ACKNOWLEDGEMENT

I owe a great many thanks to a great many people who helped and supported me

during my training Process.

I would like to express a deep sense of gratitude and thanks profusely to Mr.

SAMEER SHARMA , CDE(PE-C&I) of the NTPC NOIDA, without the wise

counsel and able guidance, it would have been impossible to complete the report in

this manner.

I am very grateful to the Maharishi Markandeshwar university, Engineering

College for providing this opportunity to carry out the six weeks industrial

training at National Thermal Power Corporation, Noida.

My deepest thanks to Mr. NIKHIL PRABHAKAR, the guide of the project for

guiding and correcting various documents of mine with attention and care. He has

taken pain to go through the project and make necessary correction as and when

needed.

I would like to thank my HOD and my class MENTOR (NIDHI MAM) and

other faculty of Department of Electronics & Communication Engineering of

MMEC for their intellectual support throughout the course of this work

Lastly, I offer my regards and blessings to all of those who supported me in any

respect during the completion of the project.

11082023 MMU Page 1

INDEX

TOPIC PAGE NO.

1.INTRODUCTION TO CONTROL AND INSTRUMENTATION 8

2. POWER STATION INSTRUMENT 9

2.1 INDICATOR

2.2 RECODER

3. SELECTION OF INSTRUMENT 10

4. INSTRUMENTATION SYSTEM 12

5. TRANSDUCER 13

5.1 DEFINITION

5.2 EFFICIENCY

5.3 SELECTING FACTORS OF TRANSDUCERS

5.4 REQUIREMENT OF GOOD TRANSDUCER

6. CLASSIFICATION OF TRANSDUCER 15

6.1 BASED ON PHYSICAL PHENOMENON 16

6.1.1 PRIMARY AND SECONDARY TRANSDUCERS

6.2 BASED ON THE POWER TYPE 18

6.2.1 PASSIVE TRANSDUCERS

6.2.2 ACTIVE TRANSDUCERS

6.3 BASED ON THE TYPES OF OUTPUT 21

6.3.1 ANALOG TRANSDUCERS

6.3.2 DIGITAL TRANSDUCERS

11082023 MMU Page 2

6.4 BASED ON ELECTRICAL PHENOMENON 23

6.4.1 RESISTIVE TRANSDUCERS

6.4.2 CAPACITIVE TRANSDUCERS

6.4.3 INDUCTIVE TRANSDUCERS

6.4.4 PIEZOELECTRIC TRANSDUCERS

6.4.5 PHOTOVOLTAIC TRANSDUCERS

6.5 BASED ON TRANSDUCTION PHENOMENON 31

6.5.1 TRANSDUCERS AND INVERSE TRANSDUCERS

7. PRESSURE 32

8. METHODS OF PRESSURE MEASUREMENT 33

8.1 HYDROSTATIC METHOD

8.1.1 PISTON

8.1.2 LIQUID COLUMN

8.2 ELECTRONIC PRESSURE SENSOR 35

8.2.1 PIEZORESISTIVE STRAIN GAUGE

8.3 MAGNETIC METHOD 36

8.3.1 OPTICAL

8.3.2 POTENTIOMETERIC

8.4 THERMAL CONDUCTIVITY METHOD 38

8.4.1 PIRANI GAUGE RANGE

8.4.2 ION GAUGE

8.4.2.1 ION GAUGE CONNECTION LEADS

8.5 WIDE RANGE PRESSURE MEASUREMENT 44

11082023 MMU Page 3

INTRODUCTION TO COMPANY

NTPC, India's largest power company, was set up in 1975 to accelerate power

development in India. It is emerging as an ‘Integrated Power Major’, with a

significant presence in the entire value chain of power generation business.

Fig. 1

NTPC ranked 317th in the ‘2009, Forbes Global 2000’ ranking of the World’s

biggest companies. With a current generating capacity of 32,194 MW, NTPC has

embarked on plans to become a 75,000 MW company by 2017.

11082023 MMU Page 4

VISION

A world class integrated power major, powering India's growth with

increasing global presence .

MISSION

Develop and provide reliable power, related products and services at

competitive prices, integrating multiple energy sources with innovative and

eco-friendly technologies and contribute to society.

Fig. 2

11082023 MMU Page 5

CORE VALUES – BCOMIT

Business Ethics

Customer Focus

Organisational & Professional Pride

Mutual Respect & Trust

Innovation & Speed

Total Quality for Excellence

EVOLUTION

1975

NTPC was set up in 1975 with 100% ownership by the Government of India. In

the last 30 years, NTPC has grown into the largest power utility in India

1997

In 1997, Government of India granted NTPC status to the Board of "Navratna"

being one of the nine jewels of India, enhancing the powers of Directors .

