Note for Metrology and Measurements - MM by Ravichandran Rao

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Chapter 1: STANDARDS OF MEASUREMENT Definition of Metrology: Metrology (from Ancient Greek metron (measure) and logos (study of)) is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. Metrology is concerned with the establishment, reproduction, conservation and transfer of units of measurement & their standards. For engineering purposes, metrology is restricted to measurements of length and angle & quantities which are expressed in linear or angular terms. Measurement is a process of comparing quantitatively an unknown magnitude with a predefined standard. Objectives of Metrology: The basic objectives of metrology are; 1. To provide accuracy at minimum cost. 2. Thorough evaluation of newly developed products, and to ensure that components are within the specified dimensions. 3. To determine the process capabilities. 4. To assess the measuring instrument capabilities and ensure that they are adequate for their specific measurements. 5. To reduce the cost of inspection & rejections and rework. 6. To standardize measuring methods. 7. To maintain the accuracy of measurements through periodical calibration of the instruments. 8. To prepare designs for gauges and special inspection fixtures. Definition of Standards: A standard is defined as “something that is set up and established by an authority as rule of the measure of quantity, weight, extent, value or quality”. For example, a meter is a standard established by an international organization for measurement of length. Industry, commerce, international trade in modern civilization would be impossible without a good system of standards. Role of Standards: The role of standards is to achieve uniform, consistent and repeatable measurements throughout the world. Today our entire industrial economy is based on the interchangeability of parts the method of manufacture. To achieve this, a measuring system adequate to define the features to the accuracy required & the standards of sufficient accuracy to support the measuring system are necessary.

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STANDARDS OF LENGTH In practice, the accurate measurement must be made by comparison with a standard of known dimension and such a standard is called “Primary Standard” The first accurate standard was made in England and was known as “Imperial Standard yard” which was followed by International Prototype meter” made in France. Since these two standards of length were made of metal alloys they are called ‘material length standards’. International Prototype meter: It is defined as the straight line distance, at 0oC, between the engraved lines of pure platinum-iridium alloy (90% platinum & 10% iridium) of 1020 mm total length and having a ‘tresca’ cross section as shown in fig. The graduations are on the upper surface of the web which coincides with the neutral axis of the section. Engravedlines 16 mm Engravedlines Neutral axis Web Platinum-iridium alloy 1 meter(at 0deg C) 1020 mm 16 mm Historical International Prototype Meter bar, made of an alloy of platinum and iridium, that was the standard from 1889 to 1960.

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The tresca cross section gives greater rigidity for the amount of material involved and is therefore economic in the use of an expensive metal. The platinum-iridium alloy is used because it is non oxidizable and retains good polished surface required for engraving good quality lines. Imperial Standard yard: 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. A round recess, 1 inch away from the two ends is cut at both ends upto the central or ‘neutral plane’ of the bar. Further, a small round recess of (1/10) inch in diameter is made below the center. Two gold plugs of (1/10) inch diameter having engravings are inserted into these holes so that the lines (engravings) are in neutral plane. 38" 36" at 62 deg F Neutral axis 1" 1" Gold plug Bronze bar 82% Cu, 13% Tin, 5% Zinc 1" Enlarged view of gold plug showing engraving Yard is defined as the distance between the two central transverse lines of the gold plug at 620F. The purpose of keeping the gold plugs in line with the neutral axis is to ensure that the neutral axis remains unaffected due to bending, and to protect the gold plugs from accidental damage.

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Bronze Yard was the official standard of length for the United States between 1855 and 1892, when the US went to metric standards. 1 yard = 0.9144 meter. The yard is used as the standard unit of field-length measurement in American, Canadian and Association football, cricket pitch dimensions, swimming pools, and in some countries, golf fairway measurements. Disadvantages of Material length standards: 1. Material length standards vary in length over the years owing to molecular changes in the alloy. 2. The exact replicas of material length standards were not available for use somewhere else. 3. If these standards are accidentally damaged or destroyed then exact copies could not be made. 4. Conversion factors have to be used for changing over to metric system. Light (Optical) wave Length Standard: Because of the problems of variation in length of material length standards, the possibility of using light as a basic unit to define primary standard has been considered. The wavelength of a selected radiation of light and is used as the basic unit of length. Since the wavelength is not a physical one, it need not be preserved & can be easily reproducible without considerable error. A krypton-filled discharge tube in the shape of the element's atomic symbol. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquefied air. The high power and relative ease of operation of krypton discharge tubes caused (from 1960 to 1983) the official meter to be defined in terms of one orange-red spectral line of krypton-86.