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Engineering Metrology is a book that. Nov 30, Engineering metrology is the measurement of dimensions: length, thickness, diameter, taper angle, flatness, profiles, etc. In engineering, there are various stages during which inspection and measurement is required. The podcast you're looking for doesn't have any content yet.
Engineering Metrology - R. Scribd is the world's largest social reading and publishing site. Open navigation menu. Upload a Thing! However, there will be an element of doubt in the value of the correction so stated. This doubt is quantitatively expressed as accuracy or overall uncertainty in assigning the value to the correction stated and will be one component of the systematic uncertainty of that instrument.
For example, in case of a metre bar, the distance between the zero and 1, mm graduation marks may be given as Then 0. This is the concept of establishing a valid calibration of a measuring instrument or measurement standard by step-by-step comparison with better standards up to an accepted or specified standard. In general, the concept of traceability implies eventual reference to an appropriate national or international standard.
A prescribed value of a quantity to which reference is made, for example, in order to define the value of an error as a proportion of this prescribed value. Usually accuracy is poor at the lower end of scale which should be avoided. In such a situation, where accurate measurement is required throughout full range, two instruments with different ranges may be used, one for lower range and other for full range.
If an instrument is precise then accuracy can be taken care of by proper calibration of the instrument. It may be mentioned that accuracy is affected by systematic errors which can be accounted for. Resolution or sensitivity, i. It represents a smallest change in the measured quantity which produces a perceptible movement of the pointer. Classification of Methods of Measurements In precision measurements various methods of measurement are followed depending upon the accuracy required and the arnount of permissible error.
There are numerous ways in which a quantity can bemeasured. Any method of measurement should be defined in such a detail and followed by such a standard practice that there is little scope for uncertainty. The nature of the procedure in some of the most common measurementsis described below. Actual measurements may employ one or more combinations of the following. In this method the value of a quantity is obtained directly by comparing the unknown with the standard.
It involves, no mathematical calculations to arrive at the results, for example, measurement of length by a graduated scale. The method is not very accurate because it depends on human insensitiveness in making judgement. For example, measurement of density by measuring mass and geometri- cal dimensions. For example, measuring a quantity directly in accordance with the definition of that quantity, or measuring a quantity indirectly by direct measurement of the quantities linked with the definition of the quantity to be measured.
This method involves comparison with either a known value of the same quantity or another quantity which is function of the quantity to be measured. In this method, the quantity to be measured is measured by direct comparison on an indicating device by replacing the measuring quantity with some other known quantity which produces same effect on the indicating device.
For example, determination of mass by Borda method. This is a method of measurement by direct comparison in which the value of the quantity to be measured is first balanced by an initial known value A of the same quantity ; next the value of the quantity to be measured is put in the place of that known value and is balanced again by a second known value B.
When the balance indicating device gives the same indication in both cases, the value of the quantity to be measured is VAB. For example, determination of amass by means of a balance and known weights, using the Gauss double weighing method.
This method involves measuring the difference between the given quantity and a known master of near about the same value. For example, determination of diameter with master cylinder on a comparator. In this differential method of measurement the very small difference between the given quantity and the reference is determined by the observation of the coincidence of scale marks. For example, measurement on vernier caliper. In this method the quantity to be measured is compared with a known source and the difference between these two is made zero.
In this method, the value of the quantity is directly indicated by deflection of a pointer on a calibrated scale. In this method, the given quantity is compared with two or more known value of near about same value ensuring at least one smaller and one bigger than the quantity to be measured and the readings interpolated. In this method, the given quantity is compared with two or more known smaller values and extrapolating the reading.
This is the method of measurement by com- parison in which the value of the quantity to be measured is combined with a known value of the same quantity so adjusted that the sum of these two values is equal to predetermined comparison value. For example, determination of the volume of a solid by liquid displacement. It involves the comparison of the actual contour of a component to be checked with its contours in maximum and minimum tolerable limits.
This method provides for the checking of the cumulative errors of the interconnected elements of the component which are controlled through a combined tolerance. In this method, the several related dimensions are gauged individually, ie, each component element is checked separately.
For example, in the case of thread, the pitch diameter, pitch, and flank angle are checked separately and then the virtual pitch diameter is caleulated. It may be noted that value of virtual pitch diameter depends on the deviations of the above thread elements. The functioning of thread depends on virtual pitch diameter lying within the specified tolerable limits.
