Chapter-1

INTRODUCTION

A transducer is a device, usually electrical,

electronic, electro-mechanical, electro-magnetic, photonic,

or photovoltaic that converts one type of energy or physical

attribute to another (generally electrical or mechanical)

for various measurement purposes including measurement or

information transfer.

The term transducer is commonly used in two senses- the

sensor used to detect a parameter as a form of energy and a

translator which translates that form to an easily

measurable form.

1

Chapter-2

REQUIREMENTS OF TRANSDUCER Main objective of a transducer is to react only for

the measurement under specified limits for which it is

designed. A transducer should have the following basic

requirements:

2.1 Linearity Its input versus output characteristics should be

linear and it should produce these characteristics in a

balanced way.

2.2 Ruggedness It should be capable of withstanding overload and some

safety arrangements must be provided with it for overload

protection.

2.3 Repeatability

2

It should be capable of reproducing the same output

signal when the same input signal is applied again and again

under unchanged environmental conditions, e.g., temperature,

pressure, humidity, etc.

2.4 High Reliability and Stability The transducer should give minimum error in measurement

for temperature variations, vibrations and other various

changes in the surroundings.

2.5 High Output Signal QualityThe quality of output signal should be good, i.e., the ratio

of signal to the noise should be high and amplitude of the

output signal should be enough.

2.6 No HysteresisIt should not give any hysteresis during measurement while

input signal is varied from its low value to its high value

and vice versa.

2.7 Residual ReformationThere should not be any deformation on removal of input

signal for a long period of use.

Chapter-3

ELECTRICAL TRANSDUCERS

3

Electrical transducers are defined as the transducers which

convert one form of energy to electrical energy for

measurement purpose. The quantities which cannot be measured

directly like pressure, displacement, temperature, etc. Are

required to be sensed and changed into electrical signal for

easy measurement.

3.1 Advantages of Electrical Transducers Power requirement is very low for controlling the

electrical or electronic system.

An amplifier may be used to amplify the electrical

signal according to requirement.

Friction effect is minimized

Mass-inertia effect is also minimized, because in case

of electrical or electronics signals the inertia effect

is due to the mass of electrons, which can be

negligible.

Output can be indicated and recorded remotely from the

sensing element.

3.2 Linear Variable Differential Transformer (LVDT)

4

This is the most widely used inductive transducer to translate

the linear motion into electrical signal. The transformer consist

of a single primary winding “P” and two secondary windings “S1”

and “S2” wound on a cylindrical former. A sinusoidal voltage of

amplitude 3 to 15 volt and frequency 50 to 20,000 Hz is used to

excite the primary. The two secondary’s have equal no. of turns

and are equally placed in either side of primary winding.

A movable soft iron core is placed inside the former. The

displacement to be measured is applied to the arm attached to the

soft iron core. The core is made up of high permeability, nickel

iron. The whole assembly is placed in stainless steel housing to

provide electrostatic and electromagnetic shielding.

5

The two secondary’s are placed or connected in series to one

another but in opposition such that the output secondary voltage

is E0 = ES1 – ES2 , where ES1 and ES2 are the induced alternating

voltages of the two secondary windings S1 and S2 respectively.

3.2.1 Operation:When the core is at the normal null position then flux linking

with both the secondary winding is equal and hence ES1 = ES2 and

therefore ES1 – ES2 = 0 and hence output voltage is zero.

Now if core is moved to the left then more flux links with the

secondary S1and less with the other winding therefore the output

ES1 is greater than ES2. The magnitude of the output voltage is

thus ES1 – ES2 and in phase with the primary voltage.

Similarly when the core is moved towards the right of null

position then ES2 will be more than ES1 and the output voltage ES1

– ES2 is 180 out of phase with the primary voltage.

Therefore the amount of change in voltage in either of the

secondaries is proportional to the amount of movement of the

core. Hence, we have an indication of the linear motion of the

core. And by noticing the output voltage is increased or

decreased, we can determine the direction of motion.

3.2.2 Advantages and Disadvantages of LVDT

3.2.2.1 Advantages

6

Fig. 3.1 LVDT

Linearity is good up to 5mm of displacement.

Output is rather high and hence immediate amplification is

not required.

Output voltage is step less and hence the resolution is very

good.

Sensitivity is high (about 40 V/mm).

It does not load the measurand mechanically.

Consumes low power and has low hysteresis loss.

3.2.2.2 Disadvantages

LVDT has a large threshold.

It is affected by stray electromagnetic field. Hence copper

shielding of device is necessary.

The ac inputs generate noise.

Its sensitivity becomes low at higher temperature.

Being a first-order instrument, its dynamic response is not

instantaneous.

3.3 Strain GuagesA strain gauge is an electrical transducer; it is used to

measure the mechanical surface tension. Strain gauge can

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detect and convert force of small mechanical displacement

into electrical signals. On the application of force a metal

conductor is stretched or compressed, its resistance changes

owing to the fact that both length and diameter of the

conductor changes. Also, there is a change in the value of

resistivity of the conductor when it is strained and this

property of the metal is known as piezoresistive effect.

Therefore resistance strain gauges are also known as

piezoresistive gauges.

