industrial instrumentation module_1

Industrial Instrumentation

Lecture 1

Wednesday, June 17, 2009 2

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


� 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



� 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



� 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



� 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



� 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


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


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


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+=


Coefficient of linear expansion αααα


Bi Metallic Strip

� Coefficient of linear expansion α is too

small

� To amplify, bond two dissimilar metals

together


Bi Metallic Strip


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


Bi Metallic Strip

� The linearity region is too limited

and depends on materials


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


Bi Metallic Strip

� Used mostly as

thermostats

� Relays and cutouts

� Temperature

compensation


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


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



� 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

=

=



� Pressure spring,

capillary and bulb

� Change in volume

due to expansion

� Pressure spring

expands



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

=

=



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


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


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


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



� 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 )



� The scale of the vapour pressure thermometer is

nonlinear and cramped at lower sides

� Different liquids are used for different ranges


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


Electrical type

� Thermocouples

� Resistance Temperature Devices

(RTDs)

� Thermistors


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.


Thermocouple

� When two dissimilar metal wires are joined

together at one end, a voltage is produced at the

other end proportional to temperature


Thermocouple

� Seebeck effect is bulk property

difference eTemperaturT

tcoefficienSeebeck

potential micalElectrocheq

E

Tq

E

s

s

=∆

=α

=

∆α=∆


Thermocouple Types


Laws of Thermocouple

� Law of homogeneous

circuit: No current flows

in a circuit made of

single metal by applying

heat alone



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



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


Law of intermediate temperatures


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.


Thermocouple


Thermocouple

We haven’t changed

the output voltage

V. It is still


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.


Thermocouple

Software Compensation:


Thermocouple


Thermopiles


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



•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,



•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°



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



•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


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.


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.




• 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




• 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


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


Thermistors


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


Thermistors

•Positive Temperature Coefficient thermistors (PTC) are mainly

used as protective elements in electric machinery

•Made from titanates of barium, lead and strontium


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


Semiconductor sensors


Digital Thermometers


Summary of Sensor Characteristics


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


Radiation Thermometry

•Stefan-Boltzman Law


Radiation Pyrometers




Optical type

•Monochromatic brightness radiation

pyrometer

•Range between 700 and 3000 °C

•The most accurate among the pyrometers



Optical type



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


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

industrial instrumentation module_1

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