Metrology PDF

METROLOGY (The Science of Measurement) National Measurement System Dr Jon Petzing  MARKET COMPETITIVENESS Proof of adherence Proof of meeting Proof of conformance to contract specifications with international legal requirements...

METROLOGY (The Science of Measurement) National Measurement System Dr Jon Petzing  MARKET COMPETITIVENESS Proof of adherence Proof of meeting Proof of conformance to contract specifications with international legal requirements specifications and standards Measurement Industry Standards Standards NPL/NEL/LGC Accredited Formal Accredited Measurement Laboratories Specification Certification Bodies UKAS Standards THE NATIONAL MEASUREMENT SYSTEM High Accuracy Research and Non-Routine Development Measurement Calibration and Standards and Training Testing Services National Standards Laboratories LGC  NATIONAL PHYSICAL LABORATORY Environmental Mass, Length and Density Optical Radiation Force and Pressure Physical Radio, Microwave Measurement Colour and Acoustics Time and Frequency Gas Standards www.npl.co.uk TÜV SÜD NATIONAL ENGINEERING LABORATORY Flow and Density Measurement Gas, Water and Oil Multiphase Mixtures www.tuvsud.com LABORATORY OF THE GOVERNMENT CHEMIST LGC Chemical and Proficiency Testing Biochemical Certified Reference Schemes Measurement Materials Gas Standards www.lgc.co.uk  LEGAL METROLOGY Department for Office for Product Business, Energy & Safety & Standards Industrial Strategy Trade Measurements Type Approval Equipment testing Traceability Trading Standards Mass, length & volume calibration EMC Testing https://www.gov.uk/government/organisations/office-for-product-safety-and-standards  LABORATORY ACCREDITATION United Kingdom Accreditation Service (UKAS) Accreditation of 500+ laboratories which provide regular calibration and testing services www.ukas.com  MEASUREMENT TRACEABILITY Ability to trace the result of a measurement to a single source that is a national, or more likely, an international standard. All industrialized countries typically have a national bureau of standards i.e. the National Physical Laboratory in the UK. The tracing between measurement and NPL (traceability chain) must be unbroken, usually involving working and transfer standards. The actual traceability chain for a company may be complex involving many steps but can be applied across different sectors. Grower / Orchard Retailer Apples You Buy  TRACEABILITY CHAIN ACCURACY NPL COST National Standards Definition of the Metre >x10 more accurate than working standard Secondary / Transfer Standards Class 0 Gauge Blocks >x10 more accurate than measuring instrument Working Standards Class 2 Gauge Blocks Instrument or Measuring Process Micrometer  MEASUREMENT TRACEABILITY One length calibration by NPL Leads to commercial services from accredited calibration and testing laboratories Traceable length calibration of hundreds of instruments for use in industry Thousands of traceable length measurements  TRANSFER STANDARDS Few companies can afford their own calibration laboratories. Most will use outside accredited laboratories for Transfer standards and calibration. Transfer standards should ideally be 10X more accurate than Working standards. Working standards at least 10X more accurate than the measurement instrument. Transfer and Working standards require periodic calibration. UKAS accredited laboratories will give guaranteed traceable calibrations to national standards, with statements of uncertainty.  CALIBRATION The purpose of calibration is to provide confidence about a measurement instrument’s accuracy and repeatability. Ideally, calibration should be traceable via UKAS accreditation. Calibration is about the instrument, but also information handling, data analysis, intervals, uncertainties/error analysis, environment, people… The error due to the calibration process should be a maximum of ten percent (1:10) of the permissible error of the instrument or measuring process being calibrated – 10X rule again.  CALIBRATION Average Measured Value True Value Accuracy Error of Measurement (Bias) Precision  CALIBRATION The calibration environment is difficult to control and is a function of: temperature pressure humidity vibration electrostatic fields electromagnetic fields human interaction The calibrator of a measuring instrument must be aware of all potential sources of error which could affect the calibration. The calibration process may just involve single measurements which are quick, but are more likely to involve many measurements across time. Stability is required in these cases.  METROLOGY (The Science of Measurement) National Measurement System Dr Jon Petzing  METROLOGY (The Science of Measurement) Coordinate Measurement Machines Dr Jon Petzing http://www.npl.co.uk/reference/measurement-units/si-base-units/the-kelvin https://youtu.be/PzoxxNefCUw  COORDINATE MEASUREMENT Coordinate measurement allows an engineer to compare one component to another using the dimensions of the two components. Sample components off the production line can be assessed with respect to a master specimen or a master set of dimensions, assumed from the design drawings. Coordinates or dimensions can be measured using Cartesian or Polar axes. Y θ r X Z  COORDINATE MEASURING MACHINES The term “Coordinate Measuring Machine” (CMM) describes metrology machines or instruments which measure coordinates. CMMs range from simple manually operated systems up to highly sophisticated computer controlled fully automatic systems. The automatic CMM’s have many design similarities to CNC machining centres in terms of rigidity, thermal stability, dimensional stability etc. The majority of these machines are now supplied with powerful ‘user- friendly’ software packages, that control the CMM, record and process data. VARIATIONS OF CMM CONFIGURATIONS Nikon Metrology Hexagon Metrology CMM EXAMPLES LK Ltd  VARIATIONS OF OPTICAL CMMs Werth GmbH OGP Inc Mitutoyo Corp. Hexagon Metrology Mahr GmbH MULTI-AXIS MEASUREMENT ARMS FARO-ARM (Platinum) ROMER 3000i MULTI-AXIS MEASUREMENT ARMS METRIS MCA  GENERAL MEASUREMENT STRATEGY The general measurement strategy for CMMs can be broken down into a number of general parts, that should ideally be followed by the user: - Selection of features on the workpiece to be measured. - Definition of the workpiece datum(s) used within the coordinate system. - Selection of the workpiece orientation. - Selection of the workpiece holding method. - Qualification of the CMM probe. - Definition of the probing strategy. - Programming of the CMM. - Analysis and recording of the results.  WORKPIECE FEATURES Manufacturing and functionality requirements will determine which features require measurement. Some features may not be measurable using a CMM !! Some features may be impracticable to measure with a CMM !! Some features may not be cost effective to measure with a CMM !! Consideration should therefore be given to the minimum number of measurements required, in order to determine component accuracy and conformity to specification. Ideally the strategy should only require one set-up to measure all features, and in reality a minimum number of set-ups should be used.  COORDINATE SYSTEMS An important issue is the coordinate system of the CMM versus the coordinate system of the component being measured. Coordinate systems may be rectilinear Z with familiar notation (X,Y,Z). CMM coordinate system However, polar coordinates may also be present, and in many cases more useful for measurement purposes (θ, r). C Object coordinate system X Care must be taken to identify the B correct coordinate systems for each measurement point on a component, and be prepared to change coordinate A systems. Y The CMM is “dumb” until the component coordinate system is identified.  