Since value of resistance is R= ρ (L/A)

The change in the value of resistance by the application

of force can be explained by normal dimensional changes of

elastic material. If a positive strain occurs, its

longitudinal dimension (x-direction) will increase while

there will be a reduction in the lateral dimension (y-

dimension). The reverse happens for a negative strain. Now

since the resistance of a conductor is directly proportional

to its length and inversely to its cross sectional area,

hence the resistance changes.

3.4 Temperature Transducers

Application of heat or its withdrawal from the body produces

many primary effects on the body such as:-

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Change in the physical or chemical state such as phase

transition.

Change in physical dimensions.

Variations in its electrical properties.

Generation of an emf at the junction of two dissimilar

metals.

Change in the intensity of emitted radiation.

Any of these effects can be used to measure the

temperature of the body.

3.5 Resistance ThermometerResistance temperature detectors or resistance thermometers

employ a sensitive element of extremely pure platinum,

copper or nickel wire that provides a definite resistance

value at each temperature within its range. The relationship

between temperature and resistance of conductors in

temperature range near 00 C can be calculated from the

equation:-

Rt = Rref ( 1 + Δt )

Where Rt= resistance of conductor at temperature t0C

Rref =resistance at reference temperature 00C

= temperature coefficient of resistance

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Δt = difference between operating and reference

temperature.

Almost all conductors have positive temperature

coefficient of resistance, that is, resistance increases

with an increase in temperature. But some materials such as

carbon and germanium have negative temperature coefficient

of resistance that signifies that the resistance decreases

with an increase in temperature.

Table 3.1 Different MetalsType Temperature

Range

Accuracy Advantages Disadvantages

Platinu

m

-30000F to

+150000F

+/-

0.10F

Low cost,

high

stability

Wide

operating

range

Relatively

slow response

time (15 sec),

not as linear

as copper

thermometersCopper -32500 F to

+25000 F

+/-

0.500F

High

linearity,

high

accuracy

High

stability

Limited

temperature

range to 25000F

Nickel -3200F to +/- Longer life, More non

10

+15000F 0.500F high

sensitivity,

high

temperature

coefficient.

linear than

copper,

limited

temperature

range to 15000F

3.6 Thermocouple In 1871, Thomas Seeback discovered that when two

dissimilar metals comes in contact, a voltage is generated

where the voltage is a function of the temperature. The

device consisting of two dissimilar metals are joined

together is known as thermocouple and the voltage generated

is known as Seeback voltage, on the name of the discoverer.

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Magnitude of this voltage depends on

i) Materials used for the wires

ii) Amount of temperature difference between the joined ends

For example, joining copper and constantan produces a

voltage of the order of a few milivolts with the positive

potential at the copper side. An increase in temperature

causes an increase in the voltage.

There are several methods of joining two dissimilar

metal:-

One is to weld the wires together, but this process

causes a brittle joints and may fracture on application

of stresses.

Another method is by soldering the two wires together,

which suffers from the disadvantage that a third

dissimilar metal is introduced.

3.7 Photoelectric Transducers These transducers use the properties of photo emissive

cells or phototube. The vacuum photoelectric cell consist of

a curved sheet of thin metal with its concave surface coated

with photo-emissive material, which forms the cathode and

the rod anode mounted at the center of curvature of the

cathode. The whole assembly ids mounted on an evacuated

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glass envelope. The material, coated cathode, emits

electrons when light radiation strikes them. The emitted

electrons from the cathode are collected by the positive

electrode (anode) forming an electric current.

Chapter-4

PRESSURE MEASUREMENT The pressure or force measurement can be done by

converting the applied force or pressure into a displacement

by elastic elements which acts as a primary transducer. Now

this displacement of the primary transducer, which is the

function of the force, can be measured by a secondary

transducer. Mechanical devices that are used to convert

applied pressure or force to displacement are called force

summing devices. Some of them are:

Flat or corrugated diaphragms

Pivot torque

Straight tube

Bellows

Circular or twisted Bourdon tube.

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Secondary transducers which are used to convert the

displacement into change of some electrical parameter are:-

Resistive transducers

Inductive transducers

Capacitive transducers

Differential transformers or LVDT

4.1 Piezoelectric Transducers

When piezoelectric crystals are under the influence of some

external force or pressure, they produce an emf. The force

or displacement or pressure to be measured is applied to the

crystal. The pressure is applied to the crystal through a

force summing device. This cause a deformation which

produces an emf which is a function of the deformation. This

output emf may be measured to know the value of applied

force and hence pressure.

4.2 Ionization TransducersIonization is the process of removing an electron from an

atom producing a free electron and a positively charged ion.

Ionization may be produced by the collision of high speed

electron from the atom.

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Electrons are emitted from heated cathode using a

filament and are accelerated towards the grid, which is

positively charged. Some of the electrons are captured by

the grid producing grid current. Electrons having high

kinetic energy pass through and cause ionization of the gas

atoms.

The positive ions so produced are attracted to the

plate, which is at negative potential and a current is

produced in the plate circuit. It is found that the pressure

of gas is proportional to the ratio of plate to grid

current.

