The Philosophy Of Metrology
by Philip Stein
In my last article, I pointed out that metrology is an activity that concentrates on details. Making measurements is pretty easy, but making accurate measurements requires knowing what quantities and factors will influence the process, quantifying those influences and ensuring that they are accounted for in the final answer. That previous article was quite technical. I'm hoping to alternate those technical tomes with more philosophical or historical ones--thus this month's topic.
I have recently been rereading a wonderful book by Forrest K. Harris. Harris was a friend, mentor and faculty advisor during my graduate work in metrology. He was dean of the staff at the National Bureau of Standards (predecessor to National Institute of Standards and Technology) for many years, heading up its work on absolute electrical measurements. His classic text, Electrical Measurements,1 is still fresh even though many technical approaches to electrical measurements have changed since it was written.
The best of the book though, is in its exposition of the philosophy of metrology. Here it is in Harris' words. I have changed almost nothing.
In considering the subject of electrical measurements, a variety of viewpoints is possible. At one extreme, measurements constitute a system of philosophy in which cause and effect can be kept in orderly array. (This viewpoint is more easily maintained at the desk than in the laboratory.) At the other extreme, measurements may be considered a contest in which the adversary is what has been called "the law of the natural cussedness of inanimate objects."
An intermediate and more profitable point of view may be taken in which the measurement is a challenge such that as the worker strives for more certainty in his results, he is confronted with problems of increasing difficulty arising chiefly from the characteristics of the apparatus and instruments he must use. These problems being solved, the worker may perhaps find himself a step nearer to the final impassable barrier set by nature--the granularity of energy. All these viewpoints occur at one time or another in varying degree to anyone who works in the field of measurements.
These words were written 50 years ago. The capability of electrical mesuring equipment has greatly increased, but the thinking required of a metrologist has not changed at all.
The author believes that the art of measurement is as much a matter of one's attitude and way of thinking as it is a collection of devices and techniques whereby one can push buttons or turn knobs and so learn how big something is or how nearly alike two things are. At the risk of being considered a bit pedantic, the author suggests that there may be profit to the reader in approaching the subject as an exercise in mental discipline rather than simply as a collection of embalmed facts or as a compendium of recipes for getting things measured.
What an important distinction! Much of what is called metrology today consists pretty much of pushing buttons, turning knobs and following recipes (documented procedures). Because of technological advances, it is now common to generate eight significant figures of nonsense where only three were possible before. The key to generating precise, accurate answers lies in the mental discipline, not in the equipment. Anyone who does not understand this is likely to fail.
The problems and difficulties of precise measurement increase very rapidly as accuracy requirements are increased. If the relations between accuracy of results and difficulty of measurement could be expressed as an analytic function, it would surely be found to have a strong exponential term. In most fields, measurements to an accuracy of a few percent are readily achieved by simple means if one uses reasonable care in selecting suitable methods and instruments. If accuracy requirements are a percent or better, careful selection of method and apparatus is essential; one must make sure that the instruments are of good quality, and one must also consider the effects of the measuring equipment on the thing that is being measured.
If the accuracy requirements are of the order of 0.10%, one has about reached the limit to which the indications of deflecting instruments can be read, and usually one must apply corrections in the most painstaking manner, taking into account, among other things, the effects of ambient conditions on the measuring apparatus. If accuracy requirements are increased beyond this point, special methods and techniques must be employed, which permit a comparison between the thing being measured and some standard whose characteristics have been studied and whose stability is known within limits that are closer than the accuracy requirements of the measurement being undertaken. These remarks apply only to quantities that are of normal magnitude. Where the thing to be measured is either extremely large or small, the difficulties encountered in its precise measurement increase much more rapidly than has been indicated at this time.
Here we can see that things have changed. Electrical measurements (DC voltage, resistance) with an accuracy of 0.05% (half of Harris' 0.10%) can be bought at your neighborhood electronics store for less than $100. Since these instruments are digital, there is no problem reading a pointer. These devices usually require negligible amounts of power from the unit being measured so as not to distort the answer. This is now a typical low accuracy capability.
At the high accuracy end are instruments that were not even contemplated when Harris wrote his book, although he lived long enough to see most of them. DC voltage can be measured commercially to a few parts per million (ppm). Although the equipment is expensive, it doesn't require a fancy laboratory or a Ph.D. to achieve that previously unheard-of accuracy.
What was said about mental discipline in 1952, however, holds to a much greater extent in 2000. Because we can display eight or nine digits of a measurement answer, we easily believe that we have achieved an accuracy concomitant with that resolution, and it just ain't so. When measuring beyond about 0.01% (100 ppm), that "cussedness of inanimate objects" will prove that some things have not changed in the last 50 years. We're talking philosophy today, so I won't go into the details of what often goes wrong. Suffice it to say that making highly accurate measurements re-quires as much care, science, engineering and mental activity as it always has. Harris wrote:
In brief, the laboratory worker who uses instruments as tools and measurements as a means of obtaining information, as well as the student of the theory of measurements, must be alert to the limitations and possibilities of various devices and techniques, to the end that the accuracy or repeatability required by the job at hand may be attained without wasting time and effort on matters that are not significant.
1 Forrest K. Harris, Electrical Measurements (New York: Wiley & Sons, 1952).
PHILIP STEIN is a metrology and quality consultant in private practice in Pennington, NJ. He holds a master's degree in measurement science from The George Washington University in Washington, DC, and is an ASQ Fellow. For more information, go to www.measurement.com.