Metrology concepts and standards

Basic concepts and standards of Metrology

1. INTRODCTION TO SUPPLY CHAIN MANAGEMENT & ROLE OF LOGISTICS POWER POINT PRESENTATION ON Metrology concepts and standards Presented by Md Tayab Ali Lecturer (Sr. Scale) Dept. of Mechanical Engineering H.R.H. The POWIET, Jorhat 6/13/2021 1

2. Meaning of Metrology The word metrology is derived from two Greek words Metro = measurement Logy = science Thus metrology is the science of measurement. Metrology may be defined as, “Comprehensive study of different measuring instruments for finding precision, accuracy, possible sources of errors, and methods for elimination of errors”. Engineering Metrology is restricted to measurement of length and angles and other quantities which are expressed in linear or angular terms. 6/13/2021 2

3. Significance of Metrology In addition to linear and angular measurements, “Metrology” covers the following aspects: (a)Manufacturing: Metrology is concerned with the manufacturing of various instruments. (b)Range and capabilities: Metrology is used to find the ranges and capabilities of various instruments used for measurements. (c) Calibration: Metrology is used to calibrate the measuring instruments according to the prescribed standards with a high degree of accuracy. (d) Method of measurements: Metrology is concerned with the different methods of measurements, essential to obtain precise measurements. (e) Maintaining and defining the standards: For accurate measurements, metrology is concerned with defining and maintaining the standards. 6/13/2021 3

4. Activities of Metrology 6/13/2021 4

5. Objectives or Necessity of Metrology To provide the required accuracy at a minimum cost. To standardize the measuring methods. To find out sources of errors. Useful in selection of proper measuring instruments and gauges. To have good accuracy and precision. To reduce cost of inspection by effective and efficient use of available facilities. To evaluate newly developed products. To enhance consumer satisfaction. To reduce rework and rejections through application of statistical quality control techniques. To prepare design for all gauges and special inspection fixtures. To maintain the accuracies of measurement. 6/13/2021 5

6. Types of Metrology Scientific Metrology- This form of metrology deals with the establishment and development of measurement standards with their maintenance. Industrial Metrology- Industrial metrology’s purpose is to ensure the ‘adequate functioning of measuring instruments’ used in ‘Industry’ as well as in ‘production and testing’ processes. Legal Metrology- Legal metrology is directed by a national organization which is called National Service of Legal Metrology. Legal metrology is concerned, to maintain uniformity of measurement throughout the world 6/13/2021 6

7. Functions of Legal Metrology 6/13/2021 7

8. Inspection Inspection means checking of all materials, products or component parts at various stages during manufacturing. It is the act of comparing materials, products or components with some established standard. 6/13/2021 8

9. Need of Inspection The need of inspection can be summarized as: To ensure that the part, material or a component conforms to the established standard. To maintain customer relation by ensuring that no faulty product reaches the customers. Provide the means of finding out shortcomings in manufacture. It also helps to purchase good quality of raw materials, tools, equipment which governs the quality of the finished products. It also helps to co-ordinate the functions of quality control, production, purchasing and other departments of the organization. 6/13/2021 9

10. PROCESS OF MEASUREMENTS The sequence of operations necessary for the execution of measurement is called process of measurement. Three important elements of measurements are: ( i ) Measurand : It is the physical quantity or property like length, angle, diameter, thickness etc. to be measured. (ii) Reference: It is the physical quantity or property to which quantitative comparisons are made. (iii) Comparator: It is the means of comparing measurand with some reference. 6/13/2021 10 Example: Measuring the length of a M.S. flat by a steel rule. Here the length of M.S. flat is a measurand ,, Steel rule is the reference and Observer’s eye can be considered as a comparator

11. METHODS OF MEASUREMENTS The methods of measurement can be classified as: 1.Direct Method 2. Indirect Method 3. Absolute or Fundamental Method 4.Comparative Method 5. Transposition Method 6. Coincidence Method 7. Deflection Method 8. Complementary Method 9. Contact Method 10. Contactless method 6/13/2021 11

12. 1.Direct Method of Measurement Measurements are directly obtained without any calculations. Example: Measurements by using scales, Vernier calipers, micrometers & bevel protractors etc . 6/13/2021 12

