INSTRUMENTATION

CHAPTER- 1

MEASURMENT

MEASURMENT-

Measurement of a quantity is the result of comparison of quantity's whose magnitude is unknown with

a predefined standard. Measurement is the process by which we convert physical parameter to

meaningful no.

Methods of Measurement-

1) Direct Method

2) Indirect Method

1) Direct Method –

In this method, the unknown quantity is directly compared against a standard. The result is expressed as

a numerical no. and a unit. This type of method is mostly common for the measurements of physical

quantities. These physical quantities include length, mass and time.

For example - We want to measure the length of a rod. Unit of length is in meter.

Indirect Method - measurement by direct VB method are not always possible feasible and practicable.

These methods are inaccurate. Hence direct methods are not preferred and are barely used in daily life.

In Indirect method we use an instrument which is called transducer. This instrument is very useful for

indirect measurement because transducer element convert the physical quantity in an analogous form.

* Basic Measurement system –

A measuring system exists to provide information about the physical value of some variable being

measured. In simple cases the system can consist of only output reading or signal according to the

magnitude of the unknown variable applied to it.

1) Measured –

It is the quantity which is to be measured by measurement system this is first sensed by the

primary sensing element whose type depends upon the type of quantity to be measured.

2) Primary sensing element –

The first contact of the measured in the measurement system is with the primary sensing

element, which is basically a transducer. It converts measured to its corresponding electrical

signal which is further processed in subsequent stages.

3) Variable conversion element –

The output of the primary sensing element is the electrical signal which is of no use as such in

measurement system. So, this electrical signal needs to be converted into a suitable from as per

the requirement of the system without distributing the original content or information of the

signal.

4) Variable Manipulation element –

As the name suggest, the functions of V.M.F is to manipulate the signal presented to it

preserving the original nature of the signal. Manipulation means there is change only in the

numerical value of the signal.

5) Data transmission element –

When the element of an instrument is actually physical separated it becomes necessary to

transmit data from one to another. The element that performs this function is called data

transmission element.

6) Data Presentation Element –

As the name suggests, this element is used to present or display the data. We know that the

information about the measurand has to be conveyed to the personal handing or the system for

monitoring control or analysis purposes. The information conveyed must be in form intelligible

to personal or to the intelligent instrumentation system. The function is done by data

presentation element.

* Display Device:-

The final stage of a measurement system is the data presentation stage consisting of data presentation

element. This stage consists of display device. It can be classified as:-

1) Analog Instrument Display Device :-

Analog display device comprise of indicating instruments which directly indicate the quality to

be measured on the scale in form of pointer deflection.

2) Digital Instrument Display Devices :-

Digital display device directly display the result on the screen. A digital display device may

receive digital information in any form but it converts that information to the decimal form.

Seven Segment Display Devices: -

This is used for numeric display consisting of seven segment a,b, c,d,e,f,g.

Chapter:-2

Transducer Transducer:-

In instrumentation system the input quantity which is generally a non-electrical device is

converted into electrical form by a device known as transducer.

Basically there are two types of transducers:-

• Electrical transducers

• Mechanical transducers

Electrical transducers: -

Electrical quantities are measured by electrical measurement. In order to measure non-

electrical quantities, a detector i.e. sensing element is used which normally converts the

physical quantities into a displacement. This displacement actuates electrical transducers which

gives output which is electrical in nature.

PARAMETER OF ELECTRICAL TRANSDUCERS:-

1. The weight and volume of transducer should be as low as possible so that its presence

does not disturb the existing condition in measurement system.

2. The transducer should be highly sensitive.

3. The output of the transducers should be compatible with the input of the measuring

system.

4. Operating range of the transducers should be avoid so that it can we used under a wide

range of measurement.

5. The relationship of the input and the output of the transducers can be linear.

ADVANTAGES OF ELECTRICAL TRANSDUCERS:-

1. The effect of friction is minimized.

2. Mass inertia effects are minimized.

3. The electrical / electronic system can be controlled with a small power.

4. Modern Digital computers make the use of these transducers absolutely essential.

TRANSDUCER ELEMENT S:-

Transducers consist of two parts:-

1. Sensing element.

2. Transudation Element.

1. SENSING ELEMENT:-

As the name suggests the part sense the changes occurring around it. It is the part of

transducer which responds to a change in physical phenomenon. The response of the sensing

element is closely related to the physical phenomenon.

2. TRANSUDATION ELEMENT:-

This element transforms the output of the sensing element to an electrical output. We can also

say that a TRANSUDATION element acts as secondary transducer.

CLASSIFICATION OF TRANSDUCERS:-

On the basis of transudation form used

1. As passive and active transducers

2. As analog and digital transducers

3. As transducers and inverse transducers

PASSIVE AND ACTIVE TRANSDUCER:-

Passive transducers are those that take the power for transduction from an externally power

source. They also derive parts of the power required for conversion from the physical quantity

under measurement. Passive transducers are also known as externally powered transducers.

