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UNIT- IMETROLOGY
UNITS AND MEASUREMENTS
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Metrology.
Metrology defines as the Science of
pure measurement. But in engineering
purposes, it in restricted tomeasurements of length and angles
and other qualities which are
expressed in linear or angular terms.
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Units and Standards
Units of Measurement:
C.G.S. System of Units
Centimeter Gram Second system of unit
M.K.S. System of Units:
Meter kilogram second system of units
International System (SI) of Units:
the meter (m), kilogram (k), second (s), and
ampere (A) of the MKSA system and, in addition,the Kelvin (K) and the candela (cd) as the units oftemperature and luminous
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Precision Degree of repetitiveness. If an
instrument is not precise it will give different
results for the same dimension for the
repeated readings.
Accuracy The maximum amount by which
the result differ from true value(ie) Closeness
to true value
Terminology in instrumentation
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Calibration
is the process of establishing the relationship
between a measuring device and the units of
measure. This is done by comparing a devise
or the output of an instrument to a standard
having known measurement characteristics.
Sensitivity
It is ratio between output signal to input signal
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Readabilityis a measure of an instrument's
ability to display incremental changes in its
output value.
True sizeTheoretical size of a dimension
which is free from errors.
Actual sizesize obtained through
measurement with permissible error
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Repeatability is the variation in measurementstaken by a single person or instrument on the sameitem and under the same conditions. Ameasurement may be said to be repeatable when
this variation is smaller than some agreed limit. Reproducibilityis one of the main principles of the
scientific method, and refers to the ability of a testor experiment to be accurately reproduced, orreplicated, by someone else workingindependently.
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Methods of measurement.
1. Direct Method
2. Indirect Method
3. Comparison Method
4. Coincidence Method.
Classification of measuring instruments.
1. Angle measuring instruments
2. Length measuring instruments
3. Instruments for surface finish
4. Instruments for deviations.
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Sources of error
Controllable Errors-
Calibration Errors ,ambient Conditions , Styluspressure, avoidable errors
Random Errors These occur randomly and the specific causes of
such errors cannot be determined, but likelysources of this type of error are small variationsin the position of setting standards andworkpiece, slight displacement of lever joints inthe measuring joints in the measuringinstrument,
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Parallax Error :
On most dials the indicating finger or pointer
lies in a plane parallel to the scale but
displaced a small distance away to allow freemovement of the pointer. It is then essential
to observe the pointer along a line normal to
the scale otherwise a reading error will occur.
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Line and End standard measurements
Line standard
Length is expressed as the distance between
two lines.
End standard
Length is expressed as the distance between
two flat parallel faces
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Linear measuring instruments
Straight edge.
Outside caliper.
Inside caliper.
Vernier caliper
Screw gauge
vernier height gauge
vernier depth gauge
Dial gauges
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Comparators Classification of comparators
Mechanical
Electrical and Electronics comparators
Optical comparators
Pneumatic comparators Fluid displacement comparators
Projection comparators.
Multi check comparators
Automatic Gauging Machines
Electro-Mech. Comparators.
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. Classification of measuring
Instruments.
According to the functions:
Length measuring instrument
Angle measuring instrument Instrument for checking deviation from
geometrical forms
Instrument for determining the quality ofsurface finish.
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According to the accuracy.
1. Most accurate instruments
Example - light interference instrument
2. Less accurate instrument
Example - Tool room Microscope,Comparators, Optimizer
3. Still less accurate instrument
Example - Dial indicator, vernier caliper.
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Angular measurements
Measuring the angle of Taper.
1. Vernier bevel Protractor
2. Tool room microscope
3. Sine bar and dial gauge
4. Auto Collimator
5. Taper measuring machine
6. Roller, Slip gauge, and micrometer.
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Angle measurement
Sine bar
Sine Centre: Sine Table
Taper Measurement
Using Precisions Balls and Rollers:-
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Slip Gauges
Direct precise measurement, where the accuracy of the work piece demands it.
For checking accuracy of venire calipers, micro metes, and such other measuringinstruments.
Setting up a comparator to specific dimension.
For measuring angle of work piece and also for angular setting in conjunction witha sine bar.
The distances of plugs, spigots, etc. on fixture are often best measured with theslip gauges or end bars for large dimensions.
To check gap between parallel locations such as in gap gauges or between twomating parts.
Slip gauges are rectangular blocks of high grade steel with exceptionally close
tolerances. These blocks are suitably hardened though out to ensure maximumresistance to wear. They are then stabilized by heating and cooling successively instages so that hardening stresses are removed.
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Surface finish measurement
Surface finish refers to the quality finish orroughness over the surface.
