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Standards Measurement is the process by which a quantitative comparison is made between a standard and a measurand. The measurand is the particular quantity of interest—the thing that is to be quantified. The standard of comparison is of the same character as the measurand and, so far as mechanical engineering is concerned, the standards are defined by law and | CHAPTER 29 MEASUREMENTS E. L. Hixson E. A. Ripperger University of Texas Austin Texas 29.1 STANDARDS AND 29.3.3 Use of Normal Distribution ACCURACY 29.1.1 Standards 917 917 to Calculate the Probable Error in X 924 29.1.2 Accuracy and Precision 29.1.3 Sensitivity or Resolution 918 918 29.3.4 External Estimates 925 29.1.4 Linearity 919 29.4 APPENDIX 29.4.1 Vibration Measurement 928 928 29.2 IMPEDANCE CONCEPTS 919 29.4.2 29.4.3 Acceleration Measurement Shock Measurement 928 928 29.3 ERROR ANALYSIS 29.3.1 Introduction 29.3.2 Internal Estimates 923 923 923 29.4.4 Sound Measurement 928 29.1 STANDARDS AND ACCURACY 29.1.1 Standards Measurement is the process by which a quantitative comparison is made between a standard and a measurand. The measurand is the particular quantity of interest the thing that is to be quantified. The standard of comparison is of the same character as the measurand and so far as mechanical engineering is concerned the standards are defined by law and maintained by the National Institute of Science and Technology NIST . The four independent standards that have been defined are length time mass and temperature.1 All other standards are derived from these four. Before 1960 the standard for length was the international prototype meter kept at Sevres France. In 1960 the meter was redefined as 1 650 763.73 wavelengths of krypton light. Then in 1983 the 17th General Conference on Weights and Measures adopted and immediately put into effect a new standard meter is the distance traveled in a vacuum by light in 1 299 792 458 seconds. 2 However there is a copy of the international prototype meter known as the National Prototype Meter kept at the National Institute of Science and Technology. Below that level there are several bars known as National Reference Standards and below that there are the working standards. Interlaboratory standards in factories and laboratories are sent to the National Institute of Science and Technology for comparison with the .