ACTIVE TRANSDUCERS:-

These are the transducers that do not require an auxiliary power source to produce their

output. They develop their own voltage or current outputs, thus they are also known as self

generating type. The energy required to produce output signal is obtained from the physical

quantity being measured.

ANALOG AND DIGITAL TRANSDUCERS:-

ANALOG TRANSDUCERS:-

These transducers convert the input quantity into an analog input which is a continuous

function of time. A strain gauge, a thermocouple thermostat are all examples of analog

transducers as they give an output which is a continuous function lf time.

DIGITAL TRANSDUCERS:-

These transducers convert the input quantity into an electrical output which is in the form of

pulse.

TRANSDUCERS AND INVERSE TRANSDUCERS:-

TRANSDUCERS:-

Transducers are a device that transforms non- electrical quantity into an electrical quantity.

INVERSE TRANSDUCERS:-

As the name suggests, it perform the reverse function of transducers. This device converts an

electrical into non electrical quantity. A current carrying coil moving in a magnetic field is an

inverse transducers becomes current carrying by it is converted into force which causes

translation or rotational displacement.

FACTOR AFFACTING THE CHOICE OF THE TRANSDUCER÷

Some of the factor which determine the choice of transducer are as follow ÷

1. Linearity the input & output characteristics of the transducers should be linear.

2. Sensitivity ÷

The transducer must be sensitive enough to produce detectable output.

3. Accuracy÷

High degree of accuracy is assured if the transducers do not require frequent calibration and

has a small value of repeatability.

4. Intensity to unwanted signal ÷

The transducer should be minimally sensitive to unwanted signal.

5. Loading effects÷

The output impedance should be high and output impedance should be low of the transducers

to avoid the loading effect.

6. Errors:-

The transducer should be maintaining the input – output relationship as described by its

transfer function so at to avoid errors.

7. Electrical Aspects:-

While selecting a transducer the electrical aspects such as length, type of cable required should

be considered. Signal noise ratio, frequency response limitations must also be taken into

account.

8. Operating principle: -

The transducer must be selected on the basis of operating principle used may resistive,

inductive, capacitor, piezoelectric etc.

9. Repeatability: -

Accurate transducers are capable of producing the same result again and again under same

environmental conditions e.g. – temperature pressure, humidity etc.

10. Residual Deformation: -

There should be no deformation of the testing material after the removal of any pressure after

long period of application.

11. Stability: -

High degree of stability is shown by transducer during its operation.

Resistive Transducers: - The transducer whose resistance varies because of the environmental

effect is called resistance transducers.

CH-3

MEASURMENT OF DISPLACEMENT AND STRAIN

DISPLACEMENT MEASURMENT- : Measurement of displacement location etc. is done

from time to time in any industrial process. The shifting of object from its desired position , function and

position of the work place in automatic milling operation has to be sensed for proper operation .

Similarly pressure is sends by pressure sensing element such as bourdon, diaphragm etc. Which convert

the pressure into displacement. The main three type of displacement sensor are -:

1. Potentiometer type.

2. Capacitive type.

3. Inductive type.

POTENTIOMETERS-: Potentiometer is a displacement transducer. It is a variable resistor with

three terminals. It consist of resistance is proportional to its length. A fixed voltage is applied across the

coil using an external voltage supply. The output signal is the D.C voltage between the movable contact

sliding on the coil. Slider displacement is proportional to the output voltage.

The movement of potential slider may be -:

1. Translation type

2. Rotational type

3. Helix type

TRANSLATION TYPE POTENTIOMETER-:

In this type of potentiometer motion of sliding contact is linear-:

ROTSTIONAL TYPE POTENTIOMETER-:

Motion in this case is rotary. In this type , motion of wiper is rotary. Such type of potentiometer.

Measure the angular displacement &are circular in shape.

the displacement may be full scale angular displacement or as small as up to 100.

HELIX TYPE POTENTIOMETER: Such type of potentiometer is used when motion is

combination of above two translational as well as rotational. Helix type pot is used when there are two

types of motion.

For measurement of translational or rotational motion, helix resistive element is used. The resistive wire

of potentiometer wire is of around 0.01 mm diameter made of nickel alloy or platinum over an insulated

former. The construction of the resistive element of pot is influenced by the resolution of the

potentiometer.

ADVANTAGE OF POTENTIOMETER:

1. These are inexpensive.

2. These are simple to operate

3. These are useful for measurement of large amplitude of displacement.

4. We get high electrical n.

DISADVANTAGE IF POTENTIOMETER-:

1. When using a linear potentiometer, a large force is required to move the sliding contact.

2. The sliding contact can wear out become misaligned &generate noise & their life decreases .v

VARIABLE RESISTANCE TRANSDUCER-: A variable resistance transducer is a rheostat

whose slider is shifted by the non electrical quantity to be measured so that the value of resistance of

the rheostat varies with the position of the solider . Thus the magnitude of the non electrical quantity

can be measured by measuring the value of resistance "R "two variable resistance transducer are shown

in fig-:

STRAIN GAUGE-: Strain gauge is thin water like device that can attach to a variety of material to

measure strain. The strain gauges passive transducer. It converts a mechanical displacement in a change

of resistance.