2004

NTPC became a listed company with majority Government ownership of 89.5%.

NTPC becomes third largest by Market Capitalization of listed companies.

11082023 MMU Page 6

2005

The company rechristened as NTPC Limited in line with its changing business portfolio and transforms itself from a thermal power utility to an integrated power utility.

2008

National Thermal Power Corporation is the largest power generation

company in India. Forbes Global 2000 for 2008 ranked it 411th in the world .

2009

NTPC- received the international Gold Star Award for Quality

on 26th October, 2009 at International Quality Convention Geneva

2009 in recognition of outstanding commitment to Quality

contributing towards the success for India in the business world .

2010

India"s Largest power company,NTPC has been conferred with the conveted

Maharatna status by the government of India for our outstanding

achievements over the years.

Ranked as the No.1 Independent Power Producer in Asia and one of the top 2

in the world as per the prestigious Platt"s Top 250 Global Company

Rankings, NTPC continues to serve the nation by providing quality power.

.

11082023 MMU Page 7

1. INTRODUCTION TO CONTROL AND INSTRUMENTATION

Control and instrumentation in any process industry,can be compared to the nerve

system in the human being.The way the nerve system controlling the operation of

various limbs of human being.

C and I in the same way controlling and operating various

motors,pumps,dampers,valves etc. and helping us to achieve our targets.

Control and Instrumentation,as the name indicates,is a branch in engineering which

deals with various measurement,indication,transmission and control in different

technical fields.The latest development made in the area of instrumentation are so

wide that it has become humanly impossible to master over all the system

individually.Even in the instrumentation there are further subgroups now.The term

instrument means "A device or combination of devices used directly or indirectly

to measure and display a variable."

Instrumentation is a measurement if various parameters with comparison to set

standards.We have been using for ages different instruments suck as weights,yard

stick,scales,measuring tapes,standard container for liquid measurement e.g Litre,

gallons etc.Each of these equipments is an instrument.Similarly ,in industries and

process plants,Instrumentation makes use of various measuring components

designed to suit the process and the purpose.As some of the big industries and

process plants needs to control different process variable from a remote distance

control room,the further measuring,transmitting indicating,Recording,abnormality

alarm system and innovated.The process of innovation is marching ahead in fast

rate.In the near future,we are certainly to enter in towards more and more

sophistication n C and I stream.

11082023 MMU Page 8

2. POWER STATIONS INSTRUMENTS

TYPES OF INSTRUMENTS

This discussion is only on the process instrumentations measuring the physical

quantities such as temperature, pressure ,level flow etc. The other types of

instruments are the Electrical Instruments measuring electrical quantities such as

current , voltage etc. The different Types of instruments normally in used are given

below.

2.1 INDICATORS

indicators are of two categories local and remote. Local indicators are self

contained and self operative and are mounted on the site . The remote indicators

are used for telemeter purposes and mounted in the centralized control room and

control panel. The indicators both local and remote are sometimes produced with

signaling contacts where ever required .The remote indicators depends on electrical

,electronics ,pneumatic or hydraulic system for their operations .And accordingly

they are named .The indicators are classified as analog and digital on the basis of

final display of the reading.

11082023 MMU Page 9

2.2 RECORDERS

Recorders are necessary wherever the operating history is required for analysing

the trends and for any future case studies or efficiency purposes.Recordes can be of

single point measuring a single parameter or multi-point measuring a number of

parameters by single instrument.

Multi-point recorders are again categorized as multipoint continuous or multipoint

dot recorders .The multipoint dot recorders select the point one after the other in

sequence where as the continuous recorders measure simultaneously all the points.

3. SELECTION OF INSTRUMENTS

Instruments engineer are required to work in close association with the system

design as well as the equipment design engineers in selecting instruments and

sensing system .After deciding the capacity of Thermal Power Station the designs

of boller turbine and auxiliary equipments such as mills,pumps,fans,deaerator,feed

heaters etc. are taken up.

Based on the design of the main and the auxiliary equipments,the parameter values

for efficient and economic operation determined load are specified.The instrument

and system design engineers decide the location for the measurement of various

parameters such as level , pressure , flow , differential pressure , temperature and

other parameters based on the system design and layout conditions.

11082023 MMU Page 10

Then , the instrument engineers select the appropriate instruments influenced by :

(1) Required accuracy of measurement.

(2) Range of measurement.

(3) The form of final data display required.

(4) Press media.

(5) Cost

(6) Calibration and repair facilities required.

(7) Layout restriction.

(8) Maintenance requirement/availability.

