Basic Principles of Metrology

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Donald A. Kelley
Professor Emil Hazarian
QAS-347: Dimensional Metrology
29 November 2012

Basic Principles of Metrology

Imagine a world with no regard for accuracy. What would happen if no one could agree on terms of time, length, weight, or the amount of electrical energy in a volt? Advances in electricity and electronics during the past century have made accurate measurement of many different properties essential. We wake up to electrical alarm clocks, cook in microwave ovens, and regulate the heat in our homes with thermostats. These devices depend on accurate voltages, currents and resistances to function properly. Metrology [1] is the science of measurement, providing accuracy we need to succeed in business and our personal lives. It is within metrology, that definitions for measurements are agreed upon. Calibration is the action of metrology. Through calibration, accuracy is measured and integrity established. Calibration is the process that provides confidence our measurement results are accurate. SI Units

An instrument often creates a measurement as a physical means of determining a quantity or variable. Measurement correlates numbers to quantities such as: a pounds, gallons, volts or miles. A quantity is a number and a unit of measure people understand and agree with. So, how are the number and units assigned? Someone had to say, “This is what is meant when I say kilogram/meter/liter/volt/ampere.” This is where definitions come into play. A standard unit (SI) is merely the agreed upon definition of a unit of measure. A unit might be mass (kilogram), length (meter), time (second), electric current (ampere), volt and so on. The “caretaker” for these internationally agreed upon defined units is the International Committee of Weights and Measures (Bureau International des Poids et Mesures (BIPM)) [2], located in France. Units are agreed upon between countries via treaty. The abbreviation SI is taken from its French name, Système International d’Unités. It was established in 1960 by the General Conference of Weights and Measures [3]. The United States and most other nations subscribe to this conference and use the SI for most legal, scientific, and technical purposes related to weights and measures. Each country has some process for deriving measurement units from the definitions. In the U. S., the National Institute of Standards and Technology (NIST) determines, maintains, and disseminates measurement standards. In Germany, the Physikalish-Techniche Bundesanstalt (PTB) is the official standards organization. Keep in mind that differences exist between the standards of various countries. These differences are measured and recognized so that there is minimal confusion.

Calibration
Industry relies heavily on measurements for standardizing components. Calibration makes it possible to achieve the accuracy, precision, and interchangeability that makes mass production possible. Calibration is the provider of confidence that components manufactured can come together. Ultimately measurements are used to help make decisions, and quality measurements result in quality decisions. It is a measure of the quality and provides the means to assess and minimize the risk and possible consequences of decisions. Calibration is the highly refined measurement process that compares test and measurement instruments of unknown accuracy with well-defined standards of greater accuracy [4]. The purpose is to detect, eliminate by adjustment, and report variation in an instrument’s accuracy. The accuracy of an electrical measurement, for example, is an expression of the closeness of its result to the true value. High accuracy indicates a close approach to the true value of the item being measured. The strongest force driving the development and practice of metrology and calibration is the need for accurate information in trade and industry. Some of the oldest laws in existence cover the need for dependable,...
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