Metallic strain gauge is manufactured from small diameter-wire of some material having

high resistivity. As the material to which the gauge is attached undergoes tension or compression its

length & diameter both change which changes its resistance. This change in resistance. This change in

resistance is proportional to the applied strain & is measured with WHEAT STONE BRIDGE.

GAUGE FACTOR -: Gauge factor is defined as the ratio of per unit change in resistance to per unit

change in length.

REQUIREMENT OF GOOD STRAIN GAUGE-:

1. High frequency response

2. Good frequency response

3. High gauge factor (K)

4. Less hysteresis effect.

5. Low temp coefficient of resistance.

TYPES OF STRAIN GAUGE-:

WIRE WOUND STRAIN GAUGE-:

(a) BONDED STRAIN GAUGE-: It is a further sub-divided into following categories-:

1. GRID TYPE

2. ROSETTE TYPE

3. TORQUE TYPE

4. HELICAL GAUGE

1. GRID TYPE -: In grid type grid is mode of thin wire. It is placed on sheet of bakelite or a paper &

then it is covered with protective layer so that it can protect the grid for mechanical injury. The carrier is

bonded with adhesive material to the structure so that strain developed in the structure be transferred

to the resistance wire. The measuring axis of strain gauges is it longitudinal axis which is parallel to the

wire element.

2. ROSSETTE GAUGE-: A rosette gauge in which the angle between any two longitudinal axis is

450

The 450 rosette gauges is the most popular used. There are different shape & size of strain gauge for

various purposes.

There is some modified grid configurations also specialized.

(B). UNBOUNDED STRAIN GAUGE-: It consist of a wire stretched b\w two points in an

insulating medium such as air the diameter of wire is about gauge .

FOIL TYPE STRAIN GAUGE -: foil type strain gauge is an extension of the wire wound strain

gauge. In foil type strain gauge we use metal foils instead of wire as is the case of wire wound strain

gauge the metals & alloys used for the foil and wire are nichrome constantan (NI+CU) (NI+CR+MO) nickel and platinum the foil type strain gauge are preferred over strain gauge because of the following

reasons -:

(1) foil gauge have much greater dissipation capacity than wire wound gauge

(2) The surface area of foil gauge is more therefore they have better bonding with the base

(3)Thy can be used at higher operating range

(4) These gauge can be fabricated more economically and a mass scale

Strain to be measured the foil type strain gauge which are used commercially are available in different

shapes

(1) UNIAXIAL SINGL ELEMENT STRAIN GAUGE -: The base of this gauge is bounded

to the structure whose strain is required to be measured. The gauge length is selected according to the

strain

(2) TWO ELEMENT ROSETTES -: The two element rosettes are these are often use -in force

transducers. The gauge are placed in a Wheat stone bridge circuit for maximum that can be so selected

that the combined output is proportional to the stress while that output of the axial element alone is

proportional to the strain.

3. THREE ELEMENT ROSSETE-: A three element rosette 600 planar foil. These are often to

determine the direction and magnitude of principle strain resulting from complex structural loading.

SEMICONDUCTOR STRAIN GAUGE:- The gauge factor low in cause of wire wound strain

gauge of foil type strain gauge. To overcome this limitation semiconductor type strain gauge are used.

Semiconductor gauge are used when a very high gauge factor is desired. They have a gauge factor 50

times as high as wire strain gauge. The resistance of semiconductor change with change in applied

strain .

Semiconductor strain gauge depends for their action upon piezoelectric effect i.e. the change in value of

their resistance due to change in resistively. For semiconductor gauge semiconductor materials such as

silicon & germanium are used

A typical strain gauge consist of a strain sensitive crystal material and leads that are sandwiched in a

protective matrix the production of these gauge employed conventional semiconductor technology

using a thickness of o.05mm and bonding them on suitable insulating such as teflon.

ADVANTAGES OF SEMICONDUCTOR STRAIN GAUGE :- (1) semiconductor strain

gauge have the advantage that they have high gauge factor of about +_ 130. This allows measurement

of very small strains of the order of 0.01 micro strains.

2. Hysteresis characteristics of semi conductor strain gauges are excellent. Some units maintain it to less

than 0.05%

3. Fatigue life is in excess of 10*106 operations and the frequency response this up to 10

12 HZ.

4. Semi conductor strain gauges can be very small ranging in length from 0.7 to 7mm. They are very

useful for measurements of local strain.