COORDINATE SYSTEMS Z r r θ α z β R R Cylindrically polar coordinate system Spherically polar coordinate system One rotary and two linear axes One linear and two rotary axes WORKPIECE DATUMS  THE TOUCH TRIGGER PROBE Renishaw TP2 Renishaw TP6 Renishaw TP1 Renishaw TP20 Renishaw Plc  THE SCANNING PROBE Renishaw SP25 Renishaw SP600 Renishaw SP80 Scanning demonstration Revo Renishaw Plc  NON-CONTACT PROBES One aim of companies such as Renishaw, has been to develop non-contacting TTPs. The advantages of these are obvious, and mainly centre around reducing the interaction with the work-piece. To date, non-contact optical probes have not seen any significant commercial success compared to their contacting competitors. Some reasons for this are: - Limited to line of sight - Limited resolution and accuracy - Engineering mistrust - Reliability - Compatibility New Scanning Renishaw Plc  STYLUS SELECTION There are some basic rules for stylus selection, bearing in mind that most probe heads will take a range of different styli: - KEEP THE STYLI SHORT AND STIFF - KEEP THE STYLUS BALL AS LARGE AS POSSIBLE Renishaw Plc A range of other stylus designs are available for more specialist applications, including star styli, pointer styli, disc styli and cylinder styli. Manual probes are prone to stylus deformation unless they are thick, short and rigid. Renishaw Plc  PROBING STRATEGY Number of contact points required Geometric Mathematical Recommended Feature Minimum Straight Line 2 5 Plane 3 9 (Approximately three lines of three) Circle 3 7 (To detect up to six lobes) Sphere 4 9 (Approximately three circles of three in three planes) Cone 6 12 (Circles in four parallel planes for information on straightness) 15 (Five points on each circle for roundness information ) Ellipse 4 12 Cylinder 5 12 (Circles in four parallel planes for information on straightness) 15 (Five points on each circle for roundness information) Cube 6 18 (At least three per face)  PROBING STRATEGY Coleman/Waters  KEY CMM ISSUES The important issues concerning CMM design, operation and use which will be further explored in Part C/D are: - Coordinate measurement principles - Encoder design - Touch Trigger Probes - GD & T principles - Errors and uncertainties - Verification and calibration - CMM measurement application - Deployment strategies  METROLOGY DATA ANALYSIS & ERRORS Dr Jon Petzing Wolfson School of Mechanical, Manufacturing & Electrical Engineering  Football Club League Position 1975 1976 1977 1978 0 5 10 Premiership (1st Division) 15 20 25 Guess the Trend ? 30 Championship (2nd Division) Guess the Team ? 35  Football Club League Position 1975 1976 1977 1978 1979 1980 0 5 10 Premiership (1st Division) 15 20 25 Guess the Trend ? 30 Championship (2nd Division) Guess the Team ? 35  Football Club League Position 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 0 5 10 Premiership (1st Division) 15 20 25 Guess the Trend ? 30 Championship (2nd Division) Guess the Team ? 35  Football Club League Position 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 0 10 Premiership (1st Division) 20 30 Championship (2nd Division) 40 50 Guess the Team ? EFL League 1 (3rd Division) 60  WHY ANALYSE MEASUREMENT DATA ? One data point does not tell us everything we need: - Measurement value - Reliability of the measurement - Measurement trend Data requires analysis in order to extract these parameters. The measurement value indicates the quality of the manufacturing process and gives a measure of conformity to specification. The reliability of the measurement indicates how good the measurement is, and how much confidence should be given to the measurement value. The measurement trend shows the change of a parameter against time.  PROCESSING OF DATA Measurement data is processed either graphically or numerically. Graphical/visual representation of data is very useful for operator analysis, immediately showing data trends and diverging data points. Numerical analysis can use a wide range of mathematical concepts: - Statistical methods - Probability based methods - Linear/non-linear regression - Least squares methods Numerical analysis provides a more detailed understanding of the measurement data and will often form the basis for on-line control of manufacturing processes.  GRAPHICAL REPRESENTATION OF DATA STUDENT MEASUREMENTS FOR H1 17.20 17.00 16.80 16.60 Measurements (mm) 16.40 16.20 16.00 15.80 15.60 15.40 15.20 15.00 0 5 10 15 20 25 30 35 40 Data Set H1 (Mic) H1 (Ver)  DATA FREQUENCY ANALYSIS 25 20 Frequency (Number of data sets) 15 10 5 0 15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 16 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 17 Data Range (mm) Micrometer Vernier  NUMERICAL DATA ANALYSIS A common method of analysing multiple data sets is to calculate the average or MEAN value. This value can always be uniquely defined assuming measurements of equal reliability: (x1 + x2 +....... xn ) Xn = n Further simple analysis is provided by identifying the minimum and maximum data points, leading to the range of the data.  NUMERICAL DATA ANALYSIS The mode of a set of values is that value which occurs most frequently. The median is that value of the data which divides the distribution into two equal parts with equal frequencies. The median can be further broken down into: - Quartiles - divide the distribution into four equal parts- - Deciles - divide the distribution into ten equal parts - Percentiles - divide the distribution into one hundred parts  NUMERICAL DATA ANALYSIS 120.00% 100.00% D8 P80 Cumulative Frequency 80.00% 60.00% M Q2 D5 P50 40.00% Q1 P25 20.00%.00% 15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 16 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 17 Measurement (mm)  NUMERICAL DATA ANALYSIS Further interpretation of the data is often presented in the form of the variance and the standard deviation. The variance is defined as: s2 = å (x - X n ) 2 s2 = åx 2 - X n2 n n The standard deviation is simply the positive square root of the variance: å (x - X ) 2 s= å - X2 x 2 s= n n n n  LEAST SQUARES METHODS y B The method of least squares ‘fits’ Qi X the ‘best’ line AB to the points by X making Σ(PiQi)2 a minimum. X X This minimises the sum of the Ri squared deviations of each X X Pi(xi, yi) X measurement point from the best A fit line. This is called the line of x regression of y on x. If Σ(PiRi)2 is minimised, then the corresponding line of best fit is called the line of regression of y on y. Equations of the lines of regression are generated providing polynomial linear or non-linear descriptions of the data trends.  IDENTIFYING ERROR SOURCES The smaller the value of the standard deviation, the more precision the measurement data has. Population or sample standard deviation is often quoted with measurement/engineering data as a form of quality index. However, this mathematical manipulation of the data does not allow the identification of sources of error. These can only be identified by understanding the measurement and the instrumentation etc. Without knowing the types of error which modulate or disrupt the measurement data, there can be no measurement confidence.  