Surface texture:
Repetitive or random deviations form thenormal surface which form the pattern of thesurface. Surface texture include roughness,waveness, lay and flows.
. Primary texture : This refers to the roughnessof a surface, as opposed to its waviness(secondary texture)
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Methods of measuring surface finish
. 1) Surface Inspection (or) comparison method
2. Direct Instrument
a) Touch Inspection
b) Visual Inspection
c) Scratch Inspection
d) Microscopic Inspection
e) Surface photograph
f) Micro - Interferometer
g) Wallace surface Dynamometer
h) Reflected light Intensity
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Surface finish measuring instruments
Profilometer.
The Tomlinson Surface Meter
Taylor-Hobson Talysurf.
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UNIT IV
TEMPERATUREMEASUREMENTS
CLASSIFICATION OF
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CLASSIFICATION OF
TEMPERATUREMEASURING
EQUIPMENTS Classification based on the Nature of Change Produced.
1. Glass thermometers
2. Pressure gauge thermometers 3. Differential expansion thermometers
4. Electrical resistance thermometers
5. Thermo couples
6. Optical pyrometers
7. Radiation pyrometers
8. Fusion pyrometers
9. Calorimetric pyrometers Based on Electrical and non-electrical Principles
1. Primarily electrical or electronic in nature
2. Not primarily electrical or electronic in nature.
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Bimetallic Thermometers:
Principle Involved : These use the principles
of metallic expansion when temperature
changes.
A bimetallic strip is shown in figure which
is straight initially. When temperature
changes, its shape also changes into an arc.
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BIMETALIC THERMOMETER USE
The displacement of the free end can be converted into an electric signalthrough use of secondary transducers like variable resistance, inductanceand capacitance transducers. Figure shows a strip of bimetal in the form ofa spiral. The curvature of the strip varies with temperature. This causesthe pointer to deflect. A scale is provided which has been calibrated toshow the temperature directly.
This kind of spiral is mostly used in devices measuring ambient
temperature and air-conditioning thermostats.
Advantages of Bimetallic Thermometers
1. Simple
2. Inexpensive
3. Accuracy of 0.5% to 2%
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RESISTANCE THERMOMETERS
Basic principle of resistance thermometers?
When an electric conductor is subjected to
temperature change the resistance of the
conductor changes. This change in resistance
of the conductor becomes a measure of the
change in temperature when calibrated.
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Thermocouples
Principles Involved : When heat is applied to
the junction of two dissimilar metals, an e.m.f.
is generated. (Figure)
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Thermistors:
Thermistor is a temperature sensitive variable resistormade of a ceramic like semiconducting material. They aremade of metal oxides and their mixtures like oxides ofcobalt, copper, nickel, etc. Unlike metals, thermistorsrespond negatively to temperature. They behave as
resistors with a high negative temperature coefficient ofresistance. Typically, for each 1C rise in temperature, theresistance of a thermistor decreases by about 5%. This highsensitivity to temperature changes makes the thermistoruseful in precision temperature measurements. The
resistance of thermistors vary from 0.5 to 0.75M .Variation of resistivity with temperature is shown in figure.
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UNIT III
FLOW MEASUREMENT
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FLOW METERS
Flow meter measures the actual flow rate.
TYPES OF FLOWMETERS
VENTURIMETER
PITOT TUBE
FLOW NOZZLE
ORIFICE PLATE
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VENTURIMETER
USES
1. Low head loss about 10% of differentialpressure head.
2. High co-efficient of discharge.
3. Capable of measuring high flow rates inpipes having very large diameter.
4. Characteristics are well established so theyare extensively used in process and otherindustries.
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VENTURI PRINCIPLE
This is just like an orifice meter. It has three distinct parts, namelyconvergent cone, throat and divergent cone. A manometermeasures the pressure difference between two sections as shownin figure.
Let a1 - Area at the inlet (1-1)
A2 - Area at the section (2-2)
x - Pressure head difference
Cd - Discharge coefficient
,
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d 1 2
2 2
1 2
C a a 2 g x
a a
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Orifice METER
Let a1Area at section I-I
a0Area of orifice
CdDischarge coefficient
Then, Flow rate
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ROTO METERS
Rotameter:
A rotameter is a variable area type flow meter. It consists ofa vertical tapered tube with a float which is free to movewithin the tube. The fluid goes from the bottom to the top.When no fluid flows, the float rests at the bottom of thetube. The float is made of such a diameter that itcompletely blocks the inlet. When flow starts in thepipeline and fluid reaches the float, the buoyant effect offluid makes the float lighter. The float passage remainsclosed until the pressure of the flowing material plus the
buoyance effect exceeds the downward pressure due to thefloat weight. Thus, depending on flow, the float assumes aposition. Thus the float gives the reading of flow rate.