DISADVANTAGES: 1. The measure disadvantage of semi conductor strain gauges is that they are

very sensitive to change in temp.

2. linearity of the semi conductor strain gauge is poor. This gauge is rather non linear at comparatively

high strains levels.

3. Semi conductor strain are more expensive and difficult to attach to the object under study.

INDUCTIVE TRANSDUCERS: Inductive electromechanical transducers is a transducer device

that converts physical motion into a change in inductance.

Inductive transducers are of 2 types self generator or the passive type. The self generating type works

on the principal of electrical generator principal i.e. a motion between a conductor & magnetic field

induced a voltage in the conductor. The relative motion between the field & the conductor is supplied

by changes in the measured. The operation of such transducer is based on the fact that inductance of a

transducer depends upon the change in position of one of its element. It is the property of coil due to

which a counter E.M.F is produced in coil whenever A.C current is passed through the coil. The

inductance depends upon the following factors:

(a) No. of turns of a coil.

(b) Relative permeability of a core.

(c) Cross sectional area of magnetic circuit.

The inductance can be classified into different ways known as :

1. Variable self inductance type.

2. Variable reluctance type.

3. Variable mutual inductance type.

WORKING PRINCIPAL OF INDUCTIVE TRANSDUCER: These inductive transducers

work on the principle that if a conducting plate is placed near a coil carrying than ac and eddy current

are produced in the conducting plate the conducting plate acts as a short circuited secondary of

transformer. The eddy current flowing in the plate produces a magnetic field of their own which acts

against the magnetic field produced by the coil .It result in reduction of flux and thus the inductance of

coil is reduced. The nears in the plate to the coil the higher the eddy currents and the so higher is

reduction the inductance of the coil. Thus the inductance of the coil changes with variation distance

between the plate and the coil.

LVDT:- LVDT is a passive inductive transducers and is commonly used to measure force in terms of

the amount & direction of displacement of an object it is used to translate linear motion into electrical

signal.

OPERATING PRINCIPLE:- The main advantage of LVDT transducers over other types of

displacement transducer is their high degree of robustness .This is derived from their principal which

there is no physical contact across the sensing element and so there is zero wear in the sensing element .

This also means that LVDTs can be made waterproof and in a format suitable for the most arduous

application.

The LVDT principle of measurement is based on magnetic transfer which

also means that the resolution of LVDT transducer is infinite. The smallest friction of movement can be

detected by suitable signal conditioning electronics.

The combination of these factor to other factor such as accuracy and repeatability has ensured that this

technology still at the for front of displacement measurement after over many years.

LVDT linear position sensors are readily available that can measure movement or small as a few

millionths of an inch up to several inches but are also capable of measuring position up to 20 inches .

CONSTRUCTION:-A differential transformer consist of a primary winding and two secondary

windings . he winding are arranged concentrically and next to each other .They are wound over a hollow

bobbin which is usually of a non-magnetic and insulating material.

When A.C excitation is applied across the primary winding then the movable core varies the coupling

between its & the two secondary winding.

WORKING:- A discussed above any physical displacement of core causes the voltage so on

secondary winding to increases while other secondary winding. At the output terminals we get the

different of two voltage which gives a measure of physical position of the core & hence the

displacement. When the core moves to the neutral position then voltage induced in the secondary

winding are equal and opposite thus they cancel each other and net output becomes negligible. When

core move in the another direction from zero position then differential voltage again increases but that

is 1800 out of phase with the input voltage. Variation of output voltage with core position is shown:-

Central position of core We can get amount & direction of the movement of core by comparing the

magnitude & phase of the output voltage with the input source. In this way displacement can be

determined.

ADVANTAGE OF LVDT:-

1. There is no wear & tear (friction) because there is no physical contact between core & coils.

2. There is no possibility of mechanically overloading because core is completely separable from

remainder of the device.

3. It repairable.

4. Insensitive to temp variations.

5. It is accurate & sensitive.

6. It has excellent linearity.

DISADVANTAGE:-

1. These transducers are sensitive it stray magnetic field.

2. They are low in power output.

3. Large displacement are required for appreciable differential output.

4. Temp. Effects the performance of transducer.

PIEZOELECTRIC TRANSDUCERS:- Piezoelectric material is one in which an electrical

potential appear across certain surface of a crystal if the dimensions of the crystal are changed by the

application of a mechanical force. The potential is produced by the displacement. The effect is reversible.

If a varying potential is applied to the proper axis of the crystal it will change the dimensions of the

crystal there by deforming it. This effect is known as piezoelectric effect.

The materials are that exhibit s significant and useful piezoelectric effects are divided into two

categories:-

1 Natural Group

2. Synthetic Group.

CHAPTER – 4

Force and Torque Measurement

Force: - Force is defined as the product mass and acceleration.