IDENTIFYING ERROR SOURCES Measuring instrument errors can generally be divided into two classes: - Systematic errors remain constant or change in a regular fashion in repeated measurements of one and the same quantity. Corrections can be introduced to remove the error - Random errors are differences between the results of separate measurements which cannot be predicted individually There are many potential sources of random errors: - Human errors - Environmental errors - Equipment errors  QUANTIFICATION OF ERROR If a measuring instrument is calibrated with respect to a traceable calibration standard (known value) then the experimental error can be deduced. This value may be stated or more likely used as an adjustment for the measurement data: - A micrometer is found to have an error of +20 microns Readings could be: 20.480mm + 0.020mm or: 20.500mm If the micrometer suffers from backlash in the thread form which is random in its presence, then this may be estimated as an uncertainty: - Readings would be: 12.740mm +/-0.050mm  NPL GPGs IN UNCERTAINTY A3468_GPG_No11_Issue2_A/W 6/3/03 3:39 pm Page 2 No. 36 No. 11 Measurement Measurement Good Practice Guide Good Practice Guide Estimating Uncertainties A Beginner's Guide in Testing to Uncertainty of Keith Birch Measurement Stephanie Bell Issue 2 The National Physical Laboratory is operated on behalf of the DTI by NPL Management Limited, a wholly owned subsidiary of Serco Group plc The National Measurement Partnership is a DTI programme managed by the National Physical Laboratory to promote good measurement practice.  THE USEMEASUREMENTS STUDENT OF ERRORFORBARS H1 17.20 17.00 16.80 16.60 Measurements (mm) 16.40 16.20 16.00 15.80 15.60 15.40 15.20 15.00 0 5 10 15 20 25 30 35 40 Data Set H1 (Mic) H1 (Ver)  THE USEMEASUREMENTS STUDENT OF ERRORFORBARS H1 17.20 17.00 16.80 16.60 Measurements (mm) 16.40 16.20 16.00 15.80 15.60 15.40 15.20 15.00 0 5 10 15 20 25 30 35 40 Data Set H1 (Mic) H1 (Ver)  THE USEMEASUREMENTS STUDENT OF ERRORFORBARS H1 17.20 17.00 16.80 16.60 Measurements (mm) 16.40 16.20 16.00 15.80 15.60 15.40 15.20 15.00 0 5 10 15 20 25 30 35 40 Data Set H1 (Mic) H1 (Ver) METROLOGY (The Science of Measurement) Metrology Errors Dr Jon Petzing https://youtu.be/WHJyKKSoCfQ https://youtu.be/LmK8ec9MruM  MEASUREMENT TERMINOLOGY The accuracy of a measuring instrument indicates the deviation of the reading from a known input. The precision of a measuring instrument indicates its ability to reproduce a certain reading with a given accuracy. The sensitivity of a measuring instrument is the ratio of the instrument scale change to the change in the measured variable. The error of a measuring instrument is the quantified deviation between the instrument reading and the known input. The uncertainty of a measuring instrument is the deviation between readings when a known reference value does not exist.  ACCURACY v PRECISION The accuracy of an instrument indicates how well it agrees with the true value. The precision of an instrument refers to the dispersion of measurements. This typically leads to four states of data characteristics. Inaccurate Inaccurate Accurate Accurate Imprecise Precise Imprecise Precise  IDENTIFYING ERROR SOURCES Measuring instrument errors can generally be divided into two classes: - Systematic errors remain constant or change in a regular fashion in repeated measurements of one and the same quantity. Corrections can be introduced to remove the error. - Random errors are differences between the results of separate measurements which cannot be predicted individually. There are many potential sources of random errors: - Human errors - Environmental errors - Equipment errors  QUANTIFICATION OF ERROR If a measuring instrument is calibrated with respect to a traceable calibration standard (known value) then the experimental error can be deduced. This value may be stated or more likely used as an adjustment for the measurement data: A micrometer is found to read high by +20 μm Readings could be: 20.480 mm - 0.020 mm or: 20.460 mm If the micrometer suffers from backlash in the thread form which is random in presence, then this may be estimated as an uncertainty: - Readings would be: 12.740 mm +/-0.050 mm  COSINE ERRORS Frequently in practical measurement, occasions arise when the alignment of certain measuring devices is known to be slightly in error. In these cases it is necessary to assess the effect on the accuracy of the measuring instrument. L = measured length T = true length T θ L T = L cosq Note: For small angles cos θ = 1  SINE ERRORS Cosine errors are often very small due to the small angle approximation. Flat measurement surfaces such as the anvils of a micrometer are designed to meet the measured artifact at right angles. If this does not happen then further Sine error contributions must be taken into account. D L = measured length T = true length T L T = L cosq - sin d δ  AIRY/BESSEL POINTS If a horizontal beam is required in a measurement instrument, then this beam will require supporting points. If the beam is incorrectly supported then it will sag or bend and the overall length of the beam will change. NA  AIRY/BESSEL POINTS The Airy points are used for two-point support and make the end faces of the beam parallel, where A = 0.5577 L L A The Bessel points are used for two-point support, and minimize the change in the overall length, where B = 0.559 L L B  THERMAL EXPANSION All materials will contract and expand when subject to changing conditions and temperatures. The use of metrology instrumentation must consider the thermal environment, including the handling of the instruments. All materials have a coefficient of thermal expansion (α), although these will not necessarily be linear with temperature. For carbon steel (0.7 - 1.4%C). Temperature 100 293 500 800 (θ) Coefficient (α) 6.7 10.7 13.7 16.2 (10-6-6 K-1-1)  THERMAL EXPANSION Knowing the coefficient of thermal expansion and the operating temperature it is then possible to quantify changes of dimension: Expansion a= Original length x temperature change Linear Expansion = L{1 + a (q2 - q1 )} θ1 should be 293K and L is the original length/dimension of concern. If the measuring instrument expands then it will measure low. If the measuring instrument contracts then it will read high.  ABBE’S PRINCIPLE Abbe’s principle states that: “Maximum accuracy may be obtained when the standard scale and the work piece are aligned on the same line of measurement” l Measuring error (E): L E E = l - L θ E = R tan θ R E = Rθ (tan θ)  PARALLAX ERROR Parallax is the change in the apparent relative positions of objects when viewed from different positions. Parallax causes measurement errors when there is a height difference between two graduated faces. Correct reading Incorrect reading Incorrect reading 1 2 3  INCLINATION ERRORS Inclination errors are due to the incorrect positioning of measurement transducers, especially when measuring internal dimensions. x θ The measurement error (Δl) is: l L If the internal micrometer is inclined in the axial direction of the pipe/tube, positive measurement errors will be produced.  