In case of composite method, all the three elements need not be checked separately and is thus useful for checking the product parts. Element method is used for checking tools and for detecting the causes of rejects in the product. In contact methods of measurements, the measuring tip of the instrament actually touches the surface to be measured. In such cases, arrangements for constant contact pressure should be provided in order to prevent errors due to excess contact pressure.
Such instruments include tool-maker's microscope and projection comparator, ete. Classification of Measuring Instruments. According to the functions, the measur- ing instruments are classified as : 1 Length measuring instruments, 2 Angle measuring instruments.
According to the accuracy of measurement, the measuring instruments are classified as follows : 1 Most accurate instruments e. Metrological characteristics of Measuring Instruments. Measuring instru- ments are usually specified by their metrological properties, such as range of measurement, scale graduation value, scale spacing, sensitivity and reading accuracy. Range of Measurement. It indicates the size values between which measurements may be made on the given instrument. Ibis the difference between the values of the measured quantities corresponding to the terminal scale marks.
Instrument range. Scale Spacing. It is the distance between the axes of two adjacent graduations on the scale. Most instruments have a constant value of scale spacing throughout the scale.
Such scales are said to be linear. In case of non-linear scales, the scale spacing value is variable within the limits of the scale. Scale Division Value. It is the measured value of the measured quantity corresponding to one division of the instrument, eg.
It is the ratio of the scale spacing to the division value. It could also be expressed as the ratio of the product of all the larger lever arms and the product of all the smaller lever arms. It is the property of a measuring instrument to respond to changes in the measured quantity. Sensitivity Threshold. It is defined as the minimum measured value which may cause any movement whatsoever of the indicating hand, It is also called the discrimination or resolving power of an instrument and is the minimum change in the quantity being measured which produces a perceptible movement of the index.
Reading Accuracy. It is the accuracy that may be attained in using a measuring instrument. Reading Error. Itis defined as the difference between the reading of the instrument and the actual value of the dimension being mieasured, Accuracy of observation, It is the accuracy attainable in reading the scale of an instrument. The width of scale mark is usually kept one-tenth of the scale spacing for accurate reading of indications.
Its apparent change in the position of the index relative to the scale marks, when the scale is observed in a direction other than perpendicular to its plane. It is the variation of indications in repeated measurements of the same dimension.
The variations may be due to clearances, friction and distortions in the instrument's mechanism. Repeatability represents the reproducibility of the readings »f an instrument when a series of measurements in carried out under fixed conditions of use. Measuring force. It is the force produced by an instrument and acting upon the measured. It is usually developed by springs whose deformation and Pressure change with the displacement of the instrument's measuring spindle. The Measurement Problem In practice, we come across four basic conditions to be controlled by tolerances, viz, a size, 6 form, c location and d conditions of assembly, operation, or function.
The difference between sizes X and Y determines the assembly condition. Size conditions are generally simple to specify and control ; the form and location conditions Form are more complex, especially where composite surfaces Location ony ze and cumulative tolerances are involved. It will be appreciated that all these four conditions inter-relate to a. Lack of true geometric Fig Product conditions, perfection makes it difficult to define and control product quality characteristics.
It may be noted that it is easy to define geometric form but difficult to produce, However, to maintain specific quality, the variation from perfect form must be defined and controlled. The geometric variations known as macro-errors concern straightness, flatness, parallelism, squareness, angular displacement, sym- metry, concentricity, eccentricity, roundness.
Lack of perfect rigidity caused due to material properties like expansion, stretching, springing, warping etc. The inter-relationships of size, form and location conditions required to define quality characteristics, coupled with production variations due to geometric form and rigidity errors, lead to a variety of complex measurement problems involving sophisticated gauging method.
Insome cases the configuration of partsis such that accurate measurement becomes difficult. In some cases, it is not possible for the standard gauge or tool to span the component.
Such cases are : parts having not even a single common plane, irregular curved surfaces, odd number spline or gear, parts with phantom dimensions e. It must beremembered that the improperly located dimensions, can often have a greater effect on correctness of dimensional measurements than errors due to insufficient indicating accuracy.
The precision of the measurement can be affected by limitations in either of the basic requirements, viz, the accuracy of the instrument at the proper location of the gauging points which determine the dimension being measured on the physical part.