F = ma

F = Force, in Newton

M = Mass, in kg

A = Acceleration, in kg/m2

Force Measurement Technique: - Now a day s, electric based measurement methods are used

where load cell is the prime element used for force measurement. The electronic signal obtain

can directly to given the computer after processing and data can be displayed.

1) Using mechanical balance. Balancing the unknown force against the known gravitational

force on a standard mass.

2) By evaluating the acceleration of a body of known mass to which the unknown force is

applied.

3) Using various types of load cell –

●Hydrauli load ell.

●P eu ati load ell.

●Strai gauge ased load ell.

●Usi g ele tri Me er.

4) Using electromagnetic based balance.

◆. Mechanical Balance: - In this type, the unknown force is balanced against the standard

known weight.

Classified into following types -

a) Analytical Balance.

b) Balance with pendulum type scale.

c) Platform type scale balance.

a) Analytical Balance :- It consist of a beam which is designed so that centre of mass is

slightly below the pivot and thus, barely equilibrium. Analytical balance is a measure

of the angular displacement Ѳ per unit unbalance in two weight w1 and w2

Balance with pendulum: - This type is used for static force measurement. The unknown force F

is applied through a section of levers. This torque is then balanced by torque produced by a

standard fixed mass arrangement as a pendulum. The deflection Q can be sensed through

angular displacement transducers to get the o/p electrical signal which is proportional to the

applied force.

Platform scale: - This type of scale is used to measure large force by using much smaller

standard weight .In this type of scale, beam of scale is brought to null by proper combination of

pan weight and adjusting the poise weight lever arm along its calibrated scale.

◆Electromagnetic type balance: - A photoelectric sensor is used which act as null detector in

the circuit. A servo system is used which consists of photo tube , and amplifier and coil on

which torque is produced.

This servo system balances the difference between unknown force F and

gravitational force as standard mass H.

ADVANTAGES:-

1) It is small in size.

2) Their response is quick.

3) Electrical output can be used for recording.

◆ Load cell :- A load cell is an electronic device that is used to convert force into an electrical

signal. Load cell utilize the elastic member as the primary transducers and strain gauge as

secondary transducers. Strain gauge attached to any elastic member. When the strain gauge –

elastic member combination is used for weighing it is called a load cell.

Strain Gauge Load cell :- This cell convert the load acting them into electrical signal.

In strain gauge load cell, a length of bar, usually steel, is used as the active element. The gauge

themselves are bonded onto a beam or structural member that deform when weight is applied.

Shown a strain gauge load cell.

As the stress is applied across the direction of S, the steel bar experience a compression along

that axis and expansion along the X and Y axis.

As the result, Gauge a experience a decrease in resistance, while gauge B undergoes an

increase in resistance. This makes the load cell sensitive to very small value of applied stress.

This change in resistance of the strain gauge causes the wheat stone bridge, which directly

proportional to the applied force bridge.

ADVANTAGES:-

1) Response time is very good.

2) Small and compact of size.

3) These are inexpensive.

4) Usually maintenance free.

Limitations:-

1) These should not be over loaded over their rating value.

2) These should be protected from non – axial load.

Hydraulic Load Cell :- Hydraulic load cell are force – balance device, measuring weight as a

change in pressure of the internal filling fluid. In a rolling diaphragm type hydraulic load cell, a

load or force acting on a loading head is transferred to a piston that in turn compresses a filling

fluid confined within an elastomeric diaphragm chamber. As force increase, the pressure of the

hydraulic fluid rises. The pressure can be locally indicated or transmitted for remote indication

or control.

Typical hydraulic load cell applications include tank, bin , and hopper weighing. The output of

the cell can be sent to a hydraulic totalize that sums the load cells signals and generates an

output representing their sum. Electronic totalizes can also be used .

Pneumatic load cell :- Pneumatic load cell also operate on the force – balance principle, This

device use multiple dampener chamber to provide higher accuracy that hydraulic device. This

load cell are often used to measure relatively small weight in industry where cleanliness and

safety of prime concern.

The advantage of this type of load cell include their being inherently explosion proof and

insensitive to temp variation.

Disadvantage: - Include relatively slow speed of response and need for clean, dry, regulated air

or nitrogen.

◆ Torque Measurement :- It is defined as the force acting on a body which tends to produce

rotation.

T = F × D

T = Torque

F = Force

D = Perpendicular Distance from the axis of rotation of line of action of force.

MEASURMENT OF TORQUE :- Torques measured by either sensing the actual shaft deflection

caused by a twisting force. The surface of a shaft under torque will experience compression

and tension to measure Torque, strain gauge element usually are mounted in pair on the shaft ,

one gauge measuring the increasing in length , the other measuring the decreases in length in

the other direction.

A number of device that are used for the measurement of torque are –

1) Strain gauge torque meter.

2) Inductive torque meter.

3) Magnetostrictive transducers

4) Digital method.

5) Proximity sensor for torque measurement.

