Measurement Standards

By Prof N D Sadaphal

Assistant Professor

Sanjivani College of Engineering, Kopargaon (Maharashtra State) 423601

Mechanical Engineering

7/17/2016

1

MechanicalEngineeringCOE, KopargaonProf N D SadaphalAssistant Professor

Engineering Metrology and Quality Control

Measurement Standards

UNIT- IMETROLOGY

AND MEASUREMENTS

7/17/2016

2

Metrology.Metrology defines as the Science of puremeasurement. But in engineering purposes,it in restricted to measurements of lengthand angles and other qualities which areexpressed in linear or angular terms.

Definition of Standards: • A standard is defined as “something that is set up and established by an authority as rule of the measure of quantity, weight, extent, value or quality”.• Rule which is universally accepted.

7/17/2016

3

Line and End standard measurements• Line standardLength is expressed as the distance between two lines.• End standardLength is expressed as the distance between two flat parallel faces• Wavelength standardWavelength of monochromatic light is used to measure length.

• Precision Degree of repetitiveness. If an instrumentis not precise it will give different results for the samedimension for the repeated readings.• Accuracy The maximum amount by which the resultdiffer from true value(ie) Closeness to true value

Terminology in Measurment

7/17/2016

4

• AccuracyAccuracy is how close a measured value is to the actual (true) value.

• PrecisionPrecision is how close the measured values are to each other.

Examples of Precision and Accuracy:

Characteristics of Measuring Instruments• Accuracy• Precision• Sensitivity-

The sensitivity denotes the smallest change in the measured variable to which the instrument responds

• Resolution-The least count of any instrument is taken as the resolution of the instrument.

• Stability-It is the ability of an instrument to retain its performance throughout isspecified operating life.

• Range or span-The minimum & maximum values of a quantity for which an instrument is designed to measure is called its range or span.

7/17/2016

5

Gauge R&RRepeatability and Reproducibility in measurement systems

Repeatability-The ability of an operator to consistently repeat the same measurement of the same part, using the

same gage, under the same conditions.Operator 1 measures a single part with Gage A 20 times, and then measures the same part with Gage B.

The solid line is the measurements from Gage A. The dashed line is the measurements from Gage B. GageA has less variation, so it is more repeatable than Gage B.

Gauge R&RRepeatability and Reproducibility in measurement systems

ReproducibilityThe ability of a gage, used by multiple operators, to consistently reproduce the same measurement of thesame part, under the same conditions.Operators 1, 2, and 3 measure the same part 20 times with the same gage.

The three lines are the measurements from Operator 1, 2, and 3. The variation in average measurementsbetween Appraisers 1 and 2 is much less than the variation between Appraisers 1 and 3. Therefore, thegauge's reproducibility is too low.

7/17/2016

6

What/why is a gage R&R study?• A gage R&R study helps you investigate: Whether your measurement system variability is small compared with the

process variability. How much variability in the measurement system is caused by differences

between operators. Whether your measurement system is capable of discriminating (good judgement) between different parts.

For example, several operators measure the diameter of screws to ensure that they meet specifications. A gage R&R study indicates whether the inspectors are consistent in their measurements of the same part (repeatability) and whether the variation between inspectors is consistent (reproducibility).

. 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 of surface finish.

7/17/2016

7

Linear measuring instruments • Straight edge (Steel rule)• Outside caliper• Inside caliper• Vernier caliper• Outside micrometer• Inside micrometer• Vernier height gauge• Vernier depth gauge• Dial gauges

Angular measurements• Measuring the angle of Taper.

1. Bevel Protractor2. Tool Makers microscope3. Sine bar4. Auto Collimator5. Sine Centre

7/17/2016

8

Measuring tools and instruments

Direct (contact) measurement(e.g. micrometer or caliper) Indirect (non-contact) measurement

(advanced methods such as optical, ultrasonic, laser, etc.)

h Calipersh Gauges and Gauge Blocksh Sine Barh Special-purpose tools

h Rulesh Vernier Calipersh Vernier Gaugesh Micrometersh Protractorsh Dial Indicators

1

Measuring tools and instruments

Graduated(either linear or angulargraduations incorporatedinto measuring system ofthe tool)

Non-graduated(gauges or adjustable tools which compare the measurements)

Imperial steel rule with various lengthshaving graduations on each side

Same rule with relatively largergraduations

Metric steel rule with various lengthshaving graduations on each side

resolution?How to read a rule:h A = 12 mm (12th graduation)h B = 22 mm (22nd graduation)h C = 31.5 mm (betweenh D = 40.5 mm (between

31st

40thand 32nd)and 41st)

2

Graduated Linear Measurement - Rules

7/17/2016

9

3

Graduated Linear Measurement - Vernier Calipers

Direct reading of an internal lengthusing digital Vernier caliper Direct reading of an external length

using digital vernier caliper

Vernier caliper with a dial indicator 4

Graduated Linear Measurement - Vernier Calipers

7/17/2016

10

Designed for use in toolrooms, workshops, inspection departments to measure or mark off verticalheights and locating center distances.

Standard Height gauge Dial Height Gauge Digital Height Gauge5

Graduated Linear Measurement - Vernier Height Gauges

Designed for use in toolrooms, workshops, inspection departments to measure depths of holes, slots,recesses, and so on.