INCLINATION ERRORS If the micrometer is inclined in the lateral direction, then negative measurement errors will be produced. x θ The measurement error (Δl) is: l L  METROLOGY (The Science of Measurement) Introduction Dr Jon Petzing VSL Dutch Metrology Institute https://youtu.be/vRnT8hIxjqk  On-line Expectations & House-Keeping Emails: On a busy day I will receive 100+ emails a day Please be patient – I will respond as soon as we can during working hours (9.00 – 17.00) Please do not expect any responses during evenings or weekends Module Information: Module Information will be communicated by the Module Noticeboard Module Information will also be communicated via Lectures On occasion Module Information may be communicated by email using the module email address Lectures will be captured and held in the Module Review Panel  Basic Module Details Metrology – Jon Petzing Subtractive Machining – Radmehr Monfared Electronics Manufacture – Patrick Webb Phase Test 1 – Week 4 – 30 multi-choice questions (20%) Phase Test 2 – Week 9 – 30 multi-choice questions (30%) Exam – Weeks 13/14 – multi-choice questions (50%)  Course Texts Primary course text is Kalpakjian (Manufacturing Engineering & Technology) – see Module Information Learn pages. – Metrology – Chapters 33 & 35 – Subtractive Machining – Chapters 21 - 27 – Electronics Manufacture – Chapter 28 – Library 4th edition (6 copies) – Library 5th edition (10 copies) – Library 6th edition (14 copies) All are relevant – Library 7th edition (11 copies) Supporting text for Electronics Manufacture is by Brindley (Newnes Electronics Assembly Handbook). – Library hard copy edition (4 copies) – Library electronic edition (multiple copies) Supporting texts for Metrology are by NPL (Measurement Good Practice Guides (see Metrology Learn pages). Module Learn Pages Please use the META module Learn pages for Manufacturing Technology: 23WS610META  Module Relationships Manufacturing technologies and DIS / DPS processes in the workplace BTEC Diploma in Engineering Typically - construction of Design & Technology / Physics A-level components, designing Part C Individual Projects factory layout, measuring physical parameters, etc IB Design / Physics Higher level Typically - construction of components, designing Part D Project Engineering (MEng Group Project) factory layout, measuring physical parameters, etc Workshop experience (machining and manufactur- ing techniques), design Integrating studies (A) development, systems thinking, critical thinking, Understanding of advanced metrology processes based Metrology (C/D) on coordinate and surface texture measurement Characteristics of materials, forming processes, deformation processes, Materials and Manufacturing Processes (A) finishing processes Manufacturing Technology Understanding of a range of advanced manufacturing Advanced Manufacturing Processes & Technology 1 & 2 (C/D) processes and applications Management principles, strategic planning, product planning, process planning, production planning, inventory Manufacturing Management (A) planning, forecasting de- Exploration of a range of laser mands, work studies based machining and materi- Laser Materials Processing (C) als modification processes Fluid properties - energy and momentum, Solid Mechanics - Development of a solution to a customer vibration, out of balance forces, Engineering Science 2 (B) need - then prototyping and develop- buckling of struts, fatigue Application of Product Design (B) ment of a manufacturing plan for production Forces, moments, Hooke's Law, structural analysis, Development of plant lay-out torsion, transverse loading, Engineering Science 1 (A) strategies, forecasting, stress, strain, heat transfer, sequencing, scheduling - all thermodynamics Manufacturing Planning & Control (B) needing understanding of manufacturing technologies  METEOROLOGY The study of the earth’s atmosphere in its relation to weather and climate This is NOT the subject of interest !! METROLOGY Metrology is the science of MEASUREMENT & CALIBRATION This is the subject I am going to talk about  METROLOGY - WHY BOTHER ?? Without measurement, there can be no fitness for purpose or conformity/compliance to design specifications Without calibration, there can be no national international measurement conformity, or measurement confidence For manufacturing companies, compliance with ISO 9000 and ISO/DIS 14253 standards is necessary  METROLOGY - WHY BOTHER ?? Metrology indicates whether a part/component/sub-assembly/ final product is acceptable or unacceptable Metrology allows comparison of the manufactured article with the original specification Metrology helps to improve product quality, reduce defects and reduce scrap rates, hence increasing market share, competitiveness, survivability and profitability  METROLOGY 1.0 2.0 3.0 4.0 Legislation & Measurement Areas Analysis Methods Installation & Standards & Equipment Dismantling Health & Safety Calibration Information & Documentation Systems  1.1 LEGISLATION & STANDARDS 1.11 1.12 1.13 Law Standards Specifications 1.121 1.122 1.123 1.124 1.125 Company (CCP) European International (OSI) British (BS) American (ANSI)  1.2 HEALTH & SAFETY 1.21 1.22 1.23 1.24 Legislation Company Regulations Employees Duties Safe Systems of Work  1.4 INFORMATION & DOCUMENTATION SYSTEMS 1.41 1.42 1.43 1.44 Information & Data Presentation Computer Applications Handover Procedures Document Handling & Communication  1.3 CALIBRATION 1.31 1.32 1.33 1.34 1.35 Operation & Care Company Specifications Environment Fault Diagnosis Calibrations Methods Monitoring & Procedures  3.0 ANALYSIS METHODS 3.1 3.2 3.3 3.4 3.5 3.6 Statistical Numerical Uncertainties Graphical Errors Probability  4.0 INSTALLATION & DISMANTLING 4.1 4.2 4.3 4.4 Configuration Installation of Dismantling of Handling Techniques Equipment Equipment Equipment  2.0 MEASUREMENT AREAS & EQUIPMENT 2.1 2.2 2.3 2.4 2.5 2.6 Dimensional Mechanics Thermal Pressure Vacuum Electrical 2.0 MEASUREMENT AREAS & EQUIPMENT 2.7 2.8 2.9 2.10 2.11 2.12 Optical Acoustics/Noise Radiation Time/Frequency Chemical Materials 2.0 MEASUREMENT AREAS & EQUIPMENT 2.13 2.14 2.15 Fluid Flow Environment Engineering Principles  METROLOGY QUOTE “When you can measure what you are speaking about and express it in numbers, you know something about it; and when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind.” Lord William Thomas Kelvin https://youtu.be/YYrnjEo90fs  THE HISTORY OF METROLOGY c. 1500BC Egyptian “Cubit” 1000AD - 1800AD Monarch based measurements 1795 Metric system established in France 1824 Yard bar adopted as the British standard of length 1834 British Yard standard destroyed by fire 1855 New Imperial Standard Yard legalized by Act of Parliament 1875 French Bureau International des Poids et Mesures founded  THE HISTORY OF METROLOGY 1900 The National Physical Laboratory (NPL) founded 1901 The Engineering Standards Committee formed 1901 The USA National Bureau of Standards founded 1932 Standard temperature changed from 62F to 20C 1958 The laser invented 1987 First ISO 9000 series of standards issued 1994 ISO 9000 standards updated 1997 ISO/DIS 14253 standard issued  MEASUREMENT UNIT SYSTEMS There are four systems of units which are still in common use world-wide: British Imperial system Centimetre-Gram-Second (CGS/cgs) system Metre-Kilogram-Second (MKS/mks) system Systeme International d’Unites (SI) The SI system is the legal measurement system in the UK, although other systems will be found world wide The use of the Metric system was santioned for use in Britain by an Act of Parliament, in 1864 ! - 159 years ago !  