Strain Gauge Torque Meter :- In this method two strain gauge are mounted on a shaft at an

analog 45 to each other. Cylindrical shaft connecting the driving and driven machine are used

primary transducers strain gauge are mounted on the surface of shaft –

We know that torque is give by –

T = πG (R4 – r 4)

gL QNm

G = Modules of rigidity; NIm2

r = inner radius of shaft; m

R = outer radius of shaft; m

L = Length of shaft; m

Q = Angular deflection of shaft.

The two gauge are in tensile mode and other two in compressive mod. All the four gauge are

available in the form of rosette. Slip rings are used for connecting the supply voltage to the

bridge.

ADVANTAGES:-

1) This arrangement compensates temperature completely.

2) Automatic compensation for bending and axial.

3) It give the maximum sensitivity for a given torque.

Inductive Torque Transducers: - In consist of a flange p which carries a coil and flange on as

iron case.

The core moves in and out of the coil according to relative displacement of the two flanges. The

inductance of the coil is changed due to relative displacement. The coil is made on arm of A.c.

Bridge. The output of a.c. bridge depends upon the inductance of the coil which in turn

depends upon the position of core and hence the displacement. Because the displacement

depends upon the torque, the bridge output can be directly calibrated to indicate the torque.

To have higher sensitivity and better linearity four inductive transducers are used with the coil

connected as four arms of an a.c. bridge. Inductance of two coil increases, while the other two

coil decreases, because of the movement of core due to torque applied to shaft in a given

direction. Sensitivity of the bridge in this arrangement is increased four times as compared to

the sensitivity obtained with a bridge using only one inductive transducers.

Digital Method: - Digital timing techniques can also be used for determination of relative

between two flanges A and B.

Let us consider the case of flanges made in the form of single toothed wheels.

C and D are the inductive pickups where voltage

Pulse is produced by the teeth. When no torque is applied to the shaft, the teeth are perfectly

aligned and when torque is applied to the shaft, there is relative displacement between the two

flanges. When a torque is applied to the shaft, a phase shift is conductive pickups C and D.

When these pulses are compared with the help of an electronic timer, it will show a time

interval between the two pulses. This time interval is proportional to the relative displacement

of two flanges which in turn is proportional to torque. In this way, torque is measured.

ADVANTAGES: - 1) Errors are eliminated.

2) Leakage of the signal.

3) No noise problem.

Magnetostrictive Transducers: - These are based on the principal that permeability of

magnetic material changes when they are subjected to strain . Permeability decrease with

positive strain and increases with negative strain.

Two a.c energized coils are would on iron cores. These are positioned closed to the shaft so that

their flux paths through the material of the shaft coincide with the directions of maximum

strain. These coils form adjacent arms of an a.c bridge. The inductance of on of the coil

increases due to increase in permeability and induct permeability. When no torque is applied,

then bridge is balanced and two coils have equal inductance. When torque is applied,

inductance in one coil increases and thus bridge becomes unbalanced.

This is due to differential change in inductance of the two coils caused by change in

permeability of flux path due to application of torque. Hence the voltage output of the a.c

bridge is inductive of the torque applied.

PROXIMITY SENSORS BASED TORQUE MEASUREMENT: These are used to measure the

torque in a rotating shaft by measuring the angular displacement between the two sections

of the rotating shaft.

Two identical toothed wheels are fixed on a shaft, a certain distance apart. A each wheel,

proximity sensor are placed.

When torque is applied, the shaft rotates and tooth of the wheel passes the proximity

sensor due to which alternating voltage are produced, whose phase difference is

proportional to the applied torque.

Measurement Of Speed (Angular Velocity) :- speed is a scalar quantity equal to the

magnitude of velocity . In industrial process, Speed as a variable refers to the revolution per

minute of Some piece of rotating equipments.

These are many method of measurement; of tachometers are the most frequency used

device. They are used for the measurement, of angular speed, usually in revolution per

minute, (RPM) , although they can be used in many other meaningful units depending upon

applications , such as feat per minute , miles per hour, yard per minute , or even in

production units per unit time .

Various tachometers are:-

1) Mechanical tachometers

2) Optical tachometers

3) Stroboscope

4) A.C tachometers

5) D.C tachometers

6) Toothed wheel rotor type Speed sensor

7) Magnetic speed sensor

Mechanical Type Hand Held Tachometers:-

It consists of linear measuring wheels, adaptor for direct contact rpm sensing. A variety

of rubber tipped wheels is present that transmit by friction the motion to be measured

to the converts input shaft.

Photoelectric Speed Sensors :-

A beam of light is allowed to fall on a rotating disc which has been marked with light and

dark areas. The disc is made to rotate at the speed of device whose speed is to be

measured.

A photocell absorbs and reflects the light. Current pulses whose frequency is

proportional to the speed of rotational are produced by the photocell . It is to be noted

that the source of light and photocell are located on opposite side of a rotating

perforated disc.