11082023 MMU Page 11

4. INSTRUMENTATION SYSTEM

A basic instrument system consists of three elements:

i SENSOR OR INPUT DEVICE

ii SIGNAL PROCESSOR

iii RECEIVER OR OUTPUT DEVICE

Most modern analogue equipment works on the following standard signal ranges.

Electric 4 to 20 mA

Pneumatic 0.2 to 1.0 bar

Older electrical equipment use 0 to 10 V. Increasingly the instruments are digital

with a binary digital encoder built in to give a binary digital output.

Pneumatic signals are commonly used in process industries for safety especially

when there is a risk of fire or explosion.

The advantage of having a standard range or using digital signals is that all

equipment may be purchased ready calibrated.

For analogue systems the minimum signal (Temperature, speed, force, pressure

and so on ) is represented by 4 mA or 0.2 bar and the maximum signal is

represented by 20 mA or 1.0 bar.

11082023 MMU Page 12

5. TRANSDUCER

5.1 DEFINITION :

A transducer is a device that converts one type of energy to another. The

conversion can be to/from electrical, electro-mechanical, electromagnetic,

photonic, photovoltaic, or any other form of energy. While the term transducer

commonly implies use as a sensor/detector, any device which converts energy can

be considered a transducer.

5.2 EFFICIENCY :

Efficiency is an important consideration in any transducer. Transducer efficiency is

defined as the ratio of the power output in the desired form to the total power input.

Mathematically, if P represents the total power input and Q represents the power

output in the desired form, then the efficiency E, as a ratio between 0 and 1, is

given by:

E = Q/P

If E% represents the efficiency as a percentage, then:

E% = 100Q/P

No transducer is 100 percent efficient; some power is always lost in the conversion

process. Usually this loss is manifested in the form of heat. 11082023 MMU Page 13

5.3 Factor to be Considered While Selecting the Transducer

• It should have high input impedance and low output impedance, to

avoid loading effect.

• It should have good resolution over is entire selected range.

• It must be highly sensitive to desired signal and insensitive to unwanted signal.

• Preferably small in size.

• It should be able to work n corrosive environment.

• It should be able to withstand pressure, shocks, vibrations etc..

• It must have high degree of accuracy and repeatability.

• Selected transducer must be free from errors.

5.4 Requirements of Good Transducers :

• Smaller in size and weight.

• High sensitivity.

• Ability to withstand environmental conditions.

• Low cost.

11082023 MMU Page 14

6. Classification of transducers

1. Based on the physical phenomenon,

Primary transducer

Secondary transducer

2. Based on the power type

Active transducer

Passive transducer

3. Based on the type of output,

Analog transducer

Digital transducer

4. Based on the electrical phenomenon,

Resistive transducer

Capacitive transducer

Inductive transducer

Phiezoelectric transducer

Photovoltaic transducer

5. Based on the transduction phenomenon,

Transducer

Inverse transducer 11082023 MMU Page 15

Based on the physical phenomenon

PRIMARY TRANSDUCER & SECONDARY TRANDUCER

Definition:

The first transducer which converts physical phenomenon into displacement,

pressure, velocity etc. which is to be accepted by next stage is known as

“Primary Transducer”.

The output of the primary transducer is converted subsequently into usable

output by a device called “Secondary Transducer”

For eq. LVDT

Fig 3

The linear variable differential transformer (LVDT) is a type of

electrical transformer used for measuring linear displacement. The transformer has

three solenoidal coils placed end-to-end around a tube. The center coil is the 11082023 MMU Page 16

primary, and the two outer coils are the secondaries. A cylindrical ferromagnetic

core, attached to the object whose position is to be measured, slides along the axis

of the tube.

An alternating current is driven through the primary, causing a voltage to be

induced in each secondary proportional to its mutual inductance with the primary.

The frequency is usually in the range 1 to 10 kHz .

As the core moves, these mutual inductances change, causing the voltages induced

in the secondaries to change. The coils are connected in reverse series, so that the

output voltage is the difference (hence "differential") between the two secondary

voltages. When the core is in its central position, equidistant between the two

secondaries, equal but opposite voltages are induced in these two coils, so the

output voltage is zero.

When the core is displaced in one direction, the voltage in one coil increases as the

other decreases, causing the output voltage to increase from zero to a maximum.

This voltage is in phase with the primary voltage. When the core moves in the

other direction, the output voltage also increases from zero to a maximum, but its

phase is opposite to that of the primary. The magnitude of the output voltage is

proportional to the distance moved by the core (up to its limit of travel), which is

why the device is described as "linear". The phase of the voltage indicates the

direction of the displacement.