CHANGING THE SI MEASUREMENT SYSTEM Continuously improving the definitions of the units ultimately makes it possible to have tighter tolerances and less waste. For example, gears will fit together better and therefore function more efficiently and manufacturing will be able to rely on the dimensions of parts to fit together. The revision to the SI was a profound change in approach, that will underlie all measurements in science and more widely. The new units are the same size as previously but defined more precisely. The new definitions (May 20th 2019) have impacted four of the base units: Kilogram – now defined in terms of the Planck constant (h) Ampere – now defined in terms of the elementary charge (e) Kelvin – now defined in terms of the Boltzmann constant (k) Mole – now defined in terms of the Avogadro constant (NA)  THE 2019 SI MEASUREMENT SYSTEM Definitions of the seven basic SI units are: 1 metre (m) = path travelled by light in 1/299 792 458 seconds (vacuum) 1 kilogram (kg) = the fixed numerical value of the Planck constant, ℎ, to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of the speed of light, 𝒸, and the hyperfine transition frequency of the caesium-133 atom, ∆ν, respectively. 1 second (s) = duration of 9 192 631 770 periods of transition between two levels of the ground state of the cesium 133 atom 1 ampere (A) = the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10−19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆ν.  THE 2019 SI MEASUREMENT SYSTEM 1 kelvin (K) = the fixed numerical value of the Boltzmann constant kB to be 1.380 649 × 10−23 when expressed in the unit J K−1, which is equal to kg m2 s−2 K−1, where the kilogram, metre and second are defined in terms of 𝘩, 𝒸 and ∆ν. 1 candela (cd) = the luminous intensity of a monochromatic source emitting at 540x10E12 Hz, with a radiant intensity of 1/683 watts per steradian 1 mole (mol) = One mole contains exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, 𝑁A, when expressed in the unit mol-1 and is called the Avogadro number. The amount of substance, symbol 𝑛, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. There are also two supplementary units: the radian and the steradian  METROLOGY (The Science of Measurement) Introduction Dr Jon Petzing VSL Dutch Metrology Institute https://youtu.be/vRnT8hIxjqk  METROLOGY (The Science of Measurement) Length Dr Jon Petzing https://youtu.be/vgqUyFaUDcI  LENGTH METROLOGY 1795 Metric system established in France 1824 Yard bar adopted as the British standard of length 1834 British Yard standard destroyed by fire 1855 New Imperial Standard Yard legalized by Act of Parliament 1864 Metric system legalized in Britain 1889 International Prototype Metre established 1892 Michelson/Benoit use wavelengths for metre measurements  DEFINITION OF THE METRE France - Google Maps 11/11/2010 14:43 Print Address g Notes 1791 French Academy of Sciences decided to redefine the metre: 1m = 1/10,000,000 of the quarter of the Earth’s circumference. Tricky part was doing this…. France - Google Maps Address g DEFINITION OF THE METRE Notes 11/11/2010 14:25 Print Jean Baptiste Joseph Delambre – Northern Expedition from Dunkerque belfry (1792). 1798 – two expeditions meet at Rodez, to complete the measurement. Pierre Méchain– Southern Expedition from Barcelona Montjuïc castle (1792).  LENGTH METROLOGY 1896 First intercomparison between the yard and metre 1932 Yard/Metre ratio frozen at the 1922 value for science & technology 1933 Work started on defining the metre as a wavelength standard 1960 Krypton-86 wavelength standard of length adopted 1963 Krypton-86 metre/yard standards legalized in Britain 1983 Metre defined as the pathlength travelled in 1/299 792 458 seconds 1998 Length standards related to Iodine stabilised HeNe lasers  END STANDARDS The first metre/yard standards were end standards manufactured from brass and platinum/iridium: 1 metre Accuracy limited by the deformation of the bar and bar supports Original definition of the metre was: “one ten-millionth (1/10 000 000) of the quadrant of the North Pole - Equator”  LINE STANDARDS Length standard designs were improved to use the line standard approach 1 metre Gold plugs inserted into length standard with rulled lines. Metre measured between lines on plugs using microscopes Neutral Axis  LINE STANDARDS The latest version of the prototype metre was designed to have an X-shaped cross section The centre of the bar is calculated to lie in the neutral plane of the cross section, providing a stable dimensional artefact  OPTICAL STANDARDS Light can be described in terms of wavelength and frequency, both of which can be accurately measured Michelson and Benoit worked with cadmium red wavelengths in 1892-93, allowing lengths up to 100mm to be measured by fringe counting 1 metre = 1 553 164.1 wavelengths of cadmium red 1 metre = 1 650 763.7 wavelengths of krypton-86 (orange in colour) 1 metre = 1 579 778.8 wavelengths of Iodine-HeNe at 633nm 1 metre 1 wavelength (0.000 000 633m) THE DIMENSIONAL METROLOGY RANGE  MEASURING TOOLS Engineers need to be able to relate the length standards to the common measurement tools such as micrometers and vernier calipers Ranges of calibrated length bars and gauge blocks are produced which range from 1.0005mm up to 100mm in length Gauges and bars can be combined together to produce any length in this range with 1.0005mm steps, if necessary  GAUGE BLOCKS Gauges usually come in sets of 47 or 88 gauges The gauge blocks are very accurately manufactured from steel alloys and ceramics Gauge blocks are “wrung” together by firmly sliding or twisting the end faces of two gauges together. Atomic interactions bond the two surfaces Gauges may be used to calibrate micrometers, verniers and other measuring instruments 40 10 Gauges may also be used with comparators GAUGE/LENGTH BAR CALIBRATION Comparison of measurement length to fixed reference length in the interferometer HeNe Laser (Iodine stabilized) Beam splitter Mirror Detector  GAUGE/LENGTH BAR CALIBRATION Calibration equipment using Iodine-HeNe laser interferometers are used to calibrate length bars and gauges Length bars and gauges will have to be sent to a specialist UKAS laboratory or to the NPL for calibration against secondary standards Length bars and gauges are TRANSFER STANDARDS and are part of the traceability chain, but come in different standards  REFERENCES Kalpakjian & Schmid, “Manufacturing Engineering & Technology”, Pearson, 5th Edition, pp11083 – 1109 Busch, Harlow & Thompson, “Fundamentals of Dimensional Metrology”, Delmar, 3rd Edition. Kennedy, Hoffman & Bond, “Inspection and Gaging”, Industrial press Inc, 6th Edition Farago & Curtiss, “Handbook of Dimensional Measurement” 3rd Edition  METROLOGY (The Science of Measurement) (Out of) Roundness Dr Jon Petzing http://www.npl.co.uk/reference/measurement-units/si-base-units/the-mole https://youtu.be/rXtOUM1Sz6Y  WHY MEASURE ROUNDNESS ? For a shaft running in a bearing, it is assumed that the shaft and bearing housing are round. If the shaft or the bearing housing or both components are not round, then smooth running will not result and bearing wear will occur. Bearing housing Shaft Lubricant Oval shaft Lobed shaft  IS IT ROUND ? Measurement of the diameter will not necessarily identify the problems associated with out-of-roundness. A lobed part will give the same reading for diameter when measured between a pair of parallel faces (micrometer/vernier caliper). D1 Diameter Maximum circle D2 Minimum circle D3  HOW IS ROUNDNESS MEASURED ? A part is said to be round in a specific cross-section, if there exists within that section a point (I.e. the centre) from which all other points on the periphery are equidistant. If the cross-section is not a perfect circle, the out-of-roundness is specified as the difference in distance of the points on the periphery from the centre. r2 r1 r1 Perfect circle Lobed circle  HOW IS ROUNDNESS MEASURED ? Specifying the out-of-roundness of an irregular profile is only possible if a centre can be determined for the profile. r1 r4 A r3 B r2 Finding a centre from which to measure the variation of the profile is an important part of the out-of-roundness assessment.  