Stroboscope:- Stroboscope is a simpler, portable manually operated device which may be used

for measurement of periodic or rotary motions. Basically, the instrument is a source of Variable

frequency flashing brilliant light , the flashing being set by the operator.

A variable frequency oscillator is fixed in the circuit that controls the flashing frequency. The

speed is measured by adjusting the frequency so that the moving objects are visible only at

specific intervals of time. Thus this method is useful for only those types of motion which occur

regularly after fixed intervals of time, such as application or rotation.

A.C. Tachometer: - An AC Tachometer, we have a rotating magnetic which may be either

permanent magnet or an electromagnet.

Am e.m.f is induced in the stator coil when magnet rotates. The amplitude and frequency of

this email e.m.f are both proportional to the speed of rotation. Thus either amplitude or

frequency of induced voltage may be used as a measure of rotational speed.

D.C Tachometer: - These consist of a small armature which is coupled to the machine whose

speed is to be measured.

Since the e.m.f. generated is proportional to the product of flux and speed , but the flux of

permanent magnet is constant, the voltage generated is proportional to speed. The e.m.f is

measured with the help of moving coil voltmeter having a uniform scale and calibrated directly

in terms of speed.

A series resistance is used in the circuit for the purpose of limiting the current from the

generator in the event of a short circuit on the output side.

Toothed wheel rotor type speed sensor :-

These sensors are of induction type.

As the speed of toothed rotor is varied, magnetic field associated with coil varies accordingly.

When toothed rotor rotates as per the speed to be measured, the teeth of rotor pass the

magnet and disturb the magnetic field, which in turn induces voltage pulse in the coil .

Magnetic Type Speed sensor: - In this technique, magnets are used to sense speed . A no. Of

magnets are mounted on rotating member. Near to the rotating magnets, an assembly of Reed

switch is kept. These reeds switch open or close when they pass thorough magnetic poles. The

frequency of open close is proportional to magnetic poles speed. The output is thus a pulse

train whose frequency is in proportion to the rotation speed.

INSTRUMENTATION

CHAPTER-5

(MEASURMENT OF PRESSURE)

* PRESSURE- Pressure is defined as the amount of force applied to a surface or distribute it and

is measured as force per unit area.

P=F/A where; P=Pressure

F= force

A= Area

SI unit of pressure is Newton per meter square.

DIFFERENT TYPE OF PREESURE - 1. GAUGE PRESSURE

2. ABSOLUTE PRESSURE

3. VACUM PRESSURE

4. STATIC PRESSURE

5. VELOCITY PRESSURE.

1. GAUGE PRESSURE- The algebraic difference between the total pressure exerted by a fluid

and the pressure exerted by atmosphere.

Pg=Pa-Ps

Where; Pg=Gauge pressure

Pa=Absolute pressure

Ps=Atmospheric pressure existing at the time and place of measurement.

2. ABSOLUTE PRESSURE- The actual total pressure exerted by a fluid.

3. VACUM PRESSURE- This is another case of differential pressure and is defined by- V=Ps-Pa

4. STATIC PRESURE- When the fluid is in equilibrium state, the pressure at a particular point is

identical in all direction and is independent of orientation.

5. VELOCITTY PRESSURE- The difference between the total pressure and the static pressure.

* BOURDAN TUBE - It is most widely used pressure summing element. It consists of a narrow

bore tube elliptical cross section sealed one end. The pressure is applied at the other end which

is open and fixed.

ADVANTAGE OF BOURDAN TUBE-

1. Simple in construction.

2. It is available in many range.

3. High accurate

4. Low cost.

DISADVANTAGE OF BOURDAN TUBE

1. Susceptibility of hysteresis

2. Susceptibility to shock and vibration

THE BOURDAN TUBE ARE FOIIOWING TYPE

A) C-TYPE

b) SPIRAL

c) TWISTED TUBE

d) HELICAL TUBE.

a) C-TYPE- The C- type of bourdon element id used for local indication and pressure

transmission and control application. The tube is oval in section is formed into an arc of 250o

and the named C for the configuration show in figure

P is the applied pressure measured in N/m2.

Therefore, the relationship between the tip and the applied pressure is non linear., each

20180410_075211pressure does not produces a corresponding movement.

The normal accuracy is about = 1%.

b SPIRAL TYPE- The displacement of the tip and varies inversely to the wall thickness and

depend upon the cross section area of the tube. It is varies directly as the length of arc . The

displacement increased by the increased in length of arc.

While using the spiral or helix type bourdon tube, there is no need for amplification and hence

the greed section is not required.

Spiral tube are made by winding a number of turns of the tube with its fleeted. When the

pressure is to be measured is applied to the spiral, it tries to uncoil and produced a relatively

long movement of the tip whose displacement can be used for indication.

c HELICAL TYPE:

A helical type bourdon type tube shown in fig. It is similar to a spiral element, except that it is

wound in the form of helix. The displacement of the helical tube is more than the spiral type

tube . A central shaft is installed within a helical element and the pointer is driven in the shaft

by connecting links. The system transmits only the circular motion of the tip to the pointer

which is directly proportional to the change in pressure.

