour pressure thermometer
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Vapour pressure thermometer
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Errors in filled in systems
� Ambient temperature effect : Change in ambient
temperature cause volume change
� Head or elevation effects: Thermometer bulb placed at
different height w.r.t bourdon tube
� Barometric effect: Error due to atmospheric pressure
change
� Immersion effect: Error due to improper immersion
� Radiation effect: Error due to temperature difference
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Electrical type
� Thermocouples
� Resistance Temperature Devices
(RTDs)
� Thermistors
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Electrical type� Seebeck effect states that the voltage produced in a
thermocouple is proportional to the temperature
between the two junctions.
� Peltier effect states that if a current flows through a
thermocouple one junction is heated and the other
junction is cooled.
� Thompson effect states that when a current flows in a
conductor along which there is a temperature difference,
heat is produced or absorbed, depending upon the
direction of the current and the variation of
temperature.
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Thermocouple
� When two dissimilar metal wires are joined
together at one end, a voltage is produced at the
other end proportional to temperature
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Thermocouple
� Seebeck effect is bulk property
difference eTemperaturT
tcoefficienSeebeck
potential micalElectrocheq
E
Tq
E
s
s
=∆
=α
=
∆α=∆
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Thermocouple Types
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Thermocouple Types
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Laws of Thermocouple
� Law of homogeneous
circuit: No current flows
in a circuit made of
single metal by applying
heat alone
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Laws of Thermocouple
Law of intermediate metals: If between any two points a
number of junctions appear but all at the same
temperature, emf between these two points is
independent of intermediate metals
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Laws of Thermocouple
Law of intermediate metals: If between any two points a
number of junctions appear but all at the same
temperature, emf between these two points is
independent of intermediate metals
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Laws of Thermocouple
Law of intermediate temperatures: EMF for a couple with junction
temperature T1 and T3 is the sum of two emfs of two couples of same metals
having junction temperature T1 and T2, and T2 and T3 respectively
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Law of intermediate temperatures
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Thermocouple
To find the
temperature at
J1, we must
know the
temperature at
J2.
To compensate
EMF due to J2, it
should be
maintained at
0oC.
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Thermocouple
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Thermocouple
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Thermocouple
We haven’t changed
the output voltage
V. It is still
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ThermocoupleThis is a useful conclusion, as it completely eliminates the
need for the iron (Fe) wire in the LO lead:
V = α (TJ1 - TREF),
where α is the Seebeck
coefficient for an Fe-C
thermocouple.
Junctions J3 and J4, take the
place of the ice bath.
These two junctions now
become the Reference
Junction.
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Thermocouple
Software Compensation:
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Thermocouple
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Thermopiles
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Resistance Temperature Detectors (RTD)
•A metallic resistance element changes its resistance
with temperature
•Pure elements have been used for measurement of
temperature by this effect
• The method is one of the most accurate ones
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Resistance Temperature Detectors (RTD)
•The relation between resistance and temperature
change ∆t is, thus, expressed by,
•Where αj ‘s are temperature coefficients of resistance.
In the narrow ranges of operation αj ‘s are negligible,
then Rt is given by,
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Resistance Temperature Detectors (RTD)
•The αj ‘s are positive for a metallic
resistance elements
• And is often determined experimentally
• Platinum with a range -190°to 660 °C
• Copper with a range -150°to 250 °C
• Nickel with a range 0°to 325°
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Resistance Temperature Detectors (RTD)
The choice of the materials are governed by
•High temperature coefficient ( larger sensitivity)
•High resistivity of the material ( ensures small length of the wire
for a high resistance value )
•Linearity of resistance-temperature relation (adds to the
convenience in measurement )
•Stability of electrical characteristics of the material and
resistance to contamination (for good repeatability )
•Sufficient mechanical strength
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Resistance Temperature Detectors (RTD)
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Resistance Temperature Detectors (RTD)
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Resistance Temperature Detectors (RTD)
•Resistance thermometers
are the most accurate of all
the temperature measuring
systems
• Accuracy of 0.0001 °C can
be obtained
•Very convenient for
measurement of a small
temperature difference
( compared to T.C )
•Bridge circuits are preferred
for the measurement of the
resistance
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Lead Configurations of RTD
ProbesLead configuration 1
• Provides one connection to each Provides one connection to each
end of the sensorend of the sensor
•This construction is suitable where
the res istance of the run of lead
w i r e m a y b e c o n s i d e r e d a s a n
additive constant in the circuit, and
particularly where the changes in
l e a d r e s i s t ance due t o amb ien t
temperature changes can be ignored.
