Ppt Auto 5sem EM

8/10/2019 Ppt Auto 5sem EM

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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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