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Pitot Tube
Principle: Transformationof kinetic energy of
a liquid into potential energy in the form of a
static head.
Figure shows a pitot tube installed in a
pipeline where it acts like a probe. The tube
consists of two concentric tubes, the inner
tube with its open ends facesthe liquid.
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Pitot tube principle
outer tube has a closed end and has four to eight holesin its wall. The pressure in the outer tube is the staticpressure in the line. Total pressure is sum of staticpressure and the pressure due to the impact of fluid.
If P - Pressure at inlet (Stagnation pressure)
Ps - Static pressure
- Density, then
Velocity v = from which flow rate is determined.
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UNIT V
FORCE MEASUREMENT
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FORCE MEASUREMENT
Force.
The mechanical quantity which changes or
tends to change the motion or shape of a
body to which it is applied is called force.
.Force measureing equipments
load cells
Load cells are devices used for force
measurement through indirect methods.
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Force measuring equipments
Scale and balance a. Equal arm balance
b. Unequal arm balance
c. Pendulum scale
2. Elastic force meter Proving ring
3. Load cell
a. Strain gauge load cell b. Hydraulic load cell
c. Pneumatic load cell
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Torque measuring equipments
Mechanical torsion meter
Optical torsion meter
Electrical torsion meter
Strain gauge torsion meter
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Types of strain gauges.
Unbonded strain gauge
Bonded strain gauge
Fine wire strain gauge
Metal foil strain gauge
Piezo-resistive strain gauge
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PROVING RING
Use of proving Rings
Proving rings are steel rings used for calibration of material testing machines
in situations where, due to their bulkness, dead weight standards cannot be used.
P ring is a circular ring of rectangular section and may support tensile or
comprehensive force across its diameter.
the change in radius in the direction of force, is given by where d is the outer diameter of the ring and
K is stiffness.
Deflection of the ring is measured using a precision micrometer. To get precise
measurements, one edge of the micrometer is mounted on a vibrating reed which
is plucked to obtain a vibratory motion. The micrometer contact is then movedforward until a noticeable damping of the vibration is observed.
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LOAD CELLS
Use of Load Cell
Force transducers intended for weighingpurposes are called load cells. Instead of using
total deflection as a measure of load, straingauge load cells measure load in terms of unitstrains. A load cell utilizes an elastic memberas the primary transducer and strain gauges as
secondary transducer. Figure shows one suchload cell arrangement.
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DYNAMO METERS
Mechanical Dynamometer:
These come under the absorption type. An example for this kind is prony brake.
In Prony brake, mechanical energy is converted into heat through dry friction between
the wooden brake blocks and the flywheel (pulley) of the machine. One block carries a lever
arm. An arrangement is provided to tighten the rope which is connected to the arm. Rope is
tightened so as to increase ht frictional resistance between the blocks and the pulley.
If F Load applied and
Power dissipated
r - Lever arm
N Speed of flywheel (rpm)
Torque T = F.r
The capacity of Prony brake is limited because: Due to wear of wooden blocks, friction coefficient varies. So, unsuitable for large powers
when used for long periods.
To limit temperature rise, cooling is to be ensured.
D C D namometer
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D.C. Dynamometer
D.C. dynamometer is usable as an absorptionas well as transmission dynamometer. So, it findsits use in I.C. Engines, steam turbines and pumps.A d.c. dynamometer is basically a d.c. motor with
a provision to run it as a d.c. generator where theinput mechanical energy, after conversion toelectrical energy, can either be dissipated througha resistance grid or recovered for use. When usedas an absorption dynamometer it acts as d.c.generator. (figure) Cradling in trunnion bearingspermits the determination of reaction torque.
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Eddy CURRENT DYNAMOMETER
Current or Inductor Dynamometers:
This is an example for absorption type dynamometers.
Principle: When a conducting material moves through a magnetic flux field,
voltage is generated, which causes current to flow. If the conductor is a wire
forming a part of a complete circuit will be caused to flow through that circuit, andwith some form of commutating device a form of a.c. or d.c. generator may result.
An eddy current dynamometer is shown in figure. It consists of a metal disc or
wheel which is rotated in the flux of a magnetic field. The field if produced by field
elements or coils excited by an external source and attached to the dynamometer
housing which is mounted in trunnion bearings. As the disc turns, eddy currents
are generated. Its reaction with the magnetic field tends to rotate the complete
housing in the trunnion bearings. Water cooling is employed.
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