6.2 BASED ON THE POWER TYPE 11082023 MMU Page 17

6.2.1 PASSIVE TRANSDUSERS :

Definition : Passive Transducers derive the power required for transduction from a

auxiliary power source.

They also derive part of the power required for conversation from the physical

quantity under measurement .

They are also known as “externally powered transducers ” .

Examples : resisitive ,inductive and capactive transducers .

A typical example of a passive transducers is a “POT” which is used for

measurement of displacement . A “POT” is a resistive transducer powered by a

source voltage ei as shown below . This “POT” is used for measurement of linear

displacement xi .

Fig 4

Suppose L is the total length of potietiometer whose total resistance Rt. The input

displacement is Xi .

Output voltage Eo = (Xi/L)Ei

Xi = (Eo/Ei)L

6.2.2 ACTIVE TRANSDUCERS :

11082023 MMU Page 18

Definition : Active Transducers are those which do not require an auxiliary power

source source to produce their output.

They are also known as “self generating type” as they develop their own voltage

or current output.

Velocity ,temperature ,light intensity and force can be transduced with the help of

active transducers .

For example : piezo electric crystal

Fig 4

The property of piezo-electric crystals is that when a force is applied to them, they

produce output voltage .The mass exerts a certain force on account of accerlation

on the crystal due to which a voltage is generated .The accerlation is applied to the

base ,due to which the mass produces a force .The mass being fixed ,the force is

proportional to accerleration .The voltage output is proportional to force and hence

is proportional to accerlation .

11082023 MMU Page 19

6.3 BASED ON THE TYPE OF OUTPUT

6.3.1 ANALOG TRANSDUCERS :

Definition : These Transducers convert the input quantity into an analog output

which is a continuos function of time .

Thus a strain gauge , an LVDT ,a thermocouple or a thermistor may be called as

“Analog Transducers ”.

Fig 5

The output of a Analog Transducers is a continuos function of time means at any

time we a certain output acc. to that time.

Eg: LVDT, thermocouple etc.

11082023 MMU Page 20

6.3.2 DIGITAL TRANSDUCER :

Definition : These Transdcers convert the input quantity into an electrical output

which is in the form of pulses .

As the binary system uses only two symbols 0 and 1 . It can be easily represented

by opaque and transparent areas on a glass scale or non conducting and

conducting areas on a metal scale .

Fig 6

CE-D Series Intelligent Transducer with Digital Display

　 　 　 　 　

CE-D Series Intelligent Transducer functions Led real time digital display,

electrical perameters measuring ,standand analog output and switching value

output .User can easily set the input threshold for output switching value, return

difference and delay time to deny input action after a switching value output acted

as your requirement. It makes the features of high cost performance and good

stability.The CE-D series intelligent transducer is an economic and practical

instrument for measuring and display of electrical parameters .

11082023 MMU Page 21

6.4 Based on the electrical phenomenon

6.4.1 Resistive Transducer :

Definition : This method involve the change in the measurement of resistance.The

resistance of the metal conductor is expressed by a simple equation that involves a

physical quantity.

The relationship is:

R= ρ L/A

Where R=resistance(ohm)

L=length of conductor(metres)

A=cross-sectional area of conductor(metre x meter)

ρ =Resistivity of conductor material (ohm-metre)

The Translational and rotational potentiometers which works on the basis of

change in the value of resistance with change in length of the conductor can be

used for the measurement of Translational or Rotatary displacements .

Strain Gauges work on the principle that the resistance of the conductor or a semi-

conductor changes when strained .This property can be used for measurement of

displacement ,force and pressure . The resisitivity of materials changes with

change of temperature thus causing a change of resistance .

Hence,this property may be used for measurement of temperature. Thus electrical

resistance transduer have wide field of application.

11082023 MMU Page 22

6.4.2 CAPACTIVE TRANSDUCER :

Definition : The principle of operation of capactive transducers is based upon the

familiar equation for capacitiance of a parral plate capacitor.

C= εr ε0 A/d

Where A = overlapping area of plates (meter x meter)

d= distance between two plates (meter)

ε = εr εo = permittivity of medium (F/m)

εr =relative permittivity

εo = permittivity of free space

A Parral plate capacitors is shown in figure below :

Fig 7

11082023 MMU Page 23

The capactive transducers work on the principle of change of capacitance which

may be caused by :

1. Change in overlapping area “A”

2. Change in the distance “d” between the plates.

3. Change in the dielectric constant

These changes are caused by physical variables like displacement , force and

pressure . The change in capacitance may be caused by change in dielectric

constant as is the case in measurement of liquid or gas levels .