4.3 Oscillation TransducersThese type of transducers use force summing device to change

the capacitance, C, or inductance, L of an LC oscillation

circuit. Its output frequency is affected by the change in

the inductance of the coil. The change in the inductance is

caused by the force summing device acting upon an inductive

device. The output of the oscillator is the modulated output

and can be demodulated and calibrated in terms of the

pressure or force applied.

4.4 Measurement of pressure

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Many techniques have been developed for the measurement of

pressure and vacuum. Instruments used to measure pressure are

called pressure gauges or vacuum gauges

A manometer is a pressure measuring instrument, usually limited

to measuring pressures near to atmospheric. It is often used to

refer specifically to liquid column hydrostatic instruments.

A vacuum gauge is used to measure the pressure in a vacuum, which

is broadly divided into two categories: high and low vacuum (and

sometimes ultra-high vacuum). Many of the different techniques

used to measure these categories have an overlap at some point in

the pressure range. By combining several different types of gauge

it is possible measure system pressure from 10 mbar down to 10e-

11 mbar.

The SI unit of pressure is the pascal (abbreviation Pa).

Atmospheric pressures are usually stated using its decimal

multiple kilopascal (kPa), where 1 kPa is close to 1.0% of

Earth's atmospheric pressure at sea level. In meteorologic

reports, hPa or mbar are the commonly used units (being by

definition 1 bar = 100 kPa). In vacuum systems, the equivalent

units torr and millimeter of mercury (mmHg) are also used, with 1

torr equaling 133.3223684 Pa above an ideal vacuum.

Other vacuum units occasionally encountered in the literature

include micrometers of mercury, the barometric scale, or as a

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percentage of atmospheric pressure in bars or atmospheres. Low

vacuum is measured in the United States also in inches of mercury

(inHg) below atmospheric pressure. "Below atmospheric" means that

the absolute pressure is equal to the atmospheric pressure (29.92

in Hg) minus the vacuum pressure in inches of mercury. (This is

effectively a gauge pressure.)Thus a vacuum of 26 in Hg is

equivalent to an absolute pressure of 29.92 in Hg − 26 in Hg =

3.92 in Hg.

4.4.1 Bourden Tube Type Pressure GuageA Bourdon gauge uses a coiled tube which as it expands due to

pressure increase causes a rotation of an arm connected to the

tube.

A combination pressure and vacuum gauge (case and viewing

glass removed)

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Fig.4.1 Indicator Side with

card and dial

Fig. 4.2 Mechanical Side with

Bourdon tube

The pressure sensing element is a closed coiled tube connected to

the chamber or pipe in which pressure is to be sensed. As the

gauge pressure increases the tube will tend to uncoil, while a

reduced gauge pressure will cause the tube to coil more tightly.

This motion is transferred through a linkage to a gear train

connected to an indicating needle. The needle is presented in

front of a card face inscribed with the pressure indications

associated with particular needle deflections. In a barometer,

the Bourdon tube is sealed at both ends and the absolute pressure

of the ambient atmosphere is sensed. Differential Bourdon gauges

use two Bourdon tubes and a mechanical linkage that compares the

readings.

In the following pictures the transparent cover face has been

removed and the mechanism removed from the case. This particular

gauge is a combination vacuum and pressure gauge used for

automotive diagnosis:

The left side of the face, used for measuring manifold

vacuum, is calibrated in centimetres of mercury on its inner

scale and inches of mercury on its outer scale.

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The right portion of the face is used to measure fuel pump

pressure and is calibrated in fractions of 1 kgf/cm² on its

inner scale and pounds per square inch on its outer scale.

Mechanical details

Fig. 4.3 Mechanical Details

Stationary parts:

A: Receiver block. This joins the inlet pipe to the fixed

end of the Bourdon tube (1) and secures the chassis plate

(B). The two holes receive screws that secure the case.

B: Chassis Plate. The face card is attached to this. It

contains bearing holes for the axles.

C: Secondary Chassis Plate. It supports the outer ends of

the axles.

D: Posts to join and space the two chassis plates.

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Moving Parts:

Stationary end of Bourdon tube. This communicates with the

inlet pipe through the receiver block.

Moving end of Bourdon tube. This end is sealed.

Pivot and pivot pin.

Link joining pivot pin to lever (5) with pins to allow joint

rotation.

Lever. This an extension of the sector gear (7).

Sector gear axle pin.

Sector gear.

Indicator needle axle. This has a spur gear that engages the

sector gear (7) and extends through the face to drive the

indicator needle. Due to the short distance between the

lever arm link boss and the pivot pin and the difference

between the effective radius of the sector gear and that of

the spur gear, any motion of the Bourdon tube is greatly

amplified. A small motion of the tube results in a large

motion of the indicator needle.

Hair spring to preload the gear train to eliminate gear lash

and hysteresis.

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4.4.2 Diaphragm Type Pressure Gauge

Fig. 4.4 Diaphragm Type Pressure Gauge

A pile of pressure capsules with corrugated diaphragms in an

aneroid barograph.