General Care of Metrological Equipment The equipment apparatus used for precision measurements is designed to fine limits of accuracy and is easily liable to be damaged by even. A great deal of careful handling is, therefore, required. As far as possible, the highly finished surfaces should not be touched by hand because the natural acids on the skin are likely to corrode the finished surface and also the temperature of body may upset the dimensions of the precision instruments.
In order to overcome this many standard metrology laboratories recommend washing of hands thoroughly and coating them with a thin film of pure petroleum jelly before handling the instruments.
For this purpose, the highly finished surfaces are first wiped with a solvent to remove any finger marks and then coated with mixture of heated petroleum jelly and petrol. This mixture spreads much more easily and is applied with cloth or with fingers. Brushing is not recommended as it is liable to trap air which, with the moisture it contains, may cause rusting.
Objectives of Metrology While the basie objective of a measurement is to provide the required accuracy at minimum cost, metrology would have further objective in a modern engineering plant with different shops like Tool Room, Machine Shop, Press Shop, Plastic Shop, Pressure Die Casting Shop, Electroplating and Painting Shop, and Assembly Shop, as also Research, Development and Engineering Depart- ment.
In such an engineering organisation, the further objectives would be as follows : a Thorough evaluation of newly developed products, to ensure that components designed are within the process and measuring instrument capabilities available in the plant.
To minimise the cost of inspection by effective and efficient use of available facilities, and to reduce the cost of rejects and rework through application of Statistical Quality Control Techni- ques.
This is achieved by laying down inspection methods for any product right at the time when production technology is prepared. P Maintenance of the accuracies of measurement.
This is achieved by periodical calibration of the metrological instruments used in the plant. Arbitration and solution of problems arising on the shop floor regarding methods of Tmeasurement. The degree of accuracy of calibration depends on the accuracy of the inspecting instruments. Devices which reduce dependence on operator skill contribute to both efficiency and accuracy. A good inspection tool should be capable of being checked against itself.
This feature increases the reliability. Standardisation and Standardising Organisations For overall higher economy, efficiency and productivity ina factory and country, itis essential that diversity be minimised and interchangeability among parts encouraged. All this is possible with standardisation. Standardisation is done at various levels, viz.
International, National, Association, Company. In India, Bureau of Indian Standards BIS is responsible for evolving standards on metrological instruments, etc, There are several sectional committees, each dealing with various main branches of industry, in BIS.
The detailed work of drawing up specifications is done by more specialised technical committees who prepare a draft standard based on practice in other countries and the needs of the country, and circulate it to relevant industries, government and service departments, research and teaching organisations, and others likely to be interested.
Comments are invited both from producer and user to consider all aspects ; meetings help to discuss the matters in depth and final standards issued. The technical committees also keep on revising the existing standards from time to time. Provide information, documentation and other services to consumers and recognised consumer organisations on such terms and conditions as may be mutually agreed upon ; Give recognition to quality assurance systems in manufacturing or processing units on such terms and conditions as mutually agreed upon ; h Bring out handbooks, guides and other special publications ; and for conformity to any other standard if so authorized.
Thus, the main funetions of the Bureau can be grouped under standards formulation, certification marking and laboratory testing, promotional and international activities, Bureau of Indian Standards has under the Mechanical Engineering Division Council, EDC, a separate Engineering Metrology Sectional Committee. This Committee was set up in and its main task is to formulate standards for the various aspects of dimensional metrological measuring instruments and accessories used in the mechanical engineering field.
Before Second World War, U. In , the LS. In fact, for engineering matters, the foremost standards organisation at international level is 1. The national standards organisation of individual countries are the members of 1. The 1. Lot of co-operative discussions in the field of standardisation have also been carried out in three countries—America, Britain and Canada known was ABC conference, The International Electro-technical Commission IEC deals with electrical engineering standards.
National Physical Laboratories NPL carry out lot of research work in various fields ; responsible for defining standards, and also issue certification marks for quality instruments. General Conference of Weights and Measures. Its objects are : To draw up and promote the decisions necessary for the propagation and perfection of an international system of units and standards of measurement..
International Committee of Weights and Measures. This Committee is placed under the authority of the General Conference of Weights and Measures and is responsible for promoting the decisions taken by the latter. Its objects are : To direct and supervise the work of the International Bureau of Weights and Measures. It is created under the Metre convention for measurement standard activities.
