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.
Recommended