13. 2.Indirect Method of Measurement The value of the quantity to be measured is obtained by measuring other quantities, which are related to required value. Example: Weight of a substance is measured by measuring the length, breath and height of the substance directly and then by using the relation Weight = Length x Breadth x Height x Density 6/13/2021 13

14. 3. Absolute or Fundamental Method It is based on the measurement of the base quantities used to define the quantity. Example: M easuring 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. 6/13/2021 14

15. 4.Comparative Method of Measurement The value of the quantity to be measured is compared with known value of the same quantity or other quantity practically related to it. So, in this method only the deviations from a master gauge are determined. Example: Dial indicators, or other comparators. 6/13/2021 15

16. 5. Transposition Method of Measurement Quantity to be measured is first balanced by an initial known value and then balanced by another new known value. Example: Determination of mass by means of a balance and known weight. 6/13/2021 16

17. 6. Coincidence Method of Measurement Measurements coincide with certain lines and signals. Example: M easurement by Vernier calliper , micrometer. 6/13/2021 17

18. 7. Deflection Method of Measurement The value of the quantity to be measured is directly indicated by a deflection of a pointer on a calibrated scale. Example: Measurement of Pressure. 6/13/2021 18

19. 8. Complementary Method of Measurement The value of quantity to be measured is combined with known value of the same quantity. Example: Determination of the volume of a solid by liquid displacement. 6/13/2021 19

20. 9.Contact Method of Measurement: In this method the sensor or measuring tip of the instrument actually touches the surface to be measured. Example: Measurements by micrometer, Vernier caliper, dial indicators etc. 6/13/2021 20

21. 10. Contactless method of Measurement There is no direct contact with the surface to be measured. Example: Measurement by optical instruments such as tool makers microscope, projection comparator etc. 6/13/2021 21

22. Basic elements of measuring system 1. Standard: The most basic element of measurement is a standard without which no measurement is possible. 2. Work piece: Once the standard is chosen select a work piece on which measurement will be performed. 3. Instrument: Then select an instrument with the help of which measurement will be done. 4. Person: T here must be some person or mechanism to carry out the measurement. 5. Environment: L astly, the measurement should be performed under standard environment. 6/13/2021 22

23. Terminologies used in measuring instruments: Precision Accuracy Sensitivity Readability Calibration Magnification Repeatability Reproducibility Range Threshold Hysteresis Backlash Resolution 6/13/2021 23

24. Precision Precision is the repeatability of the measuring process. (i.e. how closely individual measurements agree with each other) It refers to the group of measurement for the same unit of product taken under identical conditions. It indicates to what extent the identically performed measurements agree with each other. If the instrument is not precise it will give different (widely varying) results for the same dimension when measured again and again. The set of observations will scatter about the mean value. The less the scattering more precise is the instrument. 6/13/2021 24

25. Accuracy The agreement of the measured value with the true value of the measured quantity is called accuracy. The term accuracy denotes the closeness of the measured value with the true value. The difference between the measured value and the true value is the error of measurement. The lesser the error, more is the accuracy 6/13/2021 25

26. Precision Vs Accuracy 6/13/2021 26 Fig.-1

27. Several measurements are made on a component by different types of instruments (A, B and C respectively) and the results are plotted. Fig.-1 (a) shows that the instrument A is precise since the results of number of measurements are close to the average value. However, there is a large difference (error) between the true value and the average value hence it is not accurate. The readings taken by the instruments B as shown in Fig.-1(b) are scattered much from the average value and hence it is not precise but accurate as there is a small difference between the average value and true value. Fig.-1(c) shows that the instrument C is accurate as well as precise. 6/13/2021 27

28. Difference between Accuracy and Precision Accuracy Precision 1. Accuracy is the agreement of measured value with the true value of measured quantity. 1. Precision is the repeatability of the measuring process. It shows, how well identically performed measurements agree with each other. 2. Accuracy is concerned with true value. 2. Precision is concerned with mean value. Precision has no concern with true value. Precision has no meaning for only one measurement, but exists only when numbers of measurements are carried out for measuring same quantity under identical conditions. 6/13/2021 28

