Metrology – Standards

Metrology, simply, is the science of measuring things.  Metrology is used extensively in the world of industrial cleaning to both control and quantify cleaning processes.  Finding the right equipment and procedures to produce accurate and meaningful results is often a challenge for those without extensive knowledge of the science.  The upcoming blogs on metrology are intended as “food for thought” and are intended only to heighten the reader’s awareness of the fundamentals of the science.  I’ll start off with a blog discussing something called “standards.”  Standards are the foundation of metrology.

Standards –

Metrology quantifies things like weight, length, time, temperature and volume by comparing whatever is being measured to a standard.  A standard unit of measurement, in turn, is determined by comparing it to another, presumably more authoritative standard.  Eventually, the ultimate standard is reached by tracing the “heritage” of the standard to its source.  The first standards for the measurement of length, for example, were things like the length of a particular person’s arm, finger or whatever.  In many cases, the individual was a king or some other prominent individual.  Clearly, that was not a very good standard as the individual was not always available to perform his (or her) duty of a “standard.”  Furthermore, the king may not be king forever.  When the king changed, the length of the king’s arm changed as well.

To be able to measure things when the individual who was the “standard” was not available, sticks were cut to the same length as the king’s arm and used in its stead as a means of measuring.  In turn, additional sticks were cut to the same length as those that had been cut to the same length as the king’s arm to be used as “standards.”  The problem, of course, was that the accuracy of measurement was dependent on how accurately the “copy” sticks were cut.  The more copies away from the king’s arm, the more inaccurate the standard for measurement became.  We still have standards for units of measure today but they are much more accurate than the length of the king’s arm.  Today’s standards for length are based on the wavelength of a particular color of light and other natural “constants” that, at least in the world as we know it, have been and will remain the same forever.

In another example of the development and evolution of a standard, one of the earliest temperature standards was the normal temperature of the human body known as “blood heat.”  Unfortunately, this was an unreliable standard because the temperature of the human body can vary several degrees even in healthy individuals.  Eventually, more reliable constants were found in the melting and boiling points of water which were used to establish the Celsius scale of temperature on which 0 degrees is the melting point of water and 100 degrees is the boiling point of water (both at standard atmospheric pressure).

Today’s “standards” are carefully and universally defined worldwide.  In the United States, the National Bureau of Standards is responsible for maintenance of these standards.  In order to assure universal accuracy, critical measurements are always made using instruments that have been carefully calibrated to agree with the National Bureau of Standards master standards.  Since it is not possible for every instrument to be compared directly to the National Bureau of Standards master standards, procedures have been developed which assure accuracy through a “heritage” of standards traceable to the National Bureau of Standards master standards.  The accuracy of even something as simple as a carpenter’s tape measure can usually be traced to the National Bureau of Standards master standard.  Many, many years ago, I was visiting a well-known major manufacturer of steel tape measures and asked about traceability to the standard thinking surely they must just print lines on a strip of metal.  In response, I was shown a thin strip of metal about 50 feet in length kept in a temperature controlled room under constant, exact tension with several scribe marks so fine that they were invisible without powerful magnification.  That metal strip had actually been certified by the National Bureau of Standards as a standard unit of measure with an accuracy within several parts per million.  I’m sure that metal strip has since been replaced with some sort of laser measuring means but I must admit that, at the time, it was pretty impressive.  Clearly, certifying the accuracy of their tape measures was serious business to this company.

Establishing and maintaining accurate standards and the calibration of measuring instruments to those standards is an important part of metrology.  Knowing how to use the available instruments to produce meaningful results, however, is often even more important.  We’ll talk more about that in upcoming blogs.

–  FJF  –