Mechanical Measurement and Metrology Notes eBook
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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.
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.