A second type of aneroid gauge uses the deflection of a flexible

membrane that separates regions of different pressure. The amount

of deflection is repeatable for known pressures so the pressure

can be determined using by calibration. The deformation of a thin

diaphragm is dependent on the difference in pressure between its

two faces. The reference face can be open to atmosphere to

measure gauge pressure, open to a second port to measure

differential pressure, or can be sealed against a vacuum or other

fixed reference pressure to measure absolute pressure. The

deformation can be measured using mechanical, optical or

capacitive techniques. Ceramic and metallic diaphragms are used.

Useful range: above 10-2 torr [5] (roughly 1 Pa)

21

For absolute measurements, welded pressure capsules with

diaphragms on either side are often used.

Shape:

Flat

corrugated

flattened tube

capsule

4.5 Pressure Transmitter4.5.1 Operation of Pressure Transmitter:

4.5.1.1 P to F Conversion:The process fluid applied to capacitive pressure

sensor change the value of the sensor’s Cs

capacitor, thereby generating a sense frequency

Fs by the Enhanced Mode Oscillator (EMO) that is

directly proportional to the applied pressure.

4.5.1.2 F to Digital Conversion:The first of the three frequencies (Fr, Fs+r, Fs) generated by

the BMO is applied to the ASIC. When all three frequencies are

store, the micro-controller shifts the data into its serial port.

The micro-controller uses a specially developed algorithm that

cancels the effects of parasitic capacitance Cp and calculates

the true ratio Cr/Cs. When the ratio is equal to 1 the pressure

22

Fig. 4.5Pressure

Transmitter

diff between the two capacitor is known to be zero. A ratio less

than I corresponds to a +ve pressure diff. And a ratio greater

than a –ve pressure diff.

4.5.1.3 D/A Conversion and Current Signal Transmission:The pressure signal received by the micro-controller is applied

through the ASIC to a DAC. The DAC translates the digitalized

pressure signal into a PWM signal, whose PW is proportional to

the magnitude of the process pressure. The pulse are filtered and

applied to an operational amplifier, which drives V4 converter,

whose output is Darlington transistor pair acting as a pass

transistor that outputs a std. 4 to 20mA current signal to the

n/w.

A transmitter can be configured to operate in either an analog or

a digital mode, for a point-to-point or multi drops n/w res.

(A) Analog mode:A single transmitter is connected to a controller recorder or

other field device, a loop known as point to point n/w

interconnects the instruments. The transmitter `s output is the

process variable and it is sent to a controller or recorder using

astd.4-20mA analog currnt. The HART protocol is used to send all

process variable information to a HART compatible controller,

recorder or other device.

(B) Digital Mode:

23

Twisted pair cable is used to wire up to 15 parallel

connected transmitter to the multidrop ii/w. HART protocol is

used to send all process variable information to a HART

compatible controller, recorder or other device.

4.6 PRESSURE SWITCH

The device is used to control any associated parameter during the

rise or full of pressure above or below a required level. A

pressure switch turns and electric circuit “on” or at a preset

pressure. This pressure is called the “set point “of the switch.

Construction wise a bottom tube, diaphragm or bellows acts as the

sensing elements which actuates the switch, which may be a micro

switch or mercury switch. As shown in fig. There is a bellow

which receives the pressure to be measured which is in contact

with a flapper balancing spring whose tension could be adjusted

24

Fig. 4.6 PressureSwitch

with the help of range screw and its indication by a pointer on a

scale calibrated in terms of pressure. Thus, the force due to

bellows and spring tension acts against each other on the other

side of flapper there is a micro switch.

When the process pressure increase it expands the bellows which

moves the flapper end in the direction shown. When the force due

to bellows exceeds that due to the spring at the same value, the

other side of the flapper comes in contact with the micro switch

as a result of which activating the “no” or “nc” contact. The

change over to the contacts could be used to actuate a relay

which controls a final control element like a control value of

pressure switch has a “dead band” which is the range of value of

pressure during which the changing of the contacts does not take

place.

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Chapter-5

MEASUREMENT OF TEMPERATURE

There are many instruments are used for measure ment of

temperature some of them are below.

5.1 Liquid in Metal Type Temp. Guage

This is most often used

temperature gauge. In this type liquid is filled in the metal

tube. Most generally used liquids are mercury and alcohol.

5.1.1 Operation Principle:

26

Fig. 5.1 Liquid In Metal TypeTemp. Gauge

As above, the liquid filled in tube if there is any change in

tempareture occurred then the level of liquid will be increase or

decrease.

The tube connected with liquid filled bourden tube. So, as

pressure increase or decrease there will be change in the bourden

tube. As the gauge pressure increases the tube will tend to

uncoil, while a reduced gauge pressure will cause the tube to

coil more tightly. This motion is transferred through a linkage

to a gear train connected to an indicating needle. The needle is

presented in front of a card face inscribed with the pressure

indications associated with particular needle deflections.

The deflection of tube connected with pointer by link. The

movement of pointer will indicate the value of temperature. This

is easy to install, this kind of gauge has lower cost.

5.2 Thermocouple

In electronics, thermocouples are a widely used type of

temperature sensor and can also be used as a means to convert

thermal potential difference into electric potential difference.

They are cheap and interchangeable, have standard connectors, and

can measure a wide range of temperatures. The main limitation is

precision; system errors of less than 1 °C can be difficult to

achieve.