HOW IS ROUNDNESS MEASURED ? The simplest method is to use a V-block and dial gauge, with the component rotated slowly and carefully. Circular components will not alter the dial gauge reading. The dial gauge will measure surface irregularities and gross movement. Height of the irregularities and their angular spacing with respect to the V-block angle will cause variations of measurement. Variations of this three point method exist for larger components.  HOW IS ROUNDNESS MEASURED ? The V-block form of roundness measurement is always limited by the vee angle and the spacing of the irregularities. False readings can be generated by regularly spaced features. A more accurate would be to rotate the component between centres. Sag, curvature, imperfect centring  HOW IS ROUNDNESS MEASURED ? A machine designed to measure out-of-roundness (errors of roundness) must include the following features: Ø A precision bearing which will provide an accurate axis of rotation to be used as the datum for measurements. Ø A means of very precisely aligning the axes of the work piece surface and the bearing. Ø A measuring transducer capable of providing an amplified signal of the indicated errors of roundness. Ø A suitable means of recording such errors in a convenient form. Ø A means of making quantitative assessments of the errors measured.  HOW IS ROUNDNESS MEASURED ? There are two alternative forms of roundness measuring machines available commercially, both of which satisfy the above criterion. One type of machine has a stationary measuring head and a rotating work piece (1), whilst the second type has a rotating measuring head and a stationary work piece (2).  ADVANTAGES/DISADVANTAGES The rotating measuring head design is the basis for many of the Taylor Hobson Talyrond systems: Ø The precision spindle carries a light load (measuring head) therefore allowing high accuracy measurements without excessive cost. Ø The work table (stationary) can be large thereby accommodating large work pieces. Ø Computer aided centring and automatic centring is available. Ø Internal and external measurements can be made.  ADVANTAGES/DISADVANTAGES The stationary measuring head systems are also commercially available: Ø The measurement pick-up is more easily adapted to measurements of concentricity and alignment. Ø The measurement head is more free to move into constricted measurement positions. Ø Straightness measurement can be made. Ø The precision spindle of the rotating turn table limits the mass and size of the work piece being measured. Ø Computer aided centring and automatic centring is available.  RECORDING ROUNDNESS RESULTS Displacements of the stylus are movements relative to the axis of rotation of the spindle, in both designs of instrument. A perfectly centred cylindrical component will generate a perfect circle on the polar chart. With an irregular component, the measurement stylus will detect departures of roundness relative to the centre of the component.  RECORDING ROUNDNESS RESULTS The roundness variations are very small (microns) and large magnifications are used (x2000 typically) to allow accurate measurement of these variations. Note that generally, the diameter is not measured and is ignored. The equipment scales the results onto standard recording paper (or PC software) no matter what the dimension of the part is. The recorded trace is directly linked to the rotational position on the work piece, allowing the identification of irregularity positions. However, care must be taken when interpreting traces because radial magnifications will differ from circumferential magnifications.  ISO 12181:2011 Geometrical Product Specifications (GPS) — Roundness Part 1: Vocabulary and parameters of roundness This part of ISO 12181 explains how to define Roundness or Out-of-Roundness and then numerate it. Geometrical Product Specifications (GPS) — Roundness Part 2: Specification operators This part of ISO 12181 explains the filtering requirements of the profiles derived from the contact measuring equipment.  ANALYZING ROUNDNESS RESULTS ISO 12181 Part 1:2011 suggests four methods of quantifying the roundness error. All methods are based on establishing a centre and then using that centre to draw concentric circles which contain the trace. The radial separation of of peaks and/or valleys with respect to the circles, divided by the magnification will give the deviation from roundness. Reference should be made to ISO 1101:2013 which details all definitions of design tolerancing for geometric features.  MINIMUM ZONE REFERENCE CIRCLES The Minimum Zone Reference Circles are two concentric circles which just enclose the profile and have minimal radial separation. The maximum radial distance between the two circles is the value of the out-of-roundness. The centre is termed as the Minimum Zone Centre. MZC  MAXIMUM INSCRIBED REFERENCE CIRCLE The Maximum Inscribed Reference Circle (Plug Gauge Centre) is the largest circle which is just contained by the trace. The out-of-roundness is specified as the height of the largest peak above the circle. MINIMUM CIRCUMSCRIBED REFERENCE CIRCLE The Minimum Circumscribed Reference Circle (Ring Gauge Centre) is the smallest circle which will just contain the trace. The out-of-roundness is specified as the depth of the lowest valley below the circle.  LEAST SQUARES REFERENCE CIRCLE The Least Squares Reference Circle and the Minimum Zone Reference Circle methods were the preferred British Standards methods of evaluating out-of-roundness. The Maximum Inscribed Reference Circle, and, the Minimum Circumscribed Reference Circle, have both previously been described by British Standards as being non-preferred methods. ISO 1101:2013 (Section 18.3) defines roundness assessment of geometric features solely in the context of Minimum Zone Reference Circles. However, many legacy specifications, and many existing current practices may specify or require the use of the other three methods.  LEAST SQUARES REFERENCE CIRCLE The Least Squares Reference Circle can be regarded as representing the average of all the peaks and valleys (minimum departure calculation). The out-of-roundness is a function of the radial distance of the maximum peak (P) from the circle and/or the distance of the maximum valley (V) from the circle. P V LSC V: reference-to-valley P: peak-to-reference roundness deviation roundness deviation  OTHER MEASURED PARAMETERS The versatility of today’s roundness measuring equipment means that parameters can be measured and quantified: Ø Eccentricity is the term used to describe the position of the centre of the profile relative to some datum point. Denoted as a distance (mm) and an angle (degrees). Ø Concentricity is similar to eccentricity and is defined as the diameter of the circle described by the profile centre when rotated about the datum point. Ø Using additional attachments, squareness can be measured, which is defined as the minimum axial separation of two parallel planes normal to the reference axis, and which totally enclose the LS reference plane.  