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Lead Configurations of RTD Probes
Lead configuration 2
• Provides one connection to one end and Provides one connection to one end and two to the other end of the sensortwo to the other end of the sensor
•• Connected to an instrument designed to Connected to an instrument designed to accept threeaccept three--wire input, compensation is wire input, compensation is achieved for lead resistance and achieved for lead resistance and temperature change in lead resistancetemperature change in lead resistance
••This is the most commonly used This is the most commonly used configuration.configuration.
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Lead Configurations of RTD Probes
Lead configuration 2
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Lead Configurations of RTD Probes
Lead configuration 3
• Provides two connections to Provides two connections to
each end of the sensor. This each end of the sensor. This
construction is used for construction is used for
measurements of the highest measurements of the highest
precisionprecision
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Lead Configurations of RTD Probes
Lead configuration 3
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Lead Configurations of RTD Probes
Lead configuration 3
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Lead Configurations of RTD Probes
Lead configuration 3
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Lead Configurations of RTD Probes
Lead configuration 4
• Separate pair of wires are Separate pair of wires are
provided as a loop to provide provided as a loop to provide
compensation for lead compensation for lead
resistance and ambient resistance and ambient
temperature changes in lead temperature changes in lead
resistanceresistance
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Lead Configurations of RTD Probes
Lead configuration 4
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Thermistors
•Thermistors are a class of metal oxide (semiconductor material)
•High negative temperature coefficient of resistance (NTC)
•Can also be positive (PTC)
•Thermistors have high sensitivity ( 10% change /˚C )
•Most sensitive temperature elements available
•Very nonlinear characteristics
•Shapes various – disks, rods, beads….
•Size – small, Cost – low, Stability - high
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Thermistors
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Thermistors
•Main disadvantage of a thermistor is its nonlinear scale
•Compensation with a resistor parallel to the thermistor
•This reduces the sensitivity but increases the linearity
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Thermistors
•Positive Temperature Coefficient thermistors (PTC) are mainly
used as protective elements in electric machinery
•Made from titanates of barium, lead and strontium
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Semiconductor sensors
•Semiconductors have a number of parameters that vary linearly
with temperature
•Reference voltage of a zener diode or the junction voltage
variations
•Range from –50 to 150°C
• Very linear with accuracies of ±1°C
•Can be integrated onto the same dye
•High sensitivity, easy interfacing to control systems
•Rugged with good longevity and are inexpensive
•AD590,AD592,LM334,LM335,LM34,LM35
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Semiconductor sensors
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Digital Thermometers
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Summary of Sensor Characteristics
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Radiation Thermometry
•When thermometers cannot be brought in contact with
the process
•Or when the hot bodies are moving
•Any mass above absolute zero temperature will emit
electromagnetic radiation
•Measure the energy, directly or by colour comparison
•Total radiation type
•Optical type
•Ratio metric type
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Radiation Thermometry
•Stefan-Boltzman Law
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Radiation Pyrometers
•Total radiation type
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Radiation Pyrometers
•Total radiation type
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Radiation Pyrometers
Optical type
•Monochromatic brightness radiation
pyrometer
•Range between 700 and 3000 °C
•The most accurate among the pyrometers
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Radiation Pyrometers
Optical type
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Radiation Pyrometers
Optical type
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Radiation Pyrometers
Ratiometric type
•Ratio thermometer
•Two radiation pyrometers in single housing
•Lens, detectors are shared by both
•O/p wavelengths are ratioed
•Relatively independent of target size
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Quartz Crystal Thermometer
•Properly cut quartz crystal
•Resonant frequency linearly depend on Temp
•Oceanographic studies
•Measure small temp differences
•Frequency shift is small 1KHz/˚C at 10MHz
•Direct digital read out possible