27

5.2.1 Principle of Operation

When any conductor (such as a metal) is subjected to a thermal

gradient, it will generate a voltage. This is now known as the

thermoelectric effect or Seebeck effect. Any attempt to measure

this voltage necessarily involves connecting another conductor to

the "hot" end. This additional conductor will then also

experience the temperature gradient, and develop a voltage of its

own which will oppose the original. Fortunately, the magnitude of

the effect depends on the metal in use. Using a dissimilar metal

to complete the circuit will have a different voltage generated,

leaving a small difference

voltage available for

measurement, which

increases with

temperature. This

difference can typically

be between 1 and about 70 microvolts per degree Celsius for the

modern range of available metal combinations. Certain

combinations have become popular as industry standards, driven by

cost, availability, convenience, melting point, chemical

properties, stability, and output is important to note that

thermocouples measure the temperature difference between two

points, not absolute temperature. In traditional applications,

one of the junctions — the cold junction — was maintained at a known

28

Fig. 5.2 Basic Diagram

(reference) temperature, while the other end was attached to a

probe. For example, in the image above, the cold junction will be

at copper traces on the circuit board. Another temperature sensor

will measure the temperature at this point, so that the

temperature at the probe tip can be calculated.

Thermocouples can be connected in series with each other to form

a thermopile, where all the hot junctions are exposed to the

higher temperature and all the cold junctions to a lower

temperature. Thus, the voltages of the individual thermocouple

add up, which allows for a larger voltage.

Having available a known temperature cold junction, while useful

for laboratory calibrations, is simply not convenient for most

directly connected indicating and control instruments. They

incorporate into their circuits an artificial cold junction using

some other thermally sensitive device (such as a thermistor or

diode) to measure the temperature of the input connections at the

instrument, with special care being taken to minimize any

temperature gradient between terminals. Hence, the voltage from a

known cold junction can be simulated, and the appropriate

correction applied. This is known as cold junction compensation.

Additionally, cold junction compensation can be performed by software.

Device voltages can be translated into temperatures by two

29

Fig. 5.3 Thermocouple

methods. Values can either be found in look-up tables or

approximated using polynomial coefficients.

Usually the thermocouple is attached to the indicating device by

a special wire known as the compensating or extension cable. The

terms are specific. Extension cable uses wires of nominally the same

conductors as used at the thermocouple itself. These cables are

less costly than thermocouple wire, although not cheap, and are

usually produced in a convenient form for carrying over long

distances - typically as flexible insulated wiring or multicore

cables. They are usually specified for accuracy over a more

restricted temperature range than the thermocouple wires. They

are recommended for best accuracy.

Compensating cables on the other hand, are less precise, but

cheaper. They use quite different, relatively low cost alloy

conductor materials whose net thermoelectric coefficients are

similar to those of the thermocouple in question (over a limited

range of temperatures), but which do not match them quite as

faithfully as extension cables. The combination develops similar

outputs to those of the thermocouple, but the operating

temperature range of the compensating cable is restricted to keep

the mis-match errors acceptably small.Thermocouples are most

suitable for measuring over a large temperature range, up to 1800

K. They are less suitable for applications where smaller

temperature differences need to be measured with high accuracy,

30

for example the range 0–100 °C with 0.1 °C accuracy. For such

applications, thermistors and RTDs are more suitable.

5.3 Resistance Temperature Detectors

Resistance thermometers, also called resistance temperature

detectors (RTDs), are temperature sensors that exploit the

predictable change in electrical resistance of some materials

with changing temperature. As they are almost invariably made of

platinum, they are often called platinum resistance thermometers

(PRTs). They are slowly replacing the use of thermocouples in

many industrial applications below 600 °C.

5.3.1 General Description

There are two broad categories, "film" and "wire-wound" types.

Film thermometers have a layer of platinum on a substrate; the

layer may be extremely thin, perhaps 1 micrometer. Advantages of

this type are relatively low cost and fast response. Such devices

have improved in performance although the different expansion

rates of the substrate and platinum give "strain gauge" effects

and stability problems.

Wire-wound thermometers can have greater accuracy, especially for

wide temperature ranges. The coil diameter provides a compromise

between mechanical stability and allowing expansion of the wire

to minimize strain and consequential drift.

31

The current international standard which specifies tolerance and

the temperature to electrical resistance relationship for

platinum resistance thermometers is IEC 751:1983. By far the most

common devices used in industry have a nominal resistance of 100

ohms at 0 °C, and are called Pt-100 sensors ('Pt' is the symbol

for platinum). The sensitivity of a standard 100 ohm sensor is a

nominal 0.385 ohm/°C. RTDs with a sensitivity of 0.375 and 0.392

ohm/°C are also available.

5.4 How do resistance thermometers work?

Resistance thermometers are constructed in a number of forms and

offer greater stability, accuracy and repeatability in some cases

than thermocouples. While thermocouples use the Seebeck effect to

generate a voltage, resistance thermometers use electrical

resistance and require a small power source to operate. The

resistance ideally varies linearly with temperature.