It provides leadership in ensuring collaboration on metrological matters and the maintenance of an efficient worldwide measurement system.
It serves as the technical focal point to guarantee the equivalence of national standards. It is engaged in international laboratory accreditation and the standards writing bodies. A national laboratory responsible for the development and maintenance of measurement standards for the dissemination of the SI units, their multiples and sub multiples, and capable of making accurate measurements available to all users.
Legal Metrology is concerned with the statutory technical and legal requirement of units of measurements, methods of measurements and measuring instruments with a view to assure public guarantee in respect of the security and the appropriate accuracy of measurements. To study with the object of unification, statutory and regulatory problems of legal metrology the solution of which is of international interest. OIML has made a number of international recommendations.
Gz National Service of Legal Metrology. It'is the directing organisation National Institute of Legal Metrology. It is entrusted with the performance of scientific and research work National Bureau of Verification.
This SI like traditional metric system, is based on decimal arithmetic, For each physical quantity, units of different sizes are formed by multiplying or dividing a single base value by powers of Obviously this offers great advantage because the changes can be made very simply by adding zeros or shifting decimal point.
In the metric system we have been following so far, this simplicity of a series of units linked by powers of 10 is limited to plain quantities like length, and this simplicity is lost when more complex units like energy ete.
For example energy is now represented by several units like kgm, H. In contrast the SI provides only one basic unit for each physical quantity, and universality is thus achieved. This system in superior to other systems and also more convenient as it is coherent, rational and comprehensive. The SI is a coherent system, in the sense that the product or quotient of any two unit quantities in the system is the unit of the resultant quantity, eg. It is rational system sinceithas absorbed in itself the rationalised MKSA system.
It is also comprehensive because its seven base units cover all disciplines. Realising that some derived units may be compex array of base units, some units have been given special names. They are given on page 24, SI also recommends the ise of supplementary units, the only authorised supplementary units being measures for plane and spherical angles.
Though these could be expressed by base units but have been available as a convenience to users. Plane angles are represented by radians and solid angles by steradians. The two major advantages of coherency of SI units are i same system and unit of measurement are used regardless of industry, trade, or discipline ; and ii minimum of conversion factors are needed other than powers of In connection with SI units, somerules of style, abbreviations, writing, and drafting practices are applied, some of which are described below ; a No dots, commas, ete.
For example, half metre will be written as 0. N metre newton which if written as mN can be misunderstood as millinewton, c All symbols and prefixes are lowercase letters, except symbols derived from proper names, like W for watt, M, G and T for the largest three power-of prefixes. All symbols should be used as they are to avoid any confusion. The product of two or more units is preferably indicated by a dot which can be dispensed with when there is no risk of confusion with another unit symbol e.
A temperature interval can also be expressed in degree celsius. For example, 92 N is commitment 92N is not. For example 0. A sequence of four figures is generally not broken in groups. Units with names of scientists should be capitalised when written in full.
Correct the mistakes in following numbers and symbols : mps 1 Fong 82nd 45mm. C-m, M-W Kg, ker, kgs, kilo KW electric consumption B8BR88 2 Sol. NPL is the custodian of national measurement standards for physical measurement in the country. This premier national laboratory is established with the objective to strengthen and advance physics- oriented research for the overall development of science and technology in the country. NPL has the responsibility of physical measurements based on the International system SI units under the subordinate legislation of Weights and Measures Act reissued in under the Act.
NPL also has the statutory obligation to establish, maintain and update the national standards of measurement and calibration facilities for different parameters. NPL maintains seven SI base units, viz. The nodal work done by the NPL in metrication of the entire measurement system of the Country since forms the basis of legal metrology funetion in the country coordinated by the Directorate of Weights and Measures DWM Govt.
Mensureonn standards, calibration service, specialized testing back-up, technical advisory consultancy services and participation in the committees of the DWM form the bulk of NPL support for these important areas of consumer Scientific Imowledg, protection and improvement and securing of the living conditions.
Sources of Errors Instrument or indication errors may be caused by defects in manufacture of adjustment of an instrument, imperfections in design, ete. The error of measurement is the combined effect of component errors due to various causes.
