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1 0(752/2*< $1' 0($685(0(176 (For IV Sem B.E. Mechanical Engineering Students) (As per Anna University New Syllabus) Dr. A.Anderson, YVS. Karthik, S.Baskar, Dr.S.Ramachandran, AIR WALK PUBLICATIONS (Near All India Radio) 80-A, Karneeshwarar Koil Street, Mylapore, Chennai Ph.: , aishram2006@gmail.com

2 First Edition: 9th August 2014 All Rights Reserved by the Publisher This book or part thereof should not be reproduced in any form without the written permission of the author and publisher. Books will be door delivered after payment into AIR WALK PUBLICATIONS A/c No (IFSC: BKID ) Bank of India, Santhome branch, Mylapore, Chennai - 4 (or) S.Ramachandran, A/c.No (IFSC:IDIB000S201), Indian Bank, Sathyabama University Branch, Chennai Typeset by: aksharaa muthra aalayam, Chennai Ph.: Printed at: Abinayaram Printers, Chennai - 4. Ph.: ,

3 Contents C.1 &217(176 Chapter 1 GENERAL CONCEPT 1.1 Introduction Types of Metrology Legal Metrology Deterministic Metrology Significance of Measurement Need for Measurement (Inspection) Means of Measurement Methods of Measurement Generalised Measurement System Primary Stage (or) Primary Element Secondary stage or Conversion stage Manipulation Stage Final or Readout-Recording Stage Units and Standards SI System Table for derived units Definitions Standards Standard Systems of Measurement Line Standard End Standard Difference between Line standard and End standard Wavelength standard

4 C.2 Engineering Metrology and Measurements Classification of Standards Primary Standard Secondary Standard Tertiary Standards Working Standards Advantages of standards Traceability Measuring Instruments Classification of measuring instruments Sensitivity Sensitivity for linear instrument Sensitivity for non-linear Instrument Solved Problem Stability Readability Range Accuracy Precision Accuracy Vs Precision Difference between Accuracy and Precision Characteristics of a Measurement System Static And Dynamic Response Definitions Static Error Dynamic Error Percentage Error Correction Dead-zone

5 Contents C.3 Threshold Hysteresis Drift Resolution Speed of response Lag Fidelity Trueness Uncertainty Linearity Repeatability Errors in Measurement Bias or Systematic or Controllable errors Random or Precision errors Illegitimate errors Systematic Errors (a) Calibration Errors (b) Ambient Errors (c) Loading Errors (d) Errors caused by defective equipment (e) Avoidable Errors Random Errors Characteristics of Random Errors Difference between Systematic and Random Errors Correction Calibration (a) Primary calibration (b) Secondary calibration (i) Direct calibration

6 C.4 Engineering Metrology and Measurements (ii) Indirect calibration Dimensional And Geometric Tolerancing Introduction Basic Size Limits and fits ISO System of Limits of Fits Field of use of Individual Tolerances System of fit (a) Hole basis system (b) Shaft basis system Geometric Tolerancing Geometric Characteristics and Symbols Feature Control Symbol Geometric Tolerancing Explanation Form Tolerances Profile Tolerances Orientation Tolerances Location Tolerances Runout Tolerances Chapter 2 LINEAR AND ANGULAR MEASUREMENTS 2.1 Definition of Metrology Main activities of Metrology Types of Metrology Scientific Metrology Industrial Metrology Legal Metrology Deterministic Metrology...2.3

7 Contents C Elements of Measurement Linear Measurements Steel Rule Calipers Dividers Vernier Caliper Construction Working Principle of Vernier Minimum length or Thickness measurable with the Vernier calipers is called its least count Types of Vernier Calipers Limitations of Vernier Calipers Errors in Measurement with Vernier Caliper Precautions in using a Vernier Caliper Applications of Vernier caliper Care in use of Vernier Calipers Vernier Height Gauge Construction Working Important Tests on Vernier Height Gauge Sources of Errors in Vernier Height Gauges Precautions While Using A vernier Height Gauge Applications of Vernier Height Gauges

8 C.6 Engineering Metrology and Measurements Master dial indicator vernier caliper Vernier Depth Gauge Construction Working Errors in Vernier Depth Gauge Micrometer (Screw Gauge) Principle Construction Working To obtain the reading Terminology Specifications for External Micrometers Errors In Micrometers Precautions Types of Micrometer (a) Outside micrometer (b) Inside Micrometer (i) Inside Micrometer (Caliper) (ii) Inside micrometer (Tubular) Working (Method of Measurement) (iii) Self centering inside micrometer (iv) Stick Micrometer Testing of Internal Micrometers (c) Depth micrometers Special Function Micrometers (i) Thread Micrometer Caliper (ii) V-Anvil Micrometer (iii) Thickness Micrometer (iv) Blade Type Micrometer