Resistance thermometers are usually made using platinum, because

of its linear resistance-temperature relationship and its

chemical inertness. The platinum detecting wire needs to be kept

free of contamination to remain stable. A platinum wire or film

is supported on a former in such a way that it gets minimal

differential expansion or other strains from its former, yet is

reasonably resistant to vibration.

32

Commercial platinum grades are produced which exhibit a change of

resistance of 0.385 ohms/°C (European Fundamental Interval) The

sensor is usually made to have a resistance of 100Ω at 0 °C. This

is defined in BS EN 60751:1996 . The American Fundamental

Interval is 0.392 Ω/°C.

Resistance thermometers require a small current to be passed

through in order to determine the resistance. This can cause

resistive heating, and manufacturers' limits should always be

followed along with heat path considerations in design. Care

should also be taken to avoid any strains on the resistance

thermometer in its application. Lead wire resistance should be

considered, and adopting three and four wire connections can

eliminate connection lead resistance effects from measurements.

Advantages of platinum resistance thermometers:

High accuracy

Low drift

Wide operating range

Suitability for precision applications

5.4.1 Resistance thermometer elements

Resistance thermometer elements are available in a number of

forms. The most common are:

33

Wire wound in a ceramic insulator - wire spiral within sealed

ceramic cylinder, works with temperatures to 850 °C

Wire encapsulated in glass - wire around glass core with glass

fused homogenously around, resists vibration, more protection to

the detecting wire but smaller usable range

Thin film - platinum film on ceramic substrate, small and

inexpensive to mass produce, fast response to temperature change

5.4.2 Resistance thermometer construction

These elements nearly

always require insulated leads attached. At low temperatures PVC,

silicon rubber or PTFE insulators are common to 250 °C. Above

this, glass fibre or ceramic are used. The measuring point and

usually most of the leads require a housing or protection sleeve.

This is often a metal alloy which is inert to a particular

process. Often more consideration goes in to selecting and

designing protection sheaths than sensors as this is the layer

34

Fig. 5.4 Resistance Thermometer Construction

that must withstand chemical or physical attack and offer

convenient process attachment points.

Chapter-6

TEMPERATURE SWITCHESTemperature switches generally operates on one of four

principles:

1. Thermal Expansion

2. Differential Thermal Expansion

3. Vapor Pressure

35

4. Thermocouple

The thermal expansion device can be mercury filled bulb

somewhat like an ordinary laboratory glass thermometer.

In the industrial temperature switch, this would normally be

mercury filled or gas filled system, consisting of a copper or

stainless steel bulb, connected to a pressure switch through a

small capillary tube. Expansion of the mercury or gas with an

increase in temperature would increase the filled system pressure

that then actuates the switch. The vapor pressure switch is a

filled system. Having the same configuration as the mercury

thermal expansion Type.

Thermal expansion and vapor pressure temperature switch cover

a wide range of temperature and provide sample force to operate a

dependable switch mechanism.

Bimetallic temperature switches are made in many

configuration. They operate on differential thermal expansion

between two metal parts that are thermally coupled to the process

stream. The motion of the differential expansion is transferred

to the switch through and adjustable mechanism. Bimetallic

switches are generally small, low cost and useful over a wide

range of temperature. Thermocouple can be used as a temperature

measurement element because they generate a minute voltage that

is a function of the temperature these sensores good for a wide

range of temperature are very rugged and inexpensive. The

measurement voltage for a thermocouple is the difference between

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the voltage at the point of measurement and the referance

junction voltage at some other point in the circuit.

Chapter-7

FLOW MEASUREMENT

Orifice, ventures, pilot tubes, annular and nozzle are the

main primary elements. They are used to create a restriction in

the flow and create a differential pressure across them which are

sent by the secondary elements. So construction and design of

these elements are of critical importance. Orifice plates are

used frequently than other types. They are actually obstruction

within the pipe and made from stainless steel 1/8” to ½” thick.

7.1 Orifice Plate

An orifice plate is a device which measures the rate of fluid

flow. It uses the same principle as a Ventura nozzle, namely

Bernoulli's principle which says that there is a relationship

between the pressure of the fluid and the velocity of the fluid.

When the velocity increases, the pressure decreases and vice

versa.

37

An orifice plate is basically a thin plate with a hole in the

middle. It is usually placed in a pipe in which fluid flows. As

fluid flows through the pipe, it has a certain velocity and a

certain pressure. When the fluid reaches the orifice plate, with

the hole in the middle, the fluid is forced to converge to go

through the small hole; the point of maximum convergence actually

occurs shortly downstream of the physical orifice, at the so-

called vena contracta point (see drawing to the right). As it

does so, the velocity and the pressure changes. Beyond the vena

contracta, the fluid expands and the velocity and pressure change

once again. By measuring the difference in fluid pressure between

the normal pipe section and at the vena contracta, the volumetric

and mass flow rates can be obtained from Bernoulli's equation.

7.2 Venturi Tubes:

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Fig. 7.1 Orifice Plate

The venturi tube is highly accurate instrument for measuring

pressure differential. It is used for medium and high quantity

fluid flow.

The short venturi is particularly adopted to install in

pipelines, not having long Unobstructed runs. The flow of fluid

through the venture tube, established the pressure differential,

which can then be measured and related to the flow rate.