There may be errors due to method of location, environmental errors, errors due to the properties of object of measurement, viz. Static Errors. The environmental effect and other external influences on the Properties of the apparatus also contribute to static erros. Other sources of static errors could be inexactness in the calibration of the system, displaying the output of the measuring system in a-way that requires Subjective interpretation by an observer.
From above it could be concluded that static errors ster from three basic sources : reading error, characteristic error and environmental error.
In the Treasurement of length ofa surface table with a rule, these errors will be encountered when aligning the ends of the rule and surface table, and when estimating the length of the table, The static error divided by the measurement range difference between the upper and lower limits of measurement gives the measurement precision.
Reading error describes such factors as parallax, interpolation, optical resolution readability or output resolution. Reading errors apply exclusively to the readout device and have no direct relationship with other types of errors within the measuring system, Attempts have been made to reduce or eliminate the reading errors by relatively simple techniques.
Where there is possibility of error due to parallax, the use of mitror behind the readout pointer or indicator virtually eliminates occurrence of this type of error. Interpolation error ean be tackled by increasing the optical resolution by using a magnifier over the scale in the vicinity of the pointer. The use of digital readout devices is increasing tremendously for display purposes as it eliminates most of the subjective reading errors usually made by the observer.
However, there exists a Possibility of plus or minus one count error in digital readout devices also and its value can be effectively reduced by arranging full range to correspond to huge number of pulses so that one pulse has very negligible value.
Digital counting devices are capable of counting each and every pulse, however short may he the duration, but it is only during start and at stop that one pulse is likely to be missed which can lead to error. Environmental errors result from effect of surrounding temperature, pressure and humidity on measuring system. It can be reduced by controlling the atmosphere according to estipulated requirements.
External influences like magnetic or electric fields, nuclear radiation, vibration or shock, periodic or random motion ete. It is important to note that these factors affect both the measuring system and measurand, and usually the effects of these factors on each component are independent.
Thus the environmental errors of each component of the measuring system make a separate contribution to the static errors. Due to this reason, the number of environmental variables and external influences that could affect the measurement should be minimised and where it is not possible to do so then their effect should be computed and taken into account.
Characteristic error is defined as the deviation of the output of the measuring system under constant environmental conditions from the theoretically predicted performance, or from nominal performance specifications. If the theoretical output is a straight line, then linearity, hysteresis, repeatability, and resolution errors are part of the characteristic error.
Linearity errors, hysteresis and repeatability errors are present to some degree in each component ofa measuring system. Other characteristic errors include gain errors and zero offset, often collectively called calibration errors. Similar characteristic errors in each component of the measuring system tend to be additive.
However, the loading errors and dynamic errors which are generally encountered in process measurements and not in the field of Metrology, will also be diseussed in brief here to complete the subject. The effects of instrument loading are unavoidable and must be determined specifically for each measurement and measurand.
Such loading erros are often the single greatest uncertainty in a physical measurement. Therefore, measuring system should be selected such that its sensing element will minimise instrument loading error in the particular measurement involved.
In a steady state measurement, the cumulative effect of static errors and instrument loading errors determines the accuracy of the measurement.
Dynamic error is caused by time variations in the measurand and results from the inability of a measuring system to respond faithfully to a time-varying measurand. Usually the dynamic response is limited by inertia, damping, friction or other physical constraints in the sensing or readout or display system. Dynamic error is characterised by the frequency and phase response ode criterion of the system for the cyclic or periodic variations in the measurand input.
For random or transient inputs, the dynamic error is described by the time constauit of response time. In both the cases , it is essential that dynamic characteristics of the measuring system be known before putting the system to measure time varying inputs.
It is thus seen that different errors entering into any observation arise due to a variety of reasons, Many times it may not be possible to identify the source of errors. Therefore it is more fruitful to classify errors according to the effects they produce rather than on the basis of sources which produce them. For statistical study and the study of accumulation of errors, errors are categorised as controllable errors and random errors.
Systematic error is just a euphemism for ex- perimental mistakes. These are controllable in both their magnitude and sense. These can be determined and reduced, if attempts are made to analyse them. However, they can not be revealed by repeated observations, These errors either have a constant value or a value changing according to a definite law.
These can be due to: 1. Calibration Errors. The actual length of standards such as slip gauges and engraved scales will vary from nominal value by small amount.
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