29. Difference between Accuracy and Precision Accuracy Precision 3.If true value is 10 mm, then measured dimension of 9.99 mm is more accurate than 9,91 mm. 3. If true value is 10 mm, and readings obtained are 10.001, 10.002, 10.003, 10.004 and 10.005 mm, the mean value of readings will be 10.003 mm. Therefore, The measurements are said to be precise, because all the obtained readings are very close to their mean value (10.003 mm). 4. It is difficult and expensive to have good accuracy. 4. It is much easier and cheaper to achieved precision than to achieve great accuracy. 5. High accuracy cannot be obtained with low precision. 5. High precision cannot be obtained with low accuracy. 6/13/2021 29

30. Sensitivity Sensitivity refers to the ability of a measuring device to detect small variations in a quantity being measured. In other words, sensitivity is the ratio of the change in output of the instrument to a change of input or measured quantity. i.e. Sensitivity = Change in output/Change in input. Higher the ability of such detection of an instrument, more sensitive it is. Example-1: I f on a dial indicator, the scale spacing is 1.0 mm and the scale division value is 0.01 mm, then sensitivity is 100. Example-2: Sensitivity of thermometer means that it is the length of increase of the liquid per degree rise in temperature. 6/13/2021 30

31. Readability Readability refers to the ease with which the readings of a measuring instrument can be read. Fine and widely spaced graduation lines improve the readability. To make the micrometers more readable they are provided with vernier scale or magnifying devices. 6/13/2021 31

32. Calibration Calibration is a pre-measurement process, generally carried out by the manufacturer. It is the process of framing the scale of the measuring instrument by applying some standards”. It is carried out by making adjustments such that the read out device produces zero output for zero input. The accuracy of the instrument depends on the calibration. If the output of the measuring instrument is linear and repeatable, it can be easily calibrated. 6/13/2021 32

33. Magnification: Magnification is the process of enlarging magnitude of the output signal of measuring instrument many times to make it more readable. 6/13/2021 33

34. Repeatability: It is the ability of the measuring instrument to repeat the same results for the measurements for the same quantity, when the measurements are carried out: -by the same observer, - with the same instrument, - under the same conditions, - without any change in location, - without change in the method of measurement, - the measurements are carried out in short intervals of time. 6/13/2021 34

35. Reproducibility Reproducibility is the closeness of the agreement between the results of measurements of the same quantity, when individual measurements are carried out: - by different observers, - by different methods, - using different instruments, - under different conditions, locations, times etc. 6/13/2021 35

36. Range The upper and lower limits an instrument can measure a value or signal such as amps, volts and ohms. 6/13/2021 36

37. Threshold The min. value below which no output change can be detected when the input of an instrument is increased gradually from zero is called the threshold of the instrument. Threshold may be caused by backlash. 6/13/2021 37

38. Hysteresis It is defined as the magnitude of error caused in the output for a given value of input, when this value is measured from opposite direction, i.e from ascending order and then descending order. This is caused by backlash , elastic deformation but is mainly caused due to frictional effects. Hysteresis effects are best eliminated by taking observation in both the direction i.e. in ascending and then descending order values of input and then taking the arithmetic mean. 6/13/2021 38

39. 6/13/2021 39 Hysteresis of a stretched rubber band. The gap between the load and unload is the tendency of the rubber not to return to its original shape due to friction.

40. Resolution Resolution is the smallest measurement that can be measured by a measuring instrument. 6/13/2021 40 The gauge on top has finer resolution. Notice that there are more tick marks between 280 and 290 on the top gauge than on the bottom one. Finer resolution reduces rounding errors, but doesn't change a device's accuracy. However, resolution that is too coarse may add rounding errors.