9 Contents C.7 (v) Groove Micrometer Special (Advanced) Micrometer Instruments (i) Vernier Micrometer (ii) Bench Micrometer (iii) Digital Micrometers Maintenance for a digital micrometer Slip Gauges Classification of Slip Gauges Method to determine the gauge block combination Applications of Slip Gauges Care in using Slip Gauges Advantages of Ceramic Slip Gauges Slip Gauge Accessories Tool Makers Microscope Construction Working Applications Determination of the Relative Positions Measurement of Angles Comparison Measurement Comparison with a Scale Angular Measuring Instruments Types Bevel Protractor (i) Mechanical bevel protractors Type A Type B Type C

10 C.8 Engineering Metrology and Measurements (ii) Optical Bevel Protractor Universal Bevel Protractor Applications of Bevel Protractor (i) For checking the inside bevelled face of a ground surface (ii) For Checking of Vee block (iii) For measuring acute angle Spirit Level Clinometers Angle Gauges Uses of angle gauges Sine Bar (i) Locating any work to a given angle (ii) Measurement of angles of small components (iii) Measurement of angle of heavy components (iv) Checking angles greater than Sources of Errors in sine bar Precautions while using sine bars Sine Table Sine Centre Optical Angular Measuring Instruments Autocollimator Principle of Autocollimator Construction Working Factors Governing the specification of an Autocollimator (i) Focal Length

11 Contents C.9 (ii) Objective aperture size (iii) Type of Beam Splitter (iv) Fixed or variable distance setting Applications Angle Dekkor Applications of Angle Dekkor (i) Measuring the angle of a component (ii) Checking the slope angle of a V-block (iii) Angle measurement of a cone or taper gauge Angle Alignment Telescope Gauges Gauge classification Limit Gauges Plug Gauge Ring Gauge Snap Gauge (a) Rib type snap gauge (b) Plate Snap Gauges Adjustable Type Gap Gauges Miscellaneous Gauges Combined Limit Gauge Position Gauge Contour Gauges Receiver gauges Taper Gauges Feeler Gauges Gauge Design Taylor s Principle

12 C.10 Engineering Metrology and Measurements Points to be remembered for Gauge Design Material for gauges Gauge Maker s Tolerance and Wear Allowance Terms Interchangeability Parameters of Interchangeability Types of Interchangeability Chapter 3 ADVANCES IN METROLOGY 3.1 Introduction Laser Basic Concept Comparison between Laser light and light from an incandescent lamp Types of Lasers Advantages of laser Applications of Laser Laser Inspection (i) Scanning Laser Gauge (ii) Laser Telemetric system (iii) Photo Diode Array Imaging (iv) Diffraction pattern technique Laser Triangular sensors (vi) Two frequency laser interferometer Interference Principle of superposition

13 Contents C Interferometry Interferometer Applications of Interferometers Laser Interefometry Types of Laser Interferometer The Michelson Interferometer Principle of Michelson Interferometer TWYMAN-GREEN INTERFEROMETER Counting of fringes Single frequency DC Interferometer NPL Flatness Interferometer AC Laser Interferometry Description of component in A.C Laser Interferometer Two frequency laser source Optical Elements Laser head s Measurement receiver Measurement display AC Laser Interferometer (ACLI) Advantages of ACLI Heterodyne Interferometry Laser Alignment Laser equipment Straightness Coordinate Measuring Machine (CMM) NEED for CMMs Coordinate Measuring Machine Construction and Operation (a) Main Structure (b) Probing System

14 C.12 Engineering Metrology and Measurements (c) Machine Controller and Computer (d) Software Operation Types of CMM Manual CMMs CNC or DCC CMMs Bridge Type CMMs Cantilever type CMM Column Type CMMs Gantry Type CMMs Horizontal Arm CMM Benchtop CMMs Free Standing CMMs Portable CMMs Some advantages of portable CMM Advantages of CMM (a) Reduced inspection cycle time (b) Flexibility (c) Reduced operator errors (d) Improved accuracy and precisions (c) Improved productivity Probes Contact Probes Hard (or) Fixed Probes Touch trigger type Probes Measuring Type (or) Displacement Probes Non-contact Probes Optical Probes Acoustical probe