METROLOGY (The Science of Measurement) Surface Texture Dr Jon Petzing http://www.npl.co.uk/reference/measurement-units/si-base-units/the-kilogram https://youtu.be/H2QpAB-ccS4  WHY MEASURE SURFACE TEXTURE ? All manufactured/machined surfaces show deviations from absolute perfection It has long been recognized in engineering that careful finishing of components can give rise to improved: Fatigue resistance Bearing properties Interchangeability Wear resistance which all lead to components having a longer operational life, or the same life but operating in more arduous conditions For other applications a certain degree of ‘roughness’ is not only acceptable but can be essential for correct performance SURFACE TEXTURE - FATIGUE LIFE Engineering components are often subject to repeated reversals of stress, which causes fatigue The number of stress reversals that a component can withstand is called the fatigue life Failure due to fatigue is always initiated or started at a sharp corner or defect, such as the root of s surface irregularity Such fatigue failures can arise due to surface defects even on non-working surfaces of the component SURFACE TEXTURE - BEARINGS A “perfect” surface, that is one with no irregularities and therefore perfectly smooth, would not be a good bearing. A “perfect” surface would allow metal-to-metal contact and would not maintain a lubrication film. This would cause surface bonding, very similar to two slip gauges being wrung together. Traditional bearing surface were “scraped” providing large contact areas with valleys which helped to maintain the lubrication film. This form of finishing is expensive and other methods are used such as including low friction materials in the bearing surfaces. SURFACE TEXTURE - WEAR/ADHESION The rate of wear is dependent on the surface area of material contact. The larger the area of contact, the lower the load per unit area, and hence the lower the rate of wear. Adhesion in composite materials is becoming more important in manufactured components. The type of surface of the components and their surface texture have already been identified as important aspects of composite integrity. TYPICAL MACHINED SURFACES AND PROFILES Taylor Hobson Ltd  MACHINING PROCESSES Relative production time Relative production cost Surface texture measurement MACHINING PROCESSES  QUALIFYING SURFACE TEXTURE Surface texture assessment has been an engineering function which has been practiced for at least 150 years Surface texture measurement instrumentation became commonplace after 1945, designed and manufactured by companies such as Taylor Hobson Ltd The difficulty with surface texture measurement has been a common set of parameters by which the texture could be quantified Early engineers relied on visual/tactile comparative measurements Sets of roughness standards have been developed for every machining method and obtainable value known  QUANTIFYING SURFACE TEXTURE Perfect surface Slightly curved surface Reduced spacing of Curved surface the high points Wavy surface Rough surface  QUANTIFYING SURFACE TEXTURE Most surfaces will exhibit several aspects of surface texture simultaneously The categories of surface texture are divided into three form types: - ROUGHNESS - The irregularities in the surface texture arising from the inherent action of the cutting process (cutting tool on the component) - WAVINESS - The irregularities caused mainly due to machine vibration and deflections under cutting force - ERRORS OF FORM - The departure from the geometrical shape Roughness and waviness are the primary and secondary quantifiers of surface texture, whilst errors of form are often considered separately QUANTIFYING SURFACE TEXTURE  QUANTIFYING SURFACE TEXTURE Surface texture parameters attempt to put numbers to roughness and waviness which can be traced to calibrated National Standards There are many different parameters, however no single parameter can be used for every measurement case It is often necessary to specify and measure more than one surface texture parameter, these being chosen for particular applications with care The most common design of surface texture measuring instrument uses a mechanical stylus to trace the surface, providing a signal which is electronically magnified  SURFACE TEXTURE MEASUREMENT Amplifier Recorder Stylus 4mm - 5mm Direct read-out meter (analog/digital) Skid Surface under inspection The skid provides a measurement datum, the stylus provides accurate texture assessment  PORTABLE INSTRUMENTS Mahr GmbH Taylor Hobson Ltd Kosaka  LARGE INSTRUMENTS Mahr GmbH Taylor Hobson Ltd  SURFACE TEXTURE MEASUREMENT The amplified signals produce a magnified trace (plot or software) Magnification in the vertical direction is large and produces a very distorted appearance to the trace. ISO 16610 specifies the magnifications and filters  SURFACE TEXTURE MEASUREMENT Normally each surface texture measurement is taken from series of five sampling lengths It is necessary to maintain a standard sampling length for consistency It is necessary to take several traces at different positions on the work piece  SURFACE TEXTURE ASSESSMENT The assessment length (4mm) is broken down into five 0.8mm sections Parameters for each section are averaged to provide an assessment length value The surface texture parameters can be considered as falling into one of three categories: - Parameters based on the whole curve - Parameters based on the magnitude of certain features - Parameters based on the spacing of certain features Most parameters are referred to by an upper case R together with an appropriate suffix which indicates the manner of data processing All detailed in ISO 4287 SURFACE TEXTURE ASSESSMENT  AMPLITUDE PARAMETERS Ra is the universally recognised, and most used, international parameter for roughness measurement (formerly the Centre Line Average (CLA)) Ra is the arithmetic mean of the absolute departures of the roughness profile from the mean line Another commonly used parameter based on the whole curve is Rq, the root mean square (RMS) value 1 L Ra = ò | f ( x )| dx L 0 X Centre 1 L 2 (Mean) Rq = ò | f ( x )| dx Line L 0 L  MAGNITUDE PARAMETERS An alternative approach is to look at the main peaks and valleys Rp is the maxiumum peak height, Rv is the maximum valley depth Rz is the maximum peak to valley height of the profile: 1 i=n Z1 + Z2 + Z3 + Z4 + Z5 Rz = å n i =1 Zi = 5 L  METROLOGY Time Dr Jon Petzing Wolfson School of Mechanical & Manufacturing Engineering Loughborough University  TIME - CALENDERS Celestial bodies-the sun, moon, planets, and stars-have provided us a reference for measuring the passage of time throughout human existence. In every culture, some people have been preoccupied with measuring and recording the passage of time. Five thousand years ago, Sumerians in the Tigris-Euphrates valley in today's Iraq had a calendar that divided the year into 30-day months, divided the day into 12 periods (each corresponding to 2 of our hours), and divided these periods into 30 parts (each like 4 of our minutes). The earliest Egyptian calendar was based on the moon's cycles, but later the Egyptians realized that the "Dog Star" in Canis Major (Sirius), rose next to the sun every 365 days. Based on this knowledge, they devised a 365-day calendar that seems to have begun in 4236 B.C., the earliest recorded year in history.  TIME - CALENDERS In Babylonia, again in Iraq, a year of 12 alternating 29-day and 30-day lunar months was observed before 2000 B.C., giving a 354-day year. The Mayans of Central America relied not only on the sun and moon, but also the planet Venus, to establish 260-day and 365-day calendars. This culture flourished from around 2000 B.C. until about 1500 A.D. They left celestial-cycle records indicating their belief that the creation of the world occurred in 3113 B.C. Their calendars later became portions of the great Aztec calendar stones. Other civilizations, such as our own, have adopted a 365-day solar calendar with a leap year occurring every fourth year.  