41. Backlash In Mechanical Engineering, backlash, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established. 6/13/2021 41

42. System of Measurement The following systems of measurement are in use in different countries. 1.F.P.S. System 2.MKS system 3.S.I. System 1.F.P.S. System: In this system, unit of length is yard, unit of mass, weight of force is pound, unit of time is seconds and unit of temperature is degree Fahrenheit. 6/13/2021 42

43. 2.MKS system: Metric system is the predominant system in the world. It is based on metre as a unit of length, kilogram as the unit of mass and kilogram force as the unit of weight or force, unit of temperature is degree centigrade (°C). S.I. System: This S.I . ( International System of Units) provides only one basic unit for each physical quantity. It is comprehensive because its seven units cover all disciplines. For example: The units of length, mass, time, temperature, electric current, luminous intensity, quantity of substance are m, kg, s, k, a cd , and mole respectively. 6/13/2021 43

44. Derived S.I. units Units that are a combination of two or more quantities and which usually requires a compound word to name them are called compound or derived units. Example: Unit of Force is Newton. 1 N = kgm /sq.sec 6/13/2021 44

45. STANDARDS A standard is physical representation of a unit of measurement. A known accurate measure of physical quantity is termed as a standard. These standards are used to determine the values of other physical quantities by the comparison method. The standards of measurements are very useful for calibration of measuring instruments. They help in minimizing the error in the measurement systems. 6/13/2021 45

46. Standard systems of linear measurement There are two standard systems of linear measurement commonly in practice: 1.English System 2.Metric system 6/13/2021 46

47. 1.English System It is also known as British System of linear measurement. This system is based on the “Imperial Standard Yard”. The yard in its current form was first setup in 1855 in England. An imperial standard yard, shown in fig, is a bronze (82% Cu, 13% tin, 5% Zinc) bar of 1 inch square section and 38 inches long. Yard is defined as the distance between the two central transverse lines on the two golden plugs at 62° F, and is equal to 36 inches. Now-a-days, English system is limited in use. 6/13/2021 47

48. Imperial Standard Yards 6/13/2021 48

49. 2. Metric system Also known as international standard system. Based on the “International prototype meter”. This meter was setup in the year 1872 and is maintained by the International Bureau of Weights and Measures in France. The prototype meter is made of pure platinum-iridium alloy (90% platinum & 10% iridium) of 1020 mm total length and having a cross section as shown in fig. One meter is defined as the distance between the two fine lines engraved over upper surface of the web, when measured at a temperature of 0°C. This system has been adopted in India. 6/13/2021 49

50. International Prototype Meter 6/13/2021 50

51. Disadvantages of Material Standards 1. Material standards are affected by changes in environmental conditions such as temperature, pressure, humidity, and ageing, resulting in variations in length. 2. Preservation of these standards is difficult because they must have appropriate security to prevent their damage or destruction. 3. Replicas of material standards are not available for use at other places. 4. They cannot be easily reproduced. 5. Comparison and verification of the sizes of gauges pose considerable difficulty. 6. While changing to the metric system, a conversion factor is necessary. 6/13/2021 51

52. Wave Length Standard The 11th General Conference of Weights and Measures, which was held in Paris in 1960, recommended a new standard of length, known as wavelength standard, which is highly accurate and is very small unit of measure. It was decide that Krypton 86 is used in a hot cathode discharged lamp maintained at 68 °K temperature generates orange radiation can be used as ultimate wavelength standard. According to this standard, metre is defined as 1,650,763.73 × wavelengths of the red–orange radiation of a krypton 86 atom in vacuum. 6/13/2021 52

53. Advantages of Wave Length Standard It is not a material standard and hence it is not influenced by effects of variation of environmental conditions like temperature, pressure and humidity. It need not be preserved or stored under security and thus there is no fear of being destroyed as in case of meter and yard. It is not subjected to destruction by wear and tear. This standard is easily available to all standardizing laboratories and industries. Used for comparison with high accuracy. 6/13/2021 53

54. Wave Length Standard Disadvantages: Maintenance cost is high. Requires accurate wavelengths of spectral radiation. 6/13/2021 54

55. Subdivision of standards D epending upon the degree of accuracy required for the work, the standards are subdivided into four categories or grades: 1. Primary standards 2. Secondary standards 3. Territory standards 4. Working standards. 6/13/2021 55

56. 1. Primary standards They are material standard preserved under most careful conditions. These are used once in 10 to 20 years for calibration and verification of secondary standards. These are maintained at the National Standards Laboratories in different countries. For India, it is National Physical Laboratory at New Delhi. The primary standards are not available for the use out side the National Laboratory. Example: Imperial Standard Yard, International prototype meter, and international prototype of the  kilogram (IPK)   6/13/2021 56