15 Contents C Laser probes Vision probes CMM Styli and Accessories Styli Applications of Different Styli Stylus Extension Stylus tools Stylus Adapters Reference Spheres Applications of CMM Machine Vision Machine Vision System - Basic Concept General Operation of a Machine Vision System Elements of Machine Vision System (a) Delivery system (b) Light source (Illumination) (c) Lenses (optics) (d) Image sensor and digitizer (e) Preprocessor (f) Vision Processor/Controller (g) Communication Links (h) Output devices Specifications of a vision system: Sensitivity and Resolution Uses of Machine Vision Systems Application of Machine Vision System3.94

16 C.14 Engineering Metrology and Measurements Chapter 4 FORM MEASUREMENT Introduction Straightness Measurement Methods of Straightness Measurement Flatness Measurement Methods of Fltness Measurement Beam Comparator Used for Flatness Testing Faltness Measurement by Interferometry Flatness Measurement Using Laser Measurement System Flatness Measurement Electro-mechanical Gauges Surface Plate Parallelism Methods of Parallelism Measurement Using Dial Indicator and Test Mandrel Using Electro-mechanical Gauges Using an Auto collimator Autocollimator Principle of autocollimator Applications Measuring the straightness of machine components Measuring flatness Other applications Thread Measurement Nomenclature of screw threads

17 Contents C Major diameter or Nominal diameter (d) Minor diameter or core diameter or root diameter (d c ) Pitch diameter or Effective diameter (d p ) Pitch (p) Lead Crest Root Depth of thread Flank Angle of thread Slope of the thread Screw thread Flank angle (θ) Helix angle Thread per inch Addendum Deddendum Thread angle Tolerance Types of threads British association thread Whitworth (BSW) threads Metric threads Measurement of screw thread Measurement of major diameter Bench micrometer Measurement of minor diameter Measurement of effective diameter (i) One wire method

18 C.16 Engineering Metrology and Measurements (ii) Two wire method (iii) Three wire method Best size wire (iv) Thread micrometer Pitch measurement (a) Tool makers microscope Measuring principle Applications Measurement of thread form Angle Shadow protector Errors in screw thread Pitch error Types of pitch error (a) Progressive error (b) Periodic error (c) Irregular error (d) Drunken error Major diameter error Minor diameter error Effective diameter error Thread Angle error Flank angle error Gears Measurement Introduction Advantages and Limitations of Gear Drive Advantages Limitations Classification of Gears Based on position of axes of the shaft (i) Parallel Shafts

19 Contents C.17 (ii) Bevel gears for Intersecting shaft (iii) Non-parallel and non-intersecting shafts Based on type of gearing (iii) Rack and pinion: (iv) Worm and Worm Wheel Terminology and Definitions Pressure angle (or) Angle of obliquity (φ): Diametral pitch (p d): Forms of Gears Errors In Spur Gear Gear Blank Run-out Errors Gear Tooth Errors Spur Gear Measurement and checking Runout Pitch measurement (a) Tooth to Tooth pitch measurement (b) Two-Dial Gauge Method (or) Direct angular measurement Profile measurement (a) Optical projection method (b) By involute measuring machine (c) Tooth displacement method (d) Computer-controlled probe scanning method Tooth thickness measurement (a) Gear tooth vernier caliper method (b) Constant chord method (c) Base tangent method (iv) Rolling Gear Test

20 C.18 Engineering Metrology and Measurements (a) Single contact method (single flank testing) (b) Double contact method (Double flank testing) Parkinson Gear Roller Tester Measurement Over Pins Or Balls Lead checking Backlash checking Concentricity measurement Alignment checking Recent development in gear metrology Surface Finish Measurement Surface Texture Types of Irregularities (a) Primary texture (Roughness) (b) Secondary texture (Waviness) Key Words Roughness Height (or) Height of unevenness 4.73 Waviness Height Difference between Roughness and Waviness4.73 Lay Specification of surface texture Production method Reasons for measuring the surface texture Factors Affecting the surface finish Important terms Analysis of Surface Finish Form factor Measurement of Surface Finish (a) Surface Inspection by comparison method

21 Contents C Touch Inspection Visual Inspection Scratch Inspection Surface photographs Reflected Light Intensity Micro Interferometer Microscopic Inspection Wallace surface Dynamometer (b) Direct Instrument method (i) Stylus probe Instrument (a) Skid (or) Shoe (b) Stylus (or) Probe (c) Amplifying device and Indicator (d) Recording device (ii) Profilometer Types of profilometer (a) Contact profilometer (b) Non-contact profilometer (iii) Tomlinson surface meter Principle Construction Operation (iv) Taylor - Hobson Talysurf Principle Construction Operation Applications Roundness (or) Circularity Types of Irregularities of a Circular Part Causes of out-of roundness:

22 C.20 Engineering Metrology and Measurements Methods of Roundness Measurement (a) Diametral Method (b) Circumferential Confining Gauge (c) Rotating on Centers (d) V-block Method Limitation of the V-Block Method (e) Three Point Probe Method (f) Roundness measuring Spindle (g) Reference Circles (i) Least Squares Reference Circle (LSCI) (ii) Minimum Circumscribed Circle (MCCI) (iii) Minimum Zone Reference Circles (MZCI) (iv) Maximum Inscribed Circle (MICI) (h) Roundness Measuring Machines Rotating Pick up type Turn table type Modern Roundness Measuring Instruments Chapter 5 MEASUREMENT OF POWER, FLOW AND TEMPERATURE 5.1 Force Measurement Direct Force Measurement (i) Analytical Balance Method (ii) Platform Balance (iii) Unequal Arm Balance Method (iv) Pendulum Scale...5.7

23 Contents C Indirect Force Measurement (i) Accelerometers (ii) Electromagnetic Balance Method (iii) Load Cells Types of Load Cells (i) Capacitive Load Cell (ii) Magnetoelastic Load Cell Pressductor (iii) Strain Gauge Load Cell (iv) Hydraulic Load Cell (v) Pneumatic Load Cell (vi) Shear Type Load Cell Elastic Loaded Members (a) Coil Springs (b) Proving Rings (c) Load cell have been already discussed (d) Electronic Weighing System Measurement of Pressure Fluid Pressure Sensors Manometers Manometers are classified as: Mechanical Gauges Pressure Measurement Methods Bourdon gauge (C-Type) Principle Mechanism and working Other types of Bourdon gauges Diaphragm-type Pressure Gauge Principle

24 C.22 Engineering Metrology and Measurements Working Capsule Pressure Sensor Bellows Principle Features of Bellows Dead Weight Pressure Gauge Capacitive Pressure Transducer Strain Gauge Pressure Transducer Piezoelectric Sensors Tactile Sensors Simple Manometers (i) Piezometer: (ii) Simple U tube manometer (iii) Single column Manometer (a) Vertical single column Manometer (b) Inclined Single Column Manometer Pressure at A Differential Manometer Under Equilibrium To Find Pressure, Problems In Simple Manometer Pressure Head at A Pressure Head at B = h 2 s Under equilibrium, Problems In Differential Manometer Under equilibrium, Torque Measurement Torque Reaction Methods Cradled Shaft Bearing Type

25 Contents C Dynamometers (i) Hydraulic Dynamometer Description Operation (ii) Eddy Current Dynamometer Strain Gauge Type Slip Ring Type Torque Measurement Using Torsion Bar Optical Method Capacitive method Magnetostrictive Type Laser-Optic Method Proximity Sensor Method Strobescope Method Saw Method Flow Measurement Types of Fluid Flow Orifice Meter Flow Nozzle Electromagnetic Flow Meters Hot Wire Anemometer Ultrasonic Flow Meter Other Flow Measurements (i) Current Meter (a) CUP Type Current Meter (b) Propeller (or) Screw type Current Meter Venturimeter Basic Principle Description Operation

26 C.24 Engineering Metrology and Measurements Application Advantages Limitations Rotameter (Variable-area Meter) Description Operation Applications Advantages Pitot Tube Advantage of pitot tube Disadvantage of pitot tube Power Measurement Types of Dynamometers (a) Absorption dynamometers (b) Driving dynamometers (c) Transmission dynamometers Different Arrangements used to find Brake Power Rope Brake Arrangement Prony Brake Arrangement Band Brake Arrangements D.C. Dynamometer Comparators Mechanical Comparators (i) Dial Gauge (or) Dial Indicator Mechanism of dial Indicator (ii) Reed type Mechanical Comparator Merits of mechanical comparator Disadvantages Pneumatic Comparator

27 Contents C.25 (a) Flow (or) velocity type pneumatic comparator (b) Back pressure type pneumatic comparator (c) Solex Air Gauge Solex pneumatic Comparator Internal measurements: External measurements: Merits of Pneumatic Comparator Disadvantages Electrical Comparator Merits Disadvantages Temperature Measurement Temperature Temperature Scales Temperature measuring devices Temperature Measuring Instruments Bimetallic Strip Thermometer Working Thermocouple Measuring Thermocouple Voltage Thermocouple Junction Advantages of Thermocouple Disadvantages Thermocouple Specifications Thermometer A Mercury-In-Glass Thermometer Resistance Temperature Detectors (RTD) RTD s Working Principle Platinum Sensing Resistors