The capacitance may be measured with bridge circuits . The output impedance of a

capacitive transducer is

Xc = 1/ 2 πFC

Where C= capacitance

f= Frequency of excitation in hertz

The output impedance of a capacitive transducer is high .

The capacitive Transducers are commonly used for measurement of linear

displacement.These transducers use the following effects:

I. Change in capacitance due to change in overlapping area of plates.

II. Change in capacitance due to change in distance between the two plates.

11082023 MMU Page 24

6.4.3 INDUCTIVE TRANSDUCER

Definition : The inductive transducers work on the principle of the magnetic

induction of magnetic material. Just as the resistance of the electric conductor

depends on number of factors, the induction of the magnetic material depends on a

number of variables like the number of turns of the coil on the material, the size of

the magnetic material, and the permeability of the flux path. 

There are two common type inductive transducers :

 1.simple inductance type

2.Two-coil mutual inductance type

Simple Inductance Type Inductive Transducers :

In the simple inductance type of the inductive transducers simple single coil is

used as the transducer. When the mechanical element whose displacement is to

be measured is moved, it changes the permeance of the flux path generated by

the circuit, which changes the inductance of the circuit and the corresponding

output. The output from the circuit is calibrated directly against the value of the

input, thus it directly gives the valve of the parameter to be measured.

The figure below shows the single coil inductive circuit

Fig 8

11082023 MMU Page 25

Two-Coil Mutual Inductance Type Inductive Transducer

In the two coil arrangement there are two different coils. In the first coil the

excitation is generated by external source of the power and in the second coil the

output is obtained. The output is proportional to the mechanical input.

As shown in figure 9, A is the excitation coil and B is the output coil. The

inductance of the output coil changes due to change in position of the armature

which is connected to the mechanical element whose motion is to be measured. As

the armature position changes, the air gap between the fixed magnetic material and

the armature changes.

Fig 9

11082023 MMU Page 26

6.4.4 PIEZOELECTRIC TRANSDUCER

Definition :

A Piezo electric material is one in which an electric potential appears across

certain surfaces of a crystal if a dimenstion of the crystal are changed by the

application of a mechanical force. This potential is produced by a displacement of

charges .The effect is reversible that is conversely,if a varying potential is applied

to the proper axis of a crystal,it will change the dimenstions of a crystal thereby

deforming it .This effect is known as piezo electric effect.

Fig 10

The conversion of electrical pulses to mechanical vibrations and the conversion of

returned mechanical vibrations back into electrical energy is the basis for

ultrasonic testing. The active element is the heart of the transducer as it converts

the electrical energy to acoustic energy, and vice versa. The active element is

basically a piece of polarized material with electrodes attached to two of its

opposite faces. When an electric field is applied across the material, the polarized

molecules will align themselves with the electric field, resulting in induced dipoles

within the molecular or crystal structure of the material. This alignment of

molecules will cause the material to change dimensions. This phenomenon is

known as electrostriction.

11082023 MMU Page 27

6.4.5 PHOTOVOLTAIC TRANSDUCER

Definition :

Photovoltaics (PV) is a method of generating electrical power by converting solar

radiation into direct current electricity using semiconductors that exhibit the

photovoltaic effect. Photovoltaic power generation employs solar

panels comprising a number of cells containing a photovoltaic material.

Materials presently used for photovoltaics include monocrystalline

silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper

indium selenidesulfide.Due to the growing demand for renewable energy sources,

the manufacture of solar cells andphotovoltaic arrays has advanced considerably in

recent years

Fig 11

The photovoltaic effect is the creation of a voltage (or a corresponding electric

current) in a material upon exposure to light. Though the photovoltaic effect is

directly related to thephotoelectric effect, the two processes are different and

11082023 MMU Page 28

should be distinguished. In the photoelectric effect, electrons are ejected from a

material's surface upon exposure to radiation of sufficient energy. The photovoltaic

effect is different in that the generated electrons are transferred between different

bands (i.e. from the valence to conduction bands) within the material, resulting in

the buildup of a voltage between two electrodes.

In most photovoltaic applications the radiation is sunlight and for this reason the

devices are known as solar cells. In the case of a p-n junction solar cell,

illumination of the material results in the creation of an electric current as excited

electrons and the remaining holes are swept in different directions by the built-in

electric field of the depletion region .

11082023 MMU Page 29

6.5 Based on the transduction phenomenon

6.5.1 TRANSDUCER AND INVERSE TRANSDUCERS:

An inverse transducer is a device which converts an electrical quantity into a non-

electrical quantity.It is a precision actuator which has an electrical input and a low

power non-electrical output.A piezoelectric crystal acts as an inverse transducer

because when a voltage is applied across its surfaces, it changes its dimenstion

causing a mechanical displacement .