57. 2. Secondary standards Secondary standards are made as nearly as possible exactly similar to primary standards as regards design, material and length. They are compared with primary standards after long intervals and the records of deviation are noted. These standards are kept at number of places for safe custody. They are used for occasional comparison with tertiary standards whenever required. e.g :  voltmeter, a glass thermometer and pressure gauge  are examples of secondary instruments.  6/13/2021 57

58. 3. Territory standards The primary and secondary standards are applicable only as ultimate control. Tertiary standards are the first standard to be used for reference purposes in laboratories and workshops. They are made as true copy of the secondary standards. They are used for comparison at intervals with working standards. 6/13/2021 58

59. 4. Working standards. These standards are similar in design to primary, secondary and territory standards. But being less in cost and are made of low grade materials they are used for general applications in Metrology Laboratories. Both line and end working standards are used. For example, manufacturing of mechanical components such as shafts, bearings, gears etc, use a standard called working standard for checking the component dimensions. Example: Plug gauge is used for checking the bore diameter of bearings. 6/13/2021 59

60. Some times standards are also classified as: 1. Reference standards- Used for reference purposes. 2. Calibration standards - Used for calibration of inspection and working standards. 3. Inspection standards - Used by inspectors. 4. Working standards - Used by operators, during working. 6/13/2021 60

61. Types of Measurement Standards A length may be measured as the distance between two lines or at the distance between two parallel faces. So, the instruments for direct measurement of linear dimensions fall into two categories. 1. Line standards. 2. End standards. 6/13/2021 61

62. 1. Line standards When the length is measured as the distance between centers of two engraved lines, it is called line standard. Both material standards yard and meter are line standards. Example: Steel rule with divisions shown as lines marked on it. 6/13/2021 62

63. 2. End standards . When length is expressed as the distance between two flat parallel faces, it is known as end standard. Examples: Measurement by slip gauges, vernier calipers etc. The end faces are hardened, lapped flat and parallel to a very high degree of accuracy. 6/13/2021 63

64. Comparison between Line and End Standard SL No Line Standard End Standard 1 Length is expressed as the distance between two lines. Length is expressed as the distance between two flat parallel faces 2 They are accurate up to ±0.2 They are accurate up to ±0.001 3 Measurement is quick and easy Requires skill and is time-consuming. 4 Less costly More costly 5 Parallax error can occur They ate not subjected to parallax error 6 Scale markings are not subject to wear. However, significant wear may occur on leading ends. These are subjected to wear on their measuring surfaces 7 Manufacturing is simple Manufacturing is complex 8 E.g. Steel rule E.g. Micrometer 6/13/2021 64

65. ERRORS IN MEASUREMENTS It is never possible to measure the true value of a dimension there is always some error. The error in measurement is the difference between the measured value and the true value of the measured dimension. Error in measurement = Measured value – True value 6/13/2021 65

66. Classification of Errors Generally errors are classified into two types: systematic errors, random errors but they are broadly classified as : 6/13/2021 66

67. Gross errors Instrumentation misuse, calculation errors and other human mistakes (i.e. mistakes in reading, recording data results) are the main sources of Gross errors.  Gross error mainly occur due to carelessness or lack of experience of a human being or incorrect adjustments of instruments. Example: A person may reads a pressure gauge indicating 1.01 N/m2 as 1.10 N/m2 . Elimination: These errors can be minimized by 1.Taking great care while taking reading, recordings and calculating results. 2. Taking multiple readings preferably by different persons. 6/13/2021 67

68. Measurement errors: The measurement error is the result of the variation of a measurement of the true value. Usually, Measurement error consists of a random error and systematic error. Example : The best example of the measurement error is, if electronic scales are loaded with 1000 grams standard weight and the reading is 1002 grams, then The measurement error is = (1002 grams-1000 grams) = 2 grams 6/13/2021 68

69. Blunder Blunders are final source of errors. These errors are caused by faulty recording or due to a wrong value while recording a measurement, or misreading a scale or forgetting a digit while reading a scale. 6/13/2021 69

