METROLOGY FOR NON-METROLOGISTS Rocío M. Marbán Julio A. Pellecer C. 2002 iii To contact the authors: 2001 Producción y Servicios Incorporados S.A. Calzada Mateo Flores 5-55, Zona 3 de Mixco Guatemala, Centro América Tel.: (502)431-0662 Fax: (502)434-0692 email: [email protected] ISBN 99922-770-1-7 © OAS, 2002 iv This English version of the second revised edition is pub- lished under the sponsorship of SIM. The Interamerican Metrology System, SIM (Sistema Interamericano de Metrología, Normalización, Acreditación y Calidad) is the regional organization for metrology in America, comprising national metrology institutes from the 34 member nations represented at the Organization of the American States, OAS, which acts as its Executive Sec- retariat. The opinions stated in this document are not necessarily opinions of the OAS, its bodies or its staff. v vi CONTENTS Acknowledgements ix Presentation xi Introduction 1 What we measure and how 11 Characterization of metrology 19 Vocabulary 21 Applications - what is measured and what for 27 Length 27 Mass 28 Temperature 29 Time and frequency 30 Electricity and magnetism 31 Photometry and radiometry 32 Acoustics and vibrations 33 Ionizing radiation 34 Chemistry 35 Standards and reference materials Introduction 37 Length 39 Mass 45 vii INTRODUCTION The initial concept of metrology derives from its etymology: from the Greek metros - measure, and logos - treaty. This concept is certainly as old as human beings: “I have nothing”, “I have something”, “I have much”; these expressions reflect a primi- tive comparison that is still valid and presently we can say that metrology is the science of mea- surements and that to measure is to compare with something (a unit) which is taken as the basis for comparison. Measurements for primitive human beings began with the ideas of: near-far, fast-slow, light-heavy, clear-dark, hard-soft, cold-hot, quiet-noisy. At first these were personal perceptions, but experi- ence and life in common gave rise to comparisons between persons and, through the ages, to gener- ally accepted bases for comparison. Thus, after several millennia, it is easy to think of bases for comparison of personal concepts - in other words: measurements and their units. 1 Metrology for non-metrologists Some of these measurements and units are basic: MEASURE UNIT length metre (meter) mass kilogram time second temperature kelvin intensity of light candela electric current ampere amount of substance mol For other purposes, not covered by the above, it is often necessary to use other measurement units, called derived units because they use or are based on the base units. That is, using mathematical al- gorithms, a unit is expressed algebraically in terms of other units. To enter the realm of units based on one or more fundamental units, is to enter a world of scientific algorithms for specific purposes, which is why de- rived units are more numerous. A unit is a value in terms of which a quantity may be described. It must be stressed that, qua unit, it must not be broken down into its elements. Mul- 2 Introduction tiples and sub-multiples are used to express quan- tities larger or smaller than those of the unit per se. We will see later on that in the International Sys- tem of Units multiples and sub-multiples are deci- mal, that is, they use powers of 10. We mentioned using something with which to com- pare; this something is known as a measurement standard or simply a standard. Originally, a standard was considered to be a rep- resentation or physical embodiment of a unit. It was necessary to stress that the standard was a trust- worthy representation of the unit only under a set of precisely defined conditions, to make sure it was independent of environmental influences such as temperature, humidity, atmospheric pressure, etc. Because of their characteristics, physical standards were not used to directly take measurements. In- stead they were the basic reference point for the manufacture and calibration of the instruments that are used for such purposes. Today, thanks to scientific advances, we have more exact and reliable definitions for the units, based on universal physical constants, and now a stan- dard can be defined as: a materialized measure, 3 Metrology for non-metrologists measurement instrument, reference material or measurement system, whose purpose is the defi- nition, materialization, conservation, or reproduc- tion of a unit, or one or several known values of a quantity, for transmission by comparison to other measurement instruments (2). It is also important that the procedure used to mea- sure give reproducible results and, in fact, there are precise instructions on how to carry out the pro- cedure, which units to use and which standard. In the real world, we usually measure following this sequence: - we decide what we are going to measure, - we select the unit according to the measure, - we select the measuring instrument (calibrated), - we apply the accepted procedure. Before going into details of the main measures, let us have a brief, very brief look, at the