28 C.26 Engineering Metrology and Measurements Film Style Element Standard Resistance Thermometer data Rating of temperature Sensors Wiring Configuration (RTD) RTD Design Characteristics Advantages Disadvantages Limitation Thermistor Types of Thermistors Thermistors are classified as follows: (a) Positive Temperature co-efficient (PTC) thermistors (b) Negative Temperature Co-efficient (NTC) (a) Bead thermistor (b) Washer Thermistor (c) Disc Thermistor (d) Rod Thermistor (e) Probe Thermistor Applications Advantages Disadvantages Limitations Pyrometers Types of Pyrometer Optical Pyrometer Construction Working Advantages Disadvantages

29 Contents C Total Radiation Pyrometer Advantages of radiation pyrometers are: Disadvantages Fibre-optic Pyrometers Advantages of Fibre Optic Pyrometers Infrared Pyrometers Thermal Imaging

30 General Concept 1.1 Chapter 1 GENERAL CONCEPT 1.1 INTRODUCTION For anything to be understood, it has to be expressed in numbers. The process of expressing anything (object, property, etc) in terms of numbers or obtaining quantitative information about anything is called as measurement. For this an internationally accepted predefined standard i.e unit is necessary for every kind of quantity measured. Further, an instrument or apparatus is required to measure that quantity in terms of that corresponding unit. The science of measurement is called as metrology and when related to the practice of engineering, it is called engineering metrology. The basic or fundamental measuring process can be explained as follows. It consists of (i) A measurand, which is the unknown quantity or parameter being observed and quantified. This is the input to the measuring process. (ii) Comparator or measurement where the measurand is quantitatively compared with a reference or predefined standard. Both the measurand and the standard are of the same character. The standard is usually defined by a recognised agency or organization like,

31 1.2 Engineering Metrology and Measurements ISO The International organization for standardization. NBS National Bureau of Standards. ANSI The American National Standards Institute. BIS Bureau of Indian Standards, which is the National body for standardization in India. (iii) Result - The quantitative comparisons made between the standard and unknown parameter produces a result in terms of standard units. A schematic representation is shown below. Measurand Comparator Result Standard TYPES OF METROLOGY LEGAL METROLOGY The units of measurement methods and measuring instruments used for the various measuring processes are in relations to statutory, technical and legal requirements. DETERMINISTIC METROLOGY This is used for high precision manufacturing where process measurement replaces the measurement of parts. 1.2 SIGNIFICANCE OF MEASUREMENT The quantitative information obtained through the process of measurement leads to better understanding of the physical parameters being studied. This understanding is the basis for design and development of

32 General Concept 1.3 various products, devices or processes and also for improving them. The various devices or processes developed have to perform for the intended purpose. So, to check whether the actual output is in line with the derived output, the various qualities related to the operation and performance of the device or process are measured. For any control process, the basic element is measurement. Unless the controlling portion of any system has the knowledge of the difference between the actual and derived performance, it cannot act accordingly. The proper performance of any system is ensured through measurement of the various parameters related to the system. Also economics of the any system are worked out through measurement of the various inputs and costs incurred throughout the process. Hence measurement is the basis for all engineering and commercial activities and plays a vital role in the development of science and technology. 1.3 NEED FOR MEASUREMENT (INSPECTION) In the traditional approach, both the production and assembly of a product were at the same place. So, whenever there was a unfit during assembly, either of the mating part was altered there itself to fit into the assembly. Hence, no two similar parts of an assembly were the same. However, with the demand for higher quantities with better quality and cheaper price, mass production came into existence. Here different parts of a component or product are manufactured separately.

33 1.4 Engineering Metrology and Measurements So when a large number of the same parts are produced, the need for measurement arised. To ensure the interchangeability of parts, i.e any two mating parts taken at random should satisfactorily mate with each other. To ensure the efficient use of resources. To provide quality products to the end user. The actual size of the components is not necessary. It is enough if the various dimensions of a part fall within the prescribed limits to enable satisfactory mating. This leads to the usage of gauges which save a lot of time and money. Due to inspection, the quality of the products get improved. Better inspection means better products which leads to the development of precision inspection instruments. This inturn leads to the development of better tools and processing techniques. 1.4 MEANS OF MEASUREMENT (i) (ii) Standards or Reference Masters: Standard is a predefined magnitude of a given quantity or condition. Definite values of a given quantity but may be of different physical nature reproduced using these standards. Fixed gauges: These are used to check whether the dimensions of given parts fall within the specified tolerances. Also these are used to check the form and position of various features of a product.