ADVANTAGE OF HELICAL TYPE: 1. High over range capabilities in the ratio of 10:1.

2. It has the good stability in fluctuating pressure element.

The no. of coil employed in helix element depends upon the pressure to be measured.

Bourdon tube are made of different material such as ; brass, alloys, steel, stainless steel, copper,

Ni-spam c & k money.

SECONDARY TRANSDUCER: The displacement created by the force summing member is

converted into a change of some electrical parameters.

1. Resistive transducer

2. Inductive transducer

3. Capacitive transducer

4. LVDT transducer

5. Piezoelectric transducer

6. Photo electric transducer

7. Oscillation transducer

1. RESISTIVE TRANSDUCER: The electric strain gauge attached to a diaphragm is used for

measurement the pressure.

The output of these strain gauge is a function of a local strain gauge, which is a function of

diaphragm deflection and the differential pressure. The deflection generally follows a linear

variation with differential pressure p= pg -p1 change in the resistance of strain gauge due to the

application of pressure is calibrated in term of the differential pressure.

The greater disadvantage of this method is this small physical area is required for mounting the

strain gauge.

2. INDUCTIVE TRANSDUCER: Inductive transducer with a diaphragm shown in fig. Use an

arrangement which has two coils. There is an wooer and lower coil which forms the two arms

of an a.c bridge. The coil has equal no. of term.

The other arm of the bridge is formed by two equal resistances of value R each. The diaphragm

is symmetrically placed with respect to the coils. When P1= p2, the reluctance of the path of

magnetic flux for both the coil are equal.

The initial self inductance = N2\Ro

N= no. of turns

Ro = Initial reluctance of the path

3. CAPACITIVE TRANSDUCERS: this ARE USED FOR THE measurement of the pressure by

converting the pressure into displacement. The use of three terminal variable differential circuit

capacitor shown in fig.

Spherical depression of a depth of about 0.025 nm is grounded into the glass discs. These

depressions are coated with the gold and form two fixed plate of differential capacitor. A thin

stainless steel diaphragm is clamped between the discs.

With equal pressure applied (p1= p2) the diaphragm in neutral position and the bridge is

balanced. If the one pressure is made greater than the other, the diaphragm deflect in

proportional on the differential pressure , giving an output voltage from the bridge terminal .

This output voltage is proportional to the differential pressure

The use of the capacitive transducer is not common because of low sensitive. It also required

high carries frequency dynamic pressure measurement.

ADVANTAGES: 1.They has short time repine.

2. They are vibration proof.

3. They are extremely sensitive.

4. the measure static as well as dynamic pressure .

DISADVANTAGES: 1. Sensitive change with temp.

USE: 1. they are used in the distribution network.

2. They are used in processes instrumentation.

4. LVDT: The LVDT is used as a secondary transducer for the measurement of pressure. Bellows

or bourdon tube acts as a primary transducer i.e. as a force summing device. The pressure is

converted into displacement which is sensed by the LVDT and is transducer into voltage.

6. PIEZOELECTRIC TRANSDUCER:

Piezoelectric crystal produced an e.m.f when -they are deformed. The pressure to be

measured is applied to the crystal through a force summing member. The cause of

deformation which produced an e.m.f which is a function of the deformation. This

output .e.m.f may to measured to know value of applied force and hence the pressure.

ADVANTAGE:

1. Piezoelectric transducer need no external power and are therefore self generating.

2. They have a very good high frequency response.

DISADVANTAGES-

1) These transducer cannot measure static measure.

2) Output change with change in temp.

7. PHOTOELECTRIC TRANSDUCER-

The photoelectric transducer make us of the properties of a photo emissive cell or photo

tube, The cure net through the phototube produced as a reset of a higher incident light

is legible. The current is the output of the photo electric transducer.

The displacement of the force summing member modulates quantity of incident light falling on

the photo tube. (Since applied pressure change the position of the force summing member

which in turn change the position of window thus a change in incident light) this transducer use

a stable source of light or an a.c modulate light.

ADVANTAGE- 1) they have a high efficiency.

2) They can be used for static or dynamic.

DISADVANTGE - 1) They have a poor long stability.

8. OSCILLATION TRANSDUCER-

This transducer uses a force summing member to change the capacitance, C or

Induction L of an LC oscillator circuit. The basic element of LC transducer oscillator

whose output frequency is affected by a change in inductance in coil. The change in the

inductance is changed by the force summing member acting upon a inductive device.

ADVANTAGE-

1) This transducer measured both static and dynamic phenomena.

2) These transducer is very useful to telemetry application.

DISADVANTAGE-

1) It has very limited temperature range.