70. Systematic Error The Systematic errors (controllable error) are of constant or similar forms that occur due to fault in the measuring device or environmental condition etc. These errors can be removed by correcting the measurement device. Systematic errors can be classified as: Instrumental Errors Environmental Errors Observational Errors Theoretical Errors 6/13/2021 70

71. Instrumental Errors: These errors are mainly due to following four reasons: Shortcoming in the instrument -these are because of the mechanical structure of the instruments, e.g. friction in the bearings of various moving parts, irregular spring tensions, gear backlash etc. Elimination - Selecting proper instruments for the measurements - Recognize the effect of such errors and apply the proper correction factors. - Calibrate the instrument carefully against standard. 6/13/2021 71

72. Environmental Errors(due to the external condition) External conditions mainly include: Temperature changes, Pressure, Vibration, Humidity, Dust, External magnetic fields, Aging of equipments etc. Elimination Using proper correction factors and using the instrument catalogue. Using the Temperature & Pressure control methods etc. Reducing the effect of dust, humidity on the components in the instruments. The effect of external fields can be minimized by using the electrostatic shields of screens. 6/13/2021 72

73. Observational Errors This errors are produced by observer Few sources are: 1. wrong observations or reading in the instruments 2.Parallax error while reading the meter. 2.Inaccurate estimate of average reading. 3.Wrong scale reading and wrong recording the data. 4.Incorrect conversion of units between consecutive readings. Elimination Taking two or more readings repeatedly. Instrument having digital display. 6/13/2021 73

74. Theoretical Errors Theoretical errors are caused by simplification of the model system. For example, a theory states that the temperature of the system surrounding will not change the readings taken when it actually does, then this factor will begin a source of error in measurement. 6/13/2021 74

75. Random Errors (Uncontrollable Error) Random errors are caused by the sudden change in experimental conditions and noise and tiredness in the working persons. An example of the random errors is during changes in humidity, unexpected change in temperature and fluctuation in voltage. These errors may be reduced by taking the average of a large number of readings. 6/13/2021 75

76. Comparison between Systematic Errors and Random Errors Systematic error Random error 1.These errors are repetitive in nature and are of constant and similar form. 1.These are non-consistent. The sources giving rise to such errors are random. 2.These errors result from improper conditions or procedures that are consistent in nature 2. Such errors are inherent in the measuring system or measuring instruments. 3. Except personal errors, all other systematic errors can be controlled in magnitudes and sense. 3. Specific causes, magnitudes and sense of these errors cannot be determined from the knowledge of measuring system or condition. 6/13/2021 76

77. Comparison between Systematic Errors and Random Errors Systematic error Random error 4. If properly analyzed these can be determined and reduced or eliminated. 4.These errors cannot be eliminated, but the results obtained can be corrected. 5.These include calibration errors, variation in contact pressure, variation in atmospheric conditions, parallax errors, misalignment errors etc. 5.These include errors caused due to variation in position of setting standard and work-piece, errors due to displacement of lever joints of instruments, errors resulting from backlash, friction etc. 6/13/2021 77

78. Factors in selecting the measuring instruments The following are factors considered while selecting measuring instrument: 1. The important factor to be considered in selection of measuring instrument are its measuring range, Accuracy and Precision. 2. For better results instruments with higher accuracy is selected. 3. Precision is also very important feature for any measuring instrument because it provides repeatable readings. 4. The sensitivity of that instrument should remain constant through the range of its measurement. 6/13/2021 78

79. Factors in selecting the measuring instruments 5. Minimum inertia in the moving parts of the mechanism. The effect of inertia is to make the instrument sluggish (slow moving). 6. The time taken to display the final data. (as less as possible). 7. The type of data displayed. (Analog or digital or photograph) 8. The cost of measuring instrument. 9. Type of quantity to be measured constant or variable. 10. Nature of quantity being measured hot or cold 11. Resistance to environmental disturbance. 12. Simplicity in calibration when needed. 13. Safety in use. 14. Adoptability to different sizes. 6/13/2021 79

80. THANK YOU 6/13/2021 80