34 General Concept 1.5 (iii) Measuring Instruments: The values of a quantity or condition being measured are determined using these devices. Measuring instruments should be ten times more accurate than the dimension being measured. 1.5 METHODS OF MEASUREMENT The various methods of measurements are as follows: (a) Direct method: Here the value of the quantity being measured is obtained directly by comparing it with the standard. Example: measurement of length by means of graduated scale; measurement of mass on equal arm balance. (b) Indirect method: Here, different parameters of the quantity are measured directly using which the value of the quantity is determined by a mathematical relationship. Example: measurement of speed by measuring distance and time. (c) Fundamental method: Here, the measurement of base quantities which define the quantity are used for the measurement. Example: measurement of pressure determined from measurements of density, acceleration due to gravity and length which are derived from base quantities of length, mass and time. This is also known as absolute method of measurement.

35 1.6 Engineering Metrology and Measurements (d) Comparison method: Here, measurements are made by comparison with a known value of the same quantity or another quantity which is a function of the quantity to be measured. Example: measurement of pressure by bourdon tube gauge. (e) Substitution method: Here the measurement of the quantity is obtained directly on a indicating device by substituting the measuring quantity with same other known quantity which produces the same effect on the indicating device. Example: determination of temperature with beckmann thermometer. (f) Transposition method: Here, the method of measurement is by direct comparison where the quantity to be measured is first balanced by a known value and then balanced by another known value of the same quantity. From these two known values, the value of the quantity is determined Example: measurement of mass by gauss double weighing method. (g) Differential method: Here, the measurement is made by the difference between the given quantity and a known master of the same value. Example: determination of diameter with master cylinder on a comparator.

36 General Concept 1.7 (h) Coincidence method: In this method, the small difference between the given quantity and the reference is found by the observation of the coincidence of scale marks. Example: measurement of length by vernier caliper. (i) Null method: Here the measurement is made by comparing the given quantity with a known source and the difference between the two is made zero. Example: measurement of electrical resistance using wheatstone bridge and null indicator. (j) Deflection method: Here, the deflection of a pointer on a calibrated scale, directly indicates the value of the quantity. Example: measurements of length by dial indicator. (k) Complementary method: Here, the value of the unknown quantity is combined with a known value of the same quantity and the sum of these two values is adjusted so as to be equal to a predetermined comparison value. Example: determination of the volume of a solid by liquid displacement. (l) Contact and contactless methods: In contact method, the instruments measuring tip touches the surface to be measured. For contactless method, there is no contact between the instrument and quantity being measured. Example: projection comparator.

37 1.8 Engineering Metrology and Measurements 1.6 GENERALISED MEASUREMENT SYSTEM The function of a measuring system is usually to determine the values or magnitude of a particular phenomena. A general measuring system can be broadly described as below consisting of the following stages. A block diagram is shown in Fig Input Primary stage Analogous output Conversion stage Manipulation stage Amplified signal Read out Recording stage Fig. 1.1 Block diagram of the elements of a measuring system Primary or Input sensing stage Secondary stage or Conversion stage Tertiary stage or Manipulation stage Final stage or Readout - Recording stage Primary Stage (or) Primary Element Here the measurement is sensed and transduced into a signal analogous to the input signal (measured). Ideally in this stage, the sensor should select only the required input quantity to be measured and excluding all other possible inputs Secondary stage or Conversion stage Here the conversion of the transduced signal into a signal suitable to the final stage takes place without changing the information content of the input signal. Also

38 General Concept 1.9 certain basic operations, like selective filtering is done to remove unwanted input components. Not all measuring instruments necessarily go through this stage Manipulation Stage In this stage, the amplitude or power, or both of the signal is amplified without changing the nature of the signal to a level necessary for indication or recording at the final stage. Here, the nature of the signal is not changed. Also, it serves as a means of transmission of the signal to the final stage Final or Readout-Recording Stage The required information is presented in an understandable form to the human observer or controller in this stage. Usually the information is indicated in an analog or digital form. In analog form, a pointer and a scale with suitable calibration are used. In digital form, LCD or LED are used where the values are directly displayed. For some cases, the information is recorded and printed. Also the values of the quality measured may be used as an input for various computing systems or to a controller. The measuring system can be explained further with the following example of a indicating thermometer as shown in Fig 1.2. In the primary stage, heat is converted into fluid displacement by the thermometer sensing bulb. The displacement is proportional to the temperature at the bulb. In the secondary stage, the fluid displacement is converted into a displacement of a link by the bourdon tube pressure gauge. In the manipulation stage, the link displacement is amplified by suitable