A current carrying coil moving in a magnetic field is also an inverse transducer

because a current carried by it is converted into a force which causes translational

or rotational displacement.Many data indicating and recording devices are inverse

transducers.An analog ammeter or voltmeter converts current into mechanical

displacement

However,such devices which include instruments like indicating instruments,pen

recorders,oscilloscopes that converts the electrical signal to a mechanical

movement are placed at the output stage(data presentation stage) are called output

transducers.

The most useful application of inverse transducer is in feedback measuring

system.In the measuring system ,the output quantity(usually electrical in nature) if

converted to a non electrical form suitable for comparison with quantity to be

measured(non – electrical form).

NOTE: Generally a Inverse Transducer is a output transducer

Example: Indicating Instruments, Pen Recorders, Oscilloscope. 11082023 MMU Page 30

7. PRESSURE

Static pressure is uniform in all directions, so pressure measurements are

independent of direction in an immovable (static) fluid. Flow, however, applies

additional pressure on surfaces perpendicular to the flow direction, while having

little impact on surfaces parallel to the flow direction. This directional component

of pressure in a moving (dynamic) fluid is called dynamic pressure. An instrument

facing the flow direction measures the sum of the static and dynamic pressures;

this measurement is called the total pressure or stagnation pressure. Since dynamic

pressure is referenced to static pressure, it is neither gauge nor absolute; it is a

differential pressure.

While static gauge pressure is of primary importance to determining net loads on

pipe walls, Dynamic pressure is used to measure flow rates and airspeed.

Dynamic pressure can be measured by taking the differential pressure between

instruments parallel and perpendicular to the flow. Pitot-static tubes, for example

perform this measurement on airplanes to determine airspeed. The presence of the

measuring instrument inevitably acts to divert flow and create turbulence, so its

shape is critical to accuracy and the calibration curves are often non-linear.

11082023 MMU Page 31

8. METHODS OF PRESSURE MEASUREMENT

8.1 HYDROSTATIC METHOD

Hydrostatic gauges (such as the mercury column manometer) compare pressure to

the hydrostatic force per unit area at the base of a column of fluid. Hydrostatic

gauge measurements are independent of the type of gas being measured, and can

be designed to have a very linear calibration. They have poor dynamic response.

8.1.1 PISTON

Piston-type gauges counterbalance the pressure of a fluid with a solid weight or a

spring. Another name for piston gauge is deadweight tester. For example, dead-

weight testers used for calibration or tire-pressure gauges .

8.1.2 Liquid column

Liquid column gauges consist of a vertical column of liquid in a tube whose ends

are exposed to different pressures. The column will rise or fall until its weight is in

equilibrium with the pressure differential between the two ends of the tube. A very

simple version is a U-shaped tube half-full of liquid , one side of which is

connected to the region of interest while the reference  pressure (which might be

the atmospheric pressure or a vacuum) is applied to the other. The difference in

liquid level represents the applied pressure. The pressure exerted by a column of

fluid of height  h and density ρ is given by the hydrostatic pressure 11082023 MMU Page 32

equation, P = hgρ. Therefore the pressure difference between the applied

pressure Pa and the reference pressure P0 in a U-tube manometer can be found by

solving Pa − P0 = hgρ. If the fluid being measured is significantly dense,

hydrostatic corrections may have to be made for the height between the moving

surface of the manometer working fluid and the location where the pressure

measurement is desired.

Fig 12

The difference in fluid height in a liquid column manometer is proportional to the

pressure difference. 

Although any fluid can be used, mercury  is preferred for its high density (13.534

g/cm3) and low vapour pressure. For low pressure differences well above the

vapour pressure of water, water  is commonly used (and "inches of water " is a

common pressure unit). Liquid-column pressure gauges are independent of the

type of gas being measured and have a highly linear calibration. They have poor

dynamic response. When measuring vacuum, the working liquid may evaporate

and contaminate the vacuum if its vapor pressure  is too high. When measuring

liquid pressure, a loop filled with gas or a light fluid must isolate the liquids to

prevent them from mixing. Simple hydrostatic gauges can measure pressures

ranging from a few Torr (a few 100 Pa) to a few atmospheres. (Approximately

1,000,000 Pa) 11082023 MMU Page 33

8.2 ELECTRONIC PRESSURE SENSOR

8.2.1 Piezoresistive Strain Gage

Uses the piezoresistive effect of bonded or formed strain gauges to detect strain

due to applied pressure.

PIEZORESISITIVE EFFECT

The piezoresistive effect describes the changing resistivity of a semiconductor

due to applied mechanical stress. The piezoresistive effect differs from

the piezoelectric effect . In contrast to the piezoelectric effect, the piezoresistive

effect only causes a change in electrical resistance it does not produce an electric

potential.