history of measurement. Archeological finds show that very ancient civiliza- tions had well-defined concepts of weighing and measuring. Trade, land division, and taxation, 4 Introduction among others, must have required very soon the uniformity of measurements. The appearance of weights and measures systems goes far back into time. We know little of what was done in the Far East; however, there is no doubt that they existed in the Mesopotamian civilizations and - clearly - it is obvious that the construction of pyramids in Egypt (3000 to 1800 BC) required elaborate systems of measurement. We know, and in some countries we still use, some of the linear measurements of current usage in ancient Egypt (the span, the foot, the pace, the fathom, the cubit). Also in Egypt, scales were used to weigh precious metals and gems. Later on, when coins began to be used as elements of trade, they were simply pieces of gold or silver, stamped with their weight. They gave birth to a monetary system that spread throughout the whole Mediterranean area. The way we measure time is based on the sexagesimal system developed in Mesopotamia, and our calendar is derived from the original 365 days Egyptian calendar. 5 Metrology for non-metrologists Roman conquest of a large part of the European continent contributed to disseminate the systems of weights and measures. By the beginning of the second millennium AD, the different measures in use had mutiplied uncontrol- lably. There were, for instance, different measures for capacity according to the product, be it wine or beer, wheat or barley. Measures could also vary from province to province or from town to town. England used Anglo-Saxon measures and gradu- ally tried to improve and simplify its system. For many centuries, the pound-foot-second system was the preferred system in English-speaking coun- tries as well as worldwide for some commercial and technical uses; to date it has not been totally dis- carded and is still used for many activities in many countries. France created and developed a simple and logi- cal system, based on the most advanced scientific principles known at the time (the end of the eigh- teenth century) - the decimal metric system, which first came in use during the French Revolu- tion. It owes its name to the use of the decimal system for multiples and sub-multiples and to its 6 Introduction base unit: the metre, mètre in French, which is it- self derived from the Greek metron, meaning mea- sure. In its first version, the metre was defined as one ten-millionth of the length of a quadrant of the earth’s meridian (i.e. one ten-millionth of an arc representing the distance between the Equator and the North Pole) and it was determined by measur- ing an arc of meridian between Dunkerque, in France, and Barcelona, in Spain. The history, vi- cissitudes, development and application of this sys- tem are amply documented (1,18). Metrologists are very active and there are con- stantly important changes and improvements in all aspects of measurements. Growing cooperation between metrologists from different countries is also helping to establish internationally accepted work procedures. There are now uniform methods of measurement so we can all work on the basis of the same known quantity or unit, and the results of any calibration, verification and test, in any labora- tory or enterprise, are a guarantee of compatibility and quality. In consonance with the global approach, more and more countries are adopting the International Sys- tem of Units (SI) based on the decimal metric sys- 7 Metrology for non-metrologists tem, with the subsequent adoption of the corre- sponding standards and measurements tech- niques. Forty-eight countries have subscribed the Metre Convention, that adopted the International System of Units (SI). The Convention gives authority to the Conférence Générale des Poids et Mesures (CGPM - General Conference on Weights and Mea- sures), the Comité International des Poids et Mesures (CIPM - International Committee on Weights and Measures) and to the Bureau Inter- national des Poids et Mesures (BIPM - International Office of Weights and Measures), to act interna- tionally in matters pertaining to metrology. CGPM is constituted by representatives of the member countries and it holds meetings every four years in Paris, France for: discussion and exami- nation of agreements for the improvement and dis- semination of the International System of Units (SI), validation of advances and results of new funda- mental metrological determinations, scientific inter- national resolutions, and decisions pertaining to the organization and development of the BIPM. 8 Introduction In search of a world-wide unification of physical measurements, the BIPM: - establishes fundamental standards and scales for the main physical quantities, - carries out and coordinates determinations related to physical constants, - preserves international prototypes, - coordinates comparisons with standards kept at the National Laboratories of Metrology, - ensures coordination of the measurement techniques.