It consist of two small hollow pipes made in the form of cone at

the one end. The cones are joined by short parallel pipe. The

smallest diameter is called the throat section. Where lower or

downstream pressure is measured. In the parallel pipe to the

upstream, the upstream cone made steeper, to give smooth approach

to the throat section. The downstream cone recovers part of the

pressure differential, thus reducing overall pressure loss. Fluid

speed at throat is faster. It is made of cast iron or steel.

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Fig. 7.2 Venturi Tubes

Its initial cost is high but pressure loss is lower than orifice

and less wear and abrasion.

7.3 Rotameter

A Rotameter is a device that measures the flow rate of liquid or

gas in a closed tube. It is occasionally misspelled as

'rotometer'.

It belongs to a class of meters called variable area meters,

which measure flow rate by allowing the cross sectional area the

fluid travels through to vary, causing some measurable effect.

A rotameter consists of a tapered tube, typically made of glass,

with a float inside that is pushed up by flow and pulled down by

gravity. At a higher flow rate more area (between the float and

the tube) is needed to accommodate the flow, so the float rises.

Floats are made in many different shapes, with spheres and

spherical ellipses being the most common.

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The float is shaped so that it rotates axially as the fluid

passes. This allows you to tell if the float is stuck since it

will only rotate if it is not. Readings are usually taken from

the top of the float. Some manufacturers may use a different

standard, so it is always best to check the documentation

provided with the device.

Note that the 'float' does not actually float in the fluid: it

has to have a higher density than the fluid otherwise it will

float to the top even if there is no flow.

7.3.1 Advantages:

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Fig. 7.3 Rotameter

A rotameter requires no external power or fuel, it uses only

the inherent properties of the fluid, along with gravity, to

measure flow rate.

A rotameter is also a relatively simple device that can be

mass manufactured out of cheap materials, allowing for

widespread use in places such as third world countries.

7.3.2 Disadvantages:

Due to its use of gravity, a rotameter must always be

vertically oriented and right way up, with the fluid flowing

upwards.

Due to its reliance on the ability of the fluid or gas to

displace the float, the graduations on a given rotameter

will only be accurate for a given substance. The main

property of importance is the density of the fluid, however

viscosity may also be significant. Floats are ideally

designed to be insensitive to viscosity, however this is

seldom verifiable from manufacturer’s specs. Either separate

rotameters for different densities and viscosities may be

used, or multiple scales on the same rotameter can be

utilized.

Rotameters normally require the use of glass (or other

transparent material), otherwise the user cannot see the

float. This limits their use in many industries to benign

fluids, such as water.

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Rotameters are not easily adapted for reading by machine:

although magnetic floats that drive a follower outside the

tube are available.

Chapter-8

MEASUREMENT OF LEVEL

8.1 Sight Glass

8.1.1 GeneralA sight glass (gauge glass) is another method of liquid level

measurement.it is used for the continuous indication of liquid

level with a tank or vessel.

8.1.2 Construction and Working

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A sight glass instrument consist of a graduated tube of toughened

glass which is connected to the interior of a graduated tube of

bottom in which the water level is required figure shows a simple

sight matches the level of liquid in the tank. As the level of

liquid in the tank rises and falls, the level in the sight glass

the level of liquid in the tank is measured. In sight glass, it

is not necessary, to use the same liquid as in the tank. Any

other desired liquid also can be used.

Then it is desired to measure a liquid with the liquid under

pressure or vacuum, the sight glass must be connected to the tank

at the top as well at the bottom otherwise, the pressure

difference between the tank and the sight glass would cause false

reading. In this case, the glass tube is enclosed in a protective

housing, and two valves are provided for isolating the gauge from

the tank in case of breakage of the glass. The smaller valve at

the bottom is provided for blowing out the gauge for cleaning

purposes fig. Shows a high pressure sight glass. In which

measurement is made by reading the position of the level on the

calibrated scale. These safety precautions. The glass tube muses

have a small inside diameter and a thick wall

44

Fig. 8.1 Slight Glass

8.2 Differential Pressure Type Level Meter

The level also measure by measuring the upper and lower level

pressure of liquid. The

differential pressure will show

the level.

8.2.1 P to F Conversion:

The process fluid applied to capacitive pressure sensor change

the value of the sensor’s Cs capacitor, thereby generating a

sense frequency Fs by the Enhanced Mode Oscillator (EMO) that is

directly proportional to the applied pressure.

8.2.2 F to Digital Conversion:The first of the three frequencies (Fr, Fs+r, Fs) generated by the

BMO is applied to the ASIC. When all three frequencies are store,

the micro-controller shifts the data into its serial port. The

micro-controller uses a specially developed algorithm that

cancels the effects of parasitic capacitance Cp and calculates

the true ratio Cr/Cs. When the ratio is equal to 1 the pressure

45

Fig. 8.2 DifferentialPressure Type Level Meter

diff between the two capacitor is known to be zero. A ratio less

than I corresponds to a +ve pressure diff. And a ratio greater

than a –ve pressure diff.