TIME - CLOCKS 5000 to 6000 years ago great civilizations in the Middle East and North Africa initiated clock-making as opposed to calendar-making. The Egyptians formally divided their day into parts. Obelisks (slender, tapering, four-sided monuments) were built as early as 3500 B.C. Their moving shadows formed a kind of sundial, enabling citizens to partition the day into two parts by indicating noon. Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 B.C. to measure the passage of "hours." This device divided a sunlit day into 10 parts plus two "twilight hours" in the morning and evening.  RULES FOR CLOCKS A variety of ways have been devised over the past few millennia to mark the passage of time, and it is instructive to define in broad terms what constitutes a clock. All clocks must have two basic components: 1/ A regular, constant or repetitive process or action to mark off equal increments of time. Early examples of such processes included movement of the sun across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses ("hourglasses"), and in the Orient, small stone or metal mazes filled with incense that would burn at a certain pace. 2/ A means of keeping track of the increments of time and displaying the result. Our means of keeping track of time passage include the position of clock hands and a digital time display RULES FOR CLOCKS  MECHANICAL CLOCKS In 1656, Christiaan Huygens, a Dutch scientist, made the first pendulum clock, regulated by a mechanism with a "natural" period of oscillation. Huygens' pendulum clock had an error of less than 1 minute a day, the first time such accuracy had been achieved. His later refinements reduced his clock's errors to less than 10 seconds a day. In 1721 George Graham improved the pendulum clock's accuracy to 1 second a day by compensating for changes in the pendulum's length due to temperature variations.  MECHANICAL CLOCKS John Harrison refined and added new methods of reducing friction. By 1761 he had built a marine chronometer with a spring and balance wheel escapement that won the British government's 1714 prize (of over $2,000,000 in today's currency) offered for a means of determining longitude to within one-half degree after a voyage to the West Indies. It kept time on board a rolling ship to about one-fifth of a second a day, nearly as well as a pendulum clock could do on land, and 10 times better than required. In 1889 Siegmund Riefler developed a clock with a nearly free pendulum, which attained an accuracy of a hundredth of a second a day. TIME ZONES  QUARTZ CLOCKS The standard for time keeping was changed to quartz crystal clocks in the 1930s and 1940s, improving timekeeping performance far beyond that of pendulum and balance-wheel escapements. Quartz clock operation is based on the piezoelectric property of quartz crystals. If you apply an electric field to the crystal, it changes its shape, and if you squeeze it or bend it, it generates an electric field. + _ When put in a suitable electronic circuit, this interaction between mechanical stress and electric field causes the crystal to vibrate and generate a constant frequency electric signal that can be used to operate an electronic clock display.  QUARTZ CLOCKS Quartz crystal clocks were better because they had no gears or escapements to disturb their regular frequency. Even so, they still rely on a mechanical vibration whose frequency depends critically on the crystal's size and shape. Thus, no two crystals can be precisely alike, with exactly the same frequency. Such quartz clocks continue to dominate the market in numbers because their performance is excellent and they are inexpensive. But the timekeeping performance of quartz clocks has been substantially surpassed by atomic clocks.  ATOMIC CLOCKS Atoms (and molecules) have resonances; each chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies. These resonances are inherently stable over time and space. An atom of hydrogen or cesium today is exactly like one a million years ago or in another galaxy. This is a potential "pendulum" with a reproducible rate that can form the basis for more accurate clocks.  ATOMIC CLOCKS In 1949 NIST built the first atomic clock, which was based on ammonia. However, its performance wasn't much better than existing standards, and attention shifted almost immediately to more-promising, atomic-beam devices based on cesium. In 1957 NIST completed its first cesium atomic beam device. In 1967 the caesium atom's natural frequency was formally recognized as the new international unit of time. This has led to the following definition of the second: “ The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.”  REALISTIC TIMING EQUIPMENT The unit of time is realised using a caesium-beam atomic clock, based on the vibration states of the caesium-133 atom. This system allows the SI unit to be realised with an uncertainty of between 1 part in 1013 and 1014. Secondary standards are provided by rubidium gas cell resonator controlled oscillators or quartz oscillators. Rubidium oscillators provide a typical short term stability of five parts in 1012 (over 100 s) and long term stability of 1 part in 1011 (month). Quartz oscillators provide a typical short term stability of five parts in 1012 (over 1 s) and long term stability of 1 part in 108 (month). RELEVANT TIME MEASUREMENT RELEVANT TIME MEASUREMENT  RELEVANT TIME MEASUREMENT Start Finish Trigger A Trigger B 100m Olympic Sprinter Calculation = Time Golf ball flight Start Finish Trigger A Trigger B Calculation = Velocity  SINGLE SHOT TIME EQUIPMENT Channel A Display Input A Input and trigger circuitry Gate Counter Input B Input and trigger circuitry 10 MHz Channel B crystal oscillator  QUARTZ OSCILLATOR COMPENSATION The quartz crystal resonator in a timer can be uncompensated, temperature compensated or oven stabilised. The frequency stability of quartz oscillators is affected by ageing, temperature, variations in supply voltage, and changes in power supply mode. Uncompensated oscillators give sufficient accuracy for 5-6 digit measurements in most room-temperature applications. Temperature compensated oscillators can give sufficient accuracy for 6-7 digit measurements. Oven stabilised oscillators have better ageing performance and are suitable for 7-9 digit instruments.  TIME INTERVAL MEASUREMENT Start pulse (channel A) Electrical/ optical Stop pulse (channel B) Electrical/ optical Gate T 10MHz clock Counted cycles of 10MHz 100ns per pulse clock  RESOLUTION Clock Gate Single shot time interval T measurements using a 10MHz clock have a 6 pulses Counted signal (n) resolution of +/- 100 ns. Resolution will be either Clock ‘n’ or ‘n+1’. Number of counts = Time Gate (T) x Frequency of the 7 pulses T oscillator = Tf. Counted signal (n + 1)  ERRORS Accuracy of counters and timers is limited by four factors; System resolution, trigger errors, systematic errors and time-base errors. Trigger error is the absolute measurement error due to input noise causing triggering which is too early or too late: TE = +/- (Peak-to-peak noise voltage) / (Signal slew rate) Systematic error is caused by differential propagation delays in the start and stop sensors, amplified channels, or by errors in the trigger level settings of the two channels. These errors can be removed by calibration. Time-base errors are caused by deviation on the frequency of the crystal frequency from its calibrated value.