MECHANISM

In semiconductors, changes in inter-atomic spacing resulting from strain affects

the bandgaps making it easier (or harder depending on the material and strain) for

electrons to be raised into the conduction band. This results in a change in

resistivity of the semiconductor. Piezoresistivity is defined by

Where ∂ρ = Change in resistivity

ρ = Original resistivity

ε = Strain

Piezoresistivity has a much greater effect on resistance than a simple

change in geometry and so a semiconductor can be used to create a much

more sensitive strain gauge . though they are generally also more sensitive

to environmental conditions (esp. temperature).

11082023 MMU Page 34

8.3 MAGNETIC METHOD

Measures the displacement of a diaphragm by means of changes in inductance

principle .

Inductance is the property of an electrical circuit  measuring the induced electric

voltage compared to the rate of change of the electric current in the circuit. This

property also is called self inductance to discriminate it from mutual inductance,

describing the voltage induced in one electrical circuit by the rate of change of the

electric current in another circuit.

The quantitative definition of the self inductance L of an electrical circuit

in SI units (webers per ampere known as henries) is

Fig 13

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where v denotes the voltage in volts and i the current in amperes. This is a linear

relation between voltage and current akin to Ohm's law, but with an extra time

derivate. The simplest solutions of this equation are a constant current with no

voltage or a current changing linearilly in time with a constant voltage.

Fig 14

Inductance is caused by the magnetic field generated by electric currents

according to Ampere's law. To add inductance to a circuit  electronic

components called inductors are used, typically consisting of coils

ofwiretoconcentrate the magnetic field and to collect the induced voltage.

The SI unit of inductance is the henry(H),

8.3.1 Optical

Uses the physical change of an optical fiber to detect strain due applied pressure.

8.3.2 Potentiometric

Uses the motion of a wiper along a resistive mechanism to detect the strain caused

by applied pressure.

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8.4 THERMAL CONDUCTIVITY METHOD

Generally, as a real gas increases in density -which may indicate an increase

in pressure- its ability to conduct heat increases. In this type of gauge, a

wire filament is heated by running current through it. A thermocoupleor Resistance

Temperature Detector (RTD) can then be used to measure the temperature of the

filament. This temperature is dependent on the rate at which the filament loses heat

to the surrounding gas, and therefore on the thermal conductivity. A common

variant is the Pirani gauge which uses a single platinum filament as both the heated

element and RTD. These gauges are accurate from 10 Torr to 10−3 Torr, but they

are sensitive to the chemical composition of the gases being measured.

Vacuum pressure measurement is broadly dividedinto two categories: high and low

vacuum.Many of the different techniques used to measure these categories have an

overlap some point in the pressure range. By combining several different types of

gauge it is possible measure system pressure from 10 mbar down to 10-11 mbar.

For measuring in the low vacuum range (30 mbarto 10-3 mbar) we offer a range

of Pirani gauges;for pressures below this (10-3 mbar to 10-11mbar) we offer a

range of ionisation gauges.

Fig 15

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8.4.1 Pirani Gauge Range

Introduction to the Pirani Gauge Range

The Pirani gauge is a roughing pressure vacuum gauge. It uses the thermal

conductivity of gases to measure pressure. VG Scienta has two differenttypes of

Pirani gauge measuring slightly different pressure ranges; they are both availablein

a variety of styles with a range of mountingflange.

Fig 16

Principles of Operation :

The Pirani gauge head is based around a heatedwire placed in a vacuum system,

the electrical resistance of the wire being proportional to its temperature.

At atmospheric pressure, gas molecules collide with the wire and remove heat

energy from it (effectively cooling the wire). As gas moleculesare removed (when

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the system is pumped down)there are less molecules and therefore less

collisions.Fewer collisions mean that less heat isremoved from the wire and so it

heats up. As itheats up, its electrical resistance increases. A simple circuit utilising

the wire detects the changein resistance and, once calibrated, can directly correlate

the relationship between pressure andresistance.

This effect only works in the pressure regionfrom atmosphere to approx 10-3

mbar. Therefore other types of gauge have to be used to measurepressures lower

than this.

The VG Scienta range of Pirani gauges is dividedinto two types: constant current,

and constant resistance. The name refers to how the electricalmeasurement of the

wire is controlled.

The constant current type has a power supplythat gives a constant current all the

time tothe filament. Therefore the filament resistancechanges are measured.

The constant resistance type has a power supplythat changes the current supplied

to keepthe resistance of the filament the same. It has aslightly larger pressure range

but requires morecomplicated electronics to control it.