39 1.10 Engineering Metrology and Measurements Detector Transducer stage Bourdon Tube detector (Secondary stage) Fig. 1.2 Different stages of measuring system Scale (Terminating stage or Final stage) Pointer Gears (Manipulation stage) ( Intermediate (or) modifying stage) Thermometer Sensing bulb (Primary stage) Liquid (Hot) gearing to produce a sizeable deflection of the pointer. In the final stage, the deflection of a pointer over a scale indicates the temperature. 1.7 UNITS AND STANDARDS As discussed earlier, Measurement is a process of comparision of an unknown phenomena with a predefined standard. This predefined standard is called as an unit. This quantitative value or information of any dimension is expressed in terms of unit. For example for the dimension of length, meter is the unit. There may be many units for a particular dimension. For example, length can be expressed in inches, feet, meters, etc. The system of units should be standardized for universal applicability. Further, as there are various systems of

40 General Concept 1.11 units, the method of conversion from one system to another must also be established. Hence the standards are applicable to the units, system of unit and the conversion between the systems. The National Bureau of Standards (NBS), now known as the The National Institute of standards and Technology is an International Organisation responsible for the development, research and calibration of standards and devices. In India, the Bureau of Indian Standards (BIS) is the national body for standardization SI System The Eleventh General Conference on Weight and Measures established formally the SI (International System of Units (or) System International units) system. It consists of six dimensional standards for length, mass time, electric current, thermodynamic temperature and luminous intensity. Later the standard for amount of substance (mole) was added. The SI system of units is more convenient to use than other systems and comprehensive as the seven base units cover all discipline. The table below shows the seven base units in the SI System. Basic Units of SI System S. No. Physical Quantity Unit Name Unit Symbol 1. Length Meter m 2. Mass Kilogram kg

41 1.12 Engineering Metrology and Measurements S. No. Physical Quantity Unit Name Unit Symbol 3. Time Second s 4. Electric current Ampere A 5. Temperature Kelvin K 6. Luminous Intensity Candela Cd 7. Amount of Substance mole mol In addition, two supplementary units are used in the SI System for the measures of plane and spherical angles. Though these can be measured in terms of the base units, they have been added for convenience. Supplementary Units Quantity Unit Symbol Plane angle radian rad Solid angle Steradian sr From these base and supplementary units, the other physical quantities are derived. These are known as derived units. Some derived units are assigned special names. For example, the unit of force kgm/s 2 is called newton (N) and unit of work and energy kgm 2 /s is called joule J.

42 General Concept Table for derived units Parameter Symbol Unit Force F N (i.e., 1N 1 kg m/s 2 ) Pressure P N/m 2 or Pascal Density kg/m 3 Sp.wt w N/m 3 Energy E Nm or Joule Area A m 2 Velocity v, u m/s Moment or couple M N-m Angle rad Angular velocity rad/s Acceleration a m/s 2 Angular acceleration rad/s 2 Torque T N-m Power P Watts or J/S Frequency f Hz Volume V m 3 Work W 1 2 N-m Impulse kg - m/s Moment of force N-m Stress N/m 2

43 1.14 Engineering Metrology and Measurements Definitions Meter, m The unit of length is meter m. It is defined as the length of the path travelled by light in vacuum in a time interval of 1/299,792,458 of a second. Second, s The unit of time is second (s). The duration of 9,192,631,770 of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom is called a second. Kilogram, kg The unit of mass is kilogram, kg. It is defined as the mass of the International prototype kilogram, made of platinum iridium kept at the International Bureau of weights and Measures at Sevres, near paris. Kelvin, K The unit of temperature is called kelvin K. It is the fraction 1/ of the thermodynamic temperature of the triple point of water, i.e. The temperature of which the solid, liquid, and vapour phases of water coexist in equilibrium. Ampere, A The unit of electric current is ampere, A. It is defined as the constant current required (or to be maintained) to produce a force equal to Newton per meter length between two parallel rectilinear conductors of infinite length of negligible circular

44 General Concept 1.15 cross-section, placed at a distance of one metre apart in vacuum. Candela, Cd It is the unit for luminous intensity. It is defined as the luminous intensity, in the perpendicular direction, of a surface of 1/600,000 square metre of a black body at a temperature of freezing platinum under a pressure of 1,01, 325 newtons per square metre. It is also defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency Hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. Mole, mol The unit of quantity of substance is mole, mol. It is defined as the amount of any substance that contains as many elementary entities (Ex: atoms, electrons, etc) as there are atoms in kilogram of carbon 12. Radian (rad) Plane angles are represented by radians (rad). A radian is defined as the angle subtended at the center of a circle by an arc of length equal to the radius of the circle. (arc of unit length at unit radius). A radian is pure number as it a ratio of two lengths [one radian is equal to 180/ degrees]

