MEASURE FOR MEASURE
Metrology: We Use It Every Day
by Dilip Shah
I am often asked by friends and family what I do for a living, and when I respond that I consult and train on metrology issues, the familiar response is: “Oh, you do weather predictions!” They are, of course referring to meteorology, the science that deals with atmosphere and weather. We are all used to seeing weather forecasters on television, so we’re bombarded with the term. I smile and respond, “Yes, indirectly.” Because, without accurate and repeatable measurement tools, weather prediction would not be possible.
Simply put, metrology is the science of measurement.1 It is believed the science dates back to about 3000 B.C., when the royal Egyptian cubit was decreed to be equal to the length from the elbow to the tip of the middle finger of the hand of the pharaoh ruling at that time. In 1100 in England, it is believed King Henry I first defined the yard—decreed as the distance between the tip of his nose and the end of the thumb of his outstretched hand.
This quotation from William Thompson (later Lord Kelvin) describes metrology well:
In physical science, the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of science, whatever the matter may be. So therefore, if science is measurement, then without metrology there can be no science.2
In the 1970s, a television public relations campaign declared, “Without chemicals, life itself would be impossible.” Using a similar analogy, without metrology, life would be very difficult.
Imagine the following:
- Going to the gas station to fill up your vehicle and trying to figure out whether you are getting an exact gallon of gas for the price you paid.
- The scanners at grocery stores incorrectly reading the bar codes on items you are purchasing.
- Getting your outsourced parts from one country to find out they do not fit the mating part made in another country.
- Over filling (a loss to you) or under filling (illegal by trade laws) jars of peanut butter in your production facility.
- Breaking a record at a sporting event by milliseconds and not receiving credit.
- Administering a lifesaving medication when an overdose can have life threatening implications and an underdose can have no effects.
- Having vision correction surgery using a laser technique without knowing whether the laser’s power is accurate.
World Metrology Day
Every year, May 20 is World Metrology Day, a day to celebrate and be thankful that metrology makes our lives easier. Why May 20? In 2000, the Conférence Générale des Poids et Mesures, or the General Conference on Weights and Measures, dedicated May 20 to commemorate the signing of the Convention du Mètre (Meter Treaty) on that day in 1875 in Paris.3
Upon the signing of the Meter Treaty, the following agencies were created:
- The Bureau International des Poids et Mesures (BIPM), or the International Bureau of Weights and Measures, was established as the executive body, responsible for maintaining international primary standards. Located in Sèvres, France, the BIPM ensures worldwide uniformity of measurements and their traceability to the International System of Units. BIPM enjoys extraterritorial status, meaning it is exempt from the jurisdiction of local law.
- The Comité International des Poids et Mesures, or the International Committee for Weights and Measures, is the executive body of the BIPM and is made up of delegates of member countries of the General Conference on Weights and Measures.
- The Organisation Internationale de Métrologie Légale, or the International Organization of Legal Metrology, issues recommendations to guide the development of legal metrology in its member countries. Its international bureau is in Paris.
Some other organizations involved in various aspects of international metrology are:
- International Electrotechnical Commission.
- International Organization for Standardization.
- International Federation of Clinical Chemistry and Labora-tory Medicine.
- International Union of Pure and Applied Chemistry.
- International Union of Pure and Applied Physics.
The international organizations are complemented by a worldwide network of national and regional metrology organizations. In the United States, the National Institute of Standards and Technology is our national metrology institution.
The Big Seven and Traceability
Modern metrology is based on the selection of standards and artifacts to form the International System of Units, more commonly called SI Units. There are seven base SI Units:
Second: the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
Meter: the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.
Mole: the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particles.
Kelvin: the unit of thermodynamic temperature, it is 1/273.16 of the thermodynamic temperature of the triple point of water.
Candela: the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5,401,012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
Ampere: the constant current that—if maintained in two straight parallel conductors of infinite length, negligible circular cross section and placed one meter apart in a vacuum—would produce between these conductors a force equal to 2 x 10-7 Newton per meter of length.
Kilogram: the unit of mass. The word “weight” denotes a quantity of the same nature as a “force”: The weight of a body is the product of its mass and the acceleration due to gravity. In particular, the standard weight of a body is the product of its mass and the standard acceleration due to gravity.4
ISO defines traceability as “the
property of the result of a measurement or the value of a standard whereby it can
be related to stated references, usually national or international standards, through
an unbroken chain of comparisons all having stated uncertainties.”5
The traceability pyramid is shown in Figure 1.
Regardless of where one is making measurements, it is important for measurements to have upward hierarchical traceability and uncertainty of their measurement. In future columns, I will discuss the uncertainty of measurement in detail.
The Importance Today
This year we celebrated the 130th anniversary of the Meter Treaty. If it had not been for the treaty, we may have continued to define measurement by whims and fancies of kings, dictators and different government and trade association bodies without any coordination. Metrology has become a necessity for trade, technical cooperation, scientific comparison or even simple exchange of information. Imagine the resulting chaos from absence of metrology in today’s global environment.
Metrology operates transparently in all facets of our day-to-day dealings. In a world with many geopolitical conflicts, metrology works as a diplomat solving our daily problems of living. Our appreciation should go out to the people on the committees of various global standards making bodies, the countless metrologists, calibration technicians and others who keep the world of commerce running. Consider marking May 20 on your calendars and thinking of the unsung heroes who are making our lives easier everyday.
- International Vocabulary of Basic and General Terms in Metrology, International Organization for Standardization, 1993.
- William Thompson, lecture to the Institution of Civil Engineers, May 3, 1883.
- National Conference of Standards Laboratories, www.ncsli.org.
- Barry Taylor, NIST Special Publication 811: Guide for the Use of the International System of Units (SI), U.S. Government Printing Office, 1995.
- International Vocabulary of Basic and General Terms in Metrology, see reference 1.
DILIP SHAH is president of E = mc3 Solutions in Wadsworth, OH, a consulting practice that focuses on metrology, training and ISO 17025 laboratory accreditation. He is the immediate past-chair of the Measurement Quality Division and co-author of The Metrology Handbook, published by ASQ Quality Press.