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8.4.2 Ion Gauges

Introduction to Ion Gauges :

When operating below the Pirani gauge range,an ion gauge can be used to

measure pressure.There are a range of gauge heads and filamentmaterials to cover

specific pressure ranges and Vacuum requirements in this region.

Fig 17

The VG Scienta ion gauge heads all operateusing the same principle. Subtle

differences indesign and construction determine the pressurerange and robustness

of the different gauges.

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Principles of Operation

Fig 18

The ion gauge consists of three distinct parts;the filament, the grid, and the

collector. The filament produces electrons by thermonic emission. A positive

charge on the grid attracts the electrons away from the filament; they

circulatearound the grid passing through the fine structure many times until

eventually they collide withthe grid. Gas molecules inside the grid may collide

with circulating electrons. The collision canresult in the gas molecule being

ionised. The collector inside the grid is negatively charged andattracts these

positively charged ions. Likewise they are repelled from the positive grid at

thesame time. The number of ions collected by the collector is directly proportional

to the numberof molecules inside the vacuum system. By this method, measuring

the collected ion currentgives a direct reading of the pressure. 11082023 MMU Page 41

The above is a simplification of what happens. The above is a simplification of

what happens.

The design of the gauge head affects how efficientlyelectrons are produced, how

long they survive, and how likely they are to collide witha molecule. These factors

combine together to result in the gauge sensitivity. As a general rule,the higher the

sensitivity, the more efficient the operation of the gauge.There are other factors

which determine the lowest pressure that a gauge head can measure.One of these

limiting factors is the X-ray limit.When an electron collides with the grid, thereis a

probability of a photoelectron being produced.Once generated, there is also a

chancethat the photoelectron will hit the collector andproduce an electron.

Unfortunately, the collectordoes not know the electrical difference between

collecting a positive charge or losing a negativecharge. This means that every time

an electron isknocked off the collector, the electronics measureit as receiving a

positive ion instead. This effect is very small and depends on the design ofthe

gauge head. It normally generates a current measured in the picoamp range. At 10-

10 to 10-11mbar, however, this is also the current producedby the gauge head

itself. If pressure is plottedagainst current, the graph can be seen to tail offas this

X-ray current becomes the dominant effect. The X-ray current therefore limits the

lowest pressure that the ion gauge can measure.

8.4.2.1 Ion Gauge Connection Leads

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A lead is required to connect an ion gauge head to a controller. Leads are available

in both bakeable

Fig 19

and non-bakeable materials. The advantage of a bakeable lead is that the gauge can

still be operated (and pressure monitored) whilst the system is being baked (as long

as it does not exceed the temperaturelimit of the lead). It also means that the

gaugehead can be degassed whilst the system is still hot. Our standard lead length

is 3 metres; we make special length leads of up to 10 metres (the longest we

recommend that you use).

8.5 Wide Range Pressure Measurement

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Introduction to the Wide Range Gauge

Fig 20

VG Scienta wide range gauge display control unit is designed for use with the

ZPGSH and ZPGSU standard version wide range gauges,and the ZPGCH compact

version wide range gauge .

The display unit is capable of being rack mounted. The display unit will

allow continuous pressure measurement fromatmosphere to UHV, control the

degas of gauges and has up to six setpoints available.

Display and Control System

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Measuring Accuracy ±25 % (10 to 10-2 mbar)

±15 % (10-2 to 10-8 mbar)

Analogue Signal l 0 to 10 V logarithmic:

U= 0.625 V x log

(p/10-12 mbar)

Operating Voltage 24V DC +/- 10 %

Current Normal operation 0.4 A

max, 0.9 A degas max

Input Power Normal operation 10 W

max, 22 W degas max

Cathode Material Yttria-coated iridium

Emission Current 25 μA and 2 mA: degassing

20 mA

Grid Potential 240 V, 400 V degas

Ambient Temp 40 ºC max

Setpoint One set point available

Specification and Options :

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Line Voltage 230 or 115 VAC(switchable by voltageselector on main

input)

Fuses Slow-blow fuses of250 mA (230 V)/500 mA(115 V)

Outputs Analogue pressure signalof the connectedwide range gauge

(24v/1A max)

Weight 1.5 kg

Size 128 mm x 142mm x 170 mm (H x W x D)

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BIBLIOGRAPHY

1. www.ntpc.co.in

2. www.transducersdirect.com

3. www.google.com

4. www.lesker.com/newweb/menu_gauges.cfm

5. www.wisegreek.com/what-are-transducers.html

6. www.ndt-ed.org/transducertypes.html

7. www.omega.com/pressure.html

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