45 1.16 Engineering Metrology and Measurements Steradian (sr) Solid angles are represented by steradians, (sr). It is defined as the angle subtended by a unit area on the surface of a unit sphere. It is also dimensionless STANDARDS Measurement of dimensions is of primary importance in the field of engineering. So, it is necessity to prescribe standards for the basic physical quantities. The measurement of linear and angular dimensions is of fundamental importance in the production process Standard Systems of Measurement English (yard) and metric (metre) system are the two standard systems for linear measurement. NOTE: Yard or metre is defined as the distance between scribed lines on a bar of metal under certain conditions of temperature and pressure. The different standard for linear measurements are Line standard End standard Wavelength standard Line Standard When length measured is expressed as the distance between two lines, it is called line standard. Example: Rulers, etc

46 General Concept End Standard When the length measured is expressed as the distance between two parallel surfaces, it is called as End standard Difference between Line standard and End standard Line Standard End Standard Length expressed as a distance between two lines. Length expressed as a distance between two surfaces Less accurate ( 0.2 mm) Highly accurate ( mm) Low cost High cost Less skill required High skill required Subjected to parallax effect. Not subjected to parallax effect Wavelength standard The wavelength of a selected radiation of light is used to express the basic unit of length. Unlike material standards, this is not influenced by variation of environmental conditions like temperature, pressure, humidity, ageing, etc CLASSIFICATION OF STANDARDS Primary Standards Secondary Standards Tertiary Standards Working Standards

47 1.18 Engineering Metrology and Measurements Primary Standard A primary standard is precisely defined, whose value is fixed and does not change. There is only one material standard preserved under the most careful conditions. This is used for calibration and comparison with secondary standards Secondary Standard The secondary standard is obtained by comparison with the primary standard. These standards resemble closely to primary standards as regards to material, size and design. These serve as a backup in case of loss or destruction of primary standard. Also, they are kept in different countries for practical purposes, for reference to the tertiary standards. In India, these standards are maintained by the National Physical Laboratory Tertiary Standards These are obtained by comparison with secondary standards. These standards are used for reference with working standards Working Standards These standards are similar to the above standards except for the materials used. They find applications in the measuring laboratories. Standards are also classified as Reference standards used for reference purposes. Calibration standards used for calibration purposes. Inspection standards used for inspection purposes. Working standards used for operational purposes.

48 General Concept Advantages of standards By adopting standard specifications internationally parts manufactured at different locations can be used in any location, irrespective of the country of origin. For example a bolt with M6 1 mm specification would fit into every M6 1 mm nut irrespective of the manufactured location i.e interchangeability is made possible. The manufacturing costs are reduced due to increased batch sizes. Inventory can be reduced as assemblies can be done on non-selective basis which reduces costs. By standardization, uniform and better quality of components is achieved. The cost of conversion is eliminated, training of personnel is easier and fewer errors occur TRACEABILITY Traceability is a system of transferring the standardized units from the point of definition to the uses. It refers to an unbroken chain of comparisons relating to an instrument s measurements to a known standard. Measurement being a science of comparison, traceability to a known standard makes the measurement result meaningful. The standards followed in various industries must be traceable to the national standards which inturn must be traceable to international standards. The value of any measurement in any industry must be related in discrete steps and with known error

49 1.20 Engineering Metrology and Measurements to the value of the national standard. In the process of traceability, national laboratories, standardizing laboratories, etc are to be established for country, states and industries which are traceable to a single source. 1.8 MEASURING INSTRUMENTS The devices used for measuring the physical quantities of objects and events are called measuring instruments. These instruments differ according to the nature of the variables to be measured and also the accuracy required. So, a large number of measuring instruments are in use today. We will study about some of the basic and vital instruments used in the practice of mechanical engineering in this book Classification of measuring instruments They are classified as follows 1. On the basis of function (a) Length measuring instruments (b) Angle measuring instruments (c) Geometrical form checking instruments (d) Surface finish - checking instruments 2. On the basis of accuracy (a) (b) (c) Most accurate instruments Example: light interference instruments. Moderate accurate instruments Example: comparators. Below-moderate accurate instruments Example: dial indicators

50 General Concept On the basis of precision (a) Precision measuring instruments (b) Non-precision measuring instruments 1.9 SENSITIVITY Sensitivity is the degree of response of an instrument to an incoming signal. It is the ratio of the change in the output of an indicating instrument or transducer to the change in the value of the measured quantity. Sensitivity Change in instrument output Change in value of measured quantity So, a more sensitive instrument shows better response to slight changes in a quantity being measured. Sensitivity is also called the gain of the instrument. The disadvantage of high sensitivity instruments is that drifts in indication may occur due to variations in temperature draughts or other effects. Sensitivity can also be defined as ratio of the range of the instrument to the number of divisions on the scale of the instrument. Range No. of divisions The largest range of values of the measured quantity to which the instrument does not respond is called the dead zone.