8.2.3 D/A Conversion and Current Signal Transmission:The pressure signal received by the micro-controller is applied

through the ASIC to a DAC. The DAC translates the digitalized

pressure signal into a PWM signal, whose PW is proportional to

the magnitude of the process pressure. The pulse are filtered and

applied to an operational amplifier, which drives V4 converter,

whose output is Darlington transistor pair acting as a pass

transistor that outputs a std. 4 to 20mA current signal to the

n/w.

8.3 Level Switch

8.3.1 Operating Principle:Diagram illustrated the simple and fool proof magnetron operating

principle switching action is obtained, that the use of a

magnetic attractive sleeve, actuated by a float and switching

mechanism are separated by a non magnetic, pressure fifth

enclosing tube. Switching magnets are assembled to a switching

arm which operates on precision 35-pivot socket.

46

8.3.2 Operation Cycle:At “normal level” operating of liquid in the chamber the float

moves the magnetic attraction sleeve upward in the enclosed tube

and into the field of the switch mechanism magnet. As a result

the magnet is drawn in tightly to the enclosing tube causing the

switch to tilt,” making” on “breaking” and electric circuits. A

liquid level reduces the float pulls the magnetic attracting

sleeve downward until, at predetermined “low level “ the switch

magnet release and is drawn outward away from the enclosing tube

by a tension spring. This in turn tilts the switch in an opposite

direction, thus reversing switch action. When liquid level

returns to normal the float once again moves the magnetic sleeve

up the enclosing tube, causing the switch to assume its original

position.

8.4 Control Valves

Control valves are valves used within industrial plants and

elsewhere to control operating conditions such as temperature,

47

Fig. 8.3 Level Switch

pressure, flow, and liquid level by fully or partially opening or

closing in response to signals received from controllers that

compare a "set point" to a "process variable" whose value is

provided by sensors that monitor changes in such conditions. The

opening or closing of control valves is done by means of

electrical, hydraulic or pneumatic systems.

8.5 Globe ValveGlobe valve is a device for regulating flow in a pipeline,

consisting of a movable disk-type element and a stationary ring

seat in a generally spherical body.

Globe Valves are named for their spherical body shape with the

two halves of the body being separated by an

internal baffle. This has an opening that forms

a seat onto which a movable plug can be screwed in

to close (or shut) the valve. In

globe valves, the plug is connected to a

stem which is operated by screw action

in manual valves. Typically,

automated valves use sliding

stems. Globe valves have a smooth stem rather than threaded and

are opened and closed by an actuator assembly. When a globe valve

is manually operated, the stem is turned by a hand wheel.

48

Fig. 8.4 Globe Valve

Although globe valves in the past had the spherical bodies which

gave them their name, many modern globe valves do not have much

of a spherical shape. However, the term globe valve is still often

used for valves that have such an internal mechanism. In

plumbing, valves with such a mechanism are also often called stop

valves since they don't have the global appearance, but the term

stop valve may refer to valves which are used to stop flow even

when they have other mechanisms or designs.

Globe valves are used for applications requiring throttling and

frequent operation. For example, globe valves or valves with a

similar mechanism may be used as sampling valves, which are

normally shut except when liquid samples are being taken. Since

the baffle restricts flow, they're not recommended where full,

unobstructed flow is required.

Globe valves are typically two-port valves, although three port

valves are also produced. Ports are openings in the body for fluid

flowing in or out. The two ports may be oriented straight across

from each other on the body, or oriented at an angle such as a

90° angle. Globe valves with ports at such an angle are called

angle globe valves.

8.6 Butterfly ValveA butterfly valve is a type of flow control

device, typically used to regulate a fluid

49

flowing through a section of pipe. The valve is similar in

operation to a ball valve. A flat circular plate is positioned in

the center of the pipe. The plate has a rod through it connected

to a handle on the outside of the valve. Rotating the handle

turns the plate either parallel or perpendicular to the flow.

Unlike a ball valve, the plate is always present within the flow,

therefore a pressure drop is always induced in the flow

regardless of valve position.

There are three types of butterfly valve:

1. Resilient butterfly valve which has a flexible rubber seat.

Working pressure up to 1.6 megapascals (MPa)/232 pounds per

square inch (PSI)

2. High performance butterfly valve which is usually double

eccentric in design. Working pressure up to 5.0 MPa/725 PSI

3. Trimetric butterfly valve which is

usually with metal seated design.

Working pressure up to 10.0 MPa/1450 PSI

Butterfly valves are widely used in water distribution and waste

water processing (not recommended, as the debris may block the

operation of the disc). Butterfly valves can come in two body

types, affecting installation and maintenance: lugged or wafer.

Wafer style valves are more common. They are typically installed

between two flanges using bolts or studs and nuts. Lug style

50

Fig. 8.5Butterfly Valve

valves are also installed between two flanges but with a separate

set of bolts for each flange. The lug style setup makes it

possible to remove one side of the piping while the other remains

intact.

An additional application is found within the exhaust system of

automobiles. By incorporating a butterfly valve in the exhaust

system, it is possible to control the backpressure and noise

output from the muffler and catalytic converter. When in a closed

position, the valve increamount of back pressure produced and

suppresses noise. The angle of valve can be controlled in a

variety of ways, including manual control, vacuum control, as

well as being tired directly to the throttle.

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