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Standards of Measurement Ch – 3 MEC 263

Standards of Measurement Ch – 3 MEC 263. Arijit Sen Faculty Department of Mechanical Engineering IUBAT. Line Standards.

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Standards of Measurement Ch – 3 MEC 263

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  1. Standards of MeasurementCh – 3 MEC 263 ArijitSen Faculty Department of Mechanical Engineering IUBAT

  2. Line Standards • Line Standards: When length is measured as the distance between centers of two engraved lines, it is called Line Standards. Both material Standards, yard and metre are line standards. According to it, yard or metre is defined as the distance between scribed lines on a bar of metal under certain conditions of temperature and support. These are legal standards and Act of Parliament authorizes their use.

  3. Yard: • The Imperial Standard Yard is a bronze bar of one inch square cross-section and 38 inches long . A round recess, one inch away from two ends is cut at both ends up to central plane of the bar. A gold plug diameter having three lines engraved transversely, and two lines longitudinally is inserted into these holes so that the lines are in neutral plane. Yard ist hen defined as the distance between the two central transverse lines of the plug at 62°F. Secondary standards were also made as copy of the above International yard for occasional comparisons.

  4. Metre: This is the distance between the centre portions of two lines engraved on the polished surface of a bar of pure paltinum-iridium alloy (90% platinum and 10% iridium). It is inoxidisableand can have good polish required for ruling good quality of lines. The bar is kept at 0°C and under normal atmospheric pressure. It is supported by two rollers of at least 1 cm dia symmetrically situated in the same horizontal plane at a distance of 751 mm, so as to give minimum deflection. It has a shape of winged section as shown in Fig. 3.3, having a web whose surface lines are on the neutral axis. This standard is kept at BIPM at Sevres in Paris.

  5. End Standard • For all practical measurements in workshop, we employ endstandards e.g. slip gauges, gapgauges, end of micrometer anvils etc. Thus the importance of endstandards (which are actuallyused in general measurement applications) arose. Length bars and slip gauges were then madewhich were equal in length to the legal line standard. The only difficulty realised with endstandardwas that of forming two accurately parallel surfaces at the end of a bar and to heat treat the endsso that they remained stable.

  6. Characteristics of Line Standards : • i)Scale can be accurately emblemed, but the engraved lines posses thickness and it is not possible to accurately measure • (ii)Scale is used over a wide range • (iii)Scale markings are subjected to wear. However the ends are subjected to wear and this leads to undersize measurements • (iv)Scale does not posses built in datum. Therefore it is not possible to align the scale with the axis of measurement • (v)Scales are subjected to parallax errors • (vi)Assistance of magnifying glass or microscope is required.

  7. Characteristics of End Standards • (i)Highly accurate and used for measurement of closed tolerances in precision engineering as well as standard laboratories, tool rooms, inspection departments. • (ii)They require more time for measurement and measure only one dimension. • (iii)They wear at their measuring faces • (iv)They are not subjected to parallax error

  8. Wavelength Standard • A major drawback with the material standards,  that their length changes with time. Secondly, considerable difficulty is expressed while comparing the sizes of the gauges by using material standards. Jacques Babinet suggested that wave length of a monochromatic light can be used as a natural and invariable unit of length. 7thgeneral Conference of Weights and Measures approved in 1927 approved the definition of standard of length relative to meter.

  9. Orange radiation of isotope Krypton-86 was chosen for the new definition of length in 1960, by the 11thGeneral Conference of Weights and Measures. The committee recommended Krypton-86and that it should be used in hot cathode discharge lamp, maintained at a temperature of 63K. According to this standard metre was defined as equal to 165763.73 x wavelengths of the red-orange radiation of Krypton-86 isotope. A standard can now be produced to an accuracy of about 1 part of 10^9.

  10. Advantages : • (a)Not a material standard and hence it is not influenced by effects of variation of environmental conditions like temperature, pressure • (b)It need not be preserved or stored under security and thus there is not fear of being destroyed. • (c)It is subjected to destruction by wear and tear.

  11. Transfer from Line Standard to End Standard.

  12. Classification of Standard and gauge control structure National Standard National Reference Standard Working Standard Plant Laboratory Reference Standard Master gauge Inspection gauge Plant Laboratory Working Standard Shop Floor Standard Working gauge

  13. Environmental Control in Test Laboratory • Temperature • Rate of change of Temperature • Relative humidity • Barometric Pressure • Air Velocity and Air distribution • Dust Particle count • Vibration and Shock • Acoustic noise • Illumination • Electromagnetic interference. • Stray Magnetic Field • Ionization interference

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