Standard Depth Gauge Dial Depth Gauge Digital Depth Gauge6

Graduated Linear Measurement - Vernier Depth Gauges

7/17/2016

11

THIMBLE READINGVERNIER READING

Metric MicrometerSLEEVE (BARREL) READING

Metric Vernier Micrometer

7

Graduated Linear Measurement - Outside Micrometers

V-anvil Micrometer (measuring odd-fluted taps, millingcutters, reamers, and checking out of roundness)Dial-indicating Micrometer

Screw Thread Micrometer(measuring pitch diameter

of screw threads)Direct-reading

Micrometer

8

Graduated Linear Measurement - Outside Micrometers

7/17/2016

12

Standard Inside Micrometers Digital Inside Micrometers

9

Graduated Linear Measurement - Inside Micrometers

h Standard calipers have a fine adjustment screw and a quick-adjusting spring nut.h Accuracy obtained with these tools depends mostly on the inherent skill of users.h The measurements are carefully transferred to a graduated measuring tool.

Caliper for insidemeasurement

Caliper for outsidemeasurement

Caliper usedas a divider

13

Non-Graduated Linear Measurement - Calipers

7/17/2016

13

Screw Pitch Gauges (consisting of a metal case containingmany separate leaves. Each leaf has teeth corresponding to

a definite pitch. By matching the teeth with the thread on work, the correct pitch can be read directly from the leaf)

Tap and Drill Gauges (consistingof a flat rectangular steel plate with

holes accurately drilled and identified according to their size) Radius Gauges (available as individual leaves and each

leaf is marked with its radius. They are designed to check both convex and concave radii) 15

Non-Graduated Linear Measurement - Special Purpose Gauges

16

Non-Graduated Linear Measurement - Rectangular Gauge Blocks

Slip Gauge Box

7/17/2016

14

(a)(b)Simple

Protractor (measuring angles from0 to 180º) Universal Bevel Protractor (main

scale consists of 4 portions of 90º) Measuring acute (a) andobtuse (b) angles

How to read an angle on a bevel protractor:Main div. = 1º = 60´Vernier div. = 1/12th of main div. ≈ 0.0833º = 5´h The highest figure: 50 * (main div.) = 50ºh The matching figure: 4 * (vernier div.) ≈ 0.333º = 20´h The final reading is: ≈ 50.333º or 50º 20´ 11

Graduated Angular Measurement - Protractors

*

Limitations of Sine Bar:Maximum angle 45°Sine Center - 60° 18

Non-Graduated Angular Measurement - Sine Bar

7/17/2016

15

Calibration• Calibration is one of the primary processes used to maintain

instrument accuracy.• Calibration is the process of configuring an instrument to provide a

result for a sample within an acceptable range.• Measurement of Accuracy.• Establishment the relation of an instrument’s accuracy to the

international standard.

Success of Calibration• Consistency of results obtained

Need of Calibration• Quality control & quality assurance in production.• To meet requirement of ISO• To comply with requirement of global market.• To promote international recognition.Benefits of Calibration• Fulfils requirement of ISO 9000, ISO 14000. • As a proof that the instrument is working properly.• Confidence in using instrument.• Reduce rejection, failure rate.• Improved product & service quality leading to satisfied customer.• Cost saving, safety.

7/17/2016

16

Types and sources of ERRORSSystematic Errors• Systematic errors are regularly repetitive and can be eliminated. • They results from improper condition or procedure of experiment . • These error can be controlled & reduced if properly analyze, so called as Controllable errors.Errors may be of four kinds; 1. Instrumental : For example, a poorly calibrated instrument such as a thermometer that reads 102° C when immersed in boiling water and 2°C when immersed in ice water at atmospheric pressure. Such a thermometer would result in measured values that are consistently too high.

2. Observational : For example, parallax in reading a meter scale.3. Environmental : Variation in atmospheric condition i.e. temperature, pressure etc. at place of measurement.4. Stylus pressure :Variation in Force applied by anvils of micrometer on component to be measured results in different reading.

7/17/2016

17

Random Errors• Random errors in experimental measurements are caused by unknownand unpredictable changes in the experiment. These changes may occur inthe measuring instruments or in the environmental conditions.• Sources of random errors cannot always be identified. Possible sources ofrandom errors are small variations in the position of setting standards andwork piece, slight displacement of lever joints in the measuring joints inthe measuring instrument.• Examples of causes of random errors are:1. electronic noise in the circuit of an electrical instrument,2. Irregular changes in the heat loss rate from a solar collector due to changes in the wind.• These error cannot be eliminated.1. Observational : For example, errors in judgment of an observer whenreading the scale of a measuring device to the smallest division.2. Environmental : For example, unpredictable fluctuations in line voltage,temperature, or mechanical vibrations of equipment.

Parallax Error :• Parallax is a displacementor difference in the apparentposition of an object viewedalong two different lines of sight,and is measured by the angle orsemi-angle of inclinationbetween those two lines.

7/17/2016

18

Comparators• Classification of comparators1. Mechanical2. Electrical and Electronics comparators3. Optical comparators4. Pneumatic comparators5. Electro-Mech. Comparators.