“What the heck is metrology?”

… is what you were probably asking yourself not 30 seconds ago when you read my home page.

To put it simply, metrology is the study of measurement.  It deals with defining systems of units, creating standards of accuracy and precision for said units, and calibrating equipment to fit these standards.  The importance of this metrological process is that it ensures that every measurement we make, from the distance to grandma’s house to the mass of a new element, is grounded in reliable and communicable standards.  Thus, when a man in Beijing measures the height of a building in meters, I can understand the size of the building as well as his own neighbor.  This is only possible because the meter is an internationally defined unit.  The Beijinger’s tape measure is connected to my meter stick through the international and unchanging definition of the meter.

Taking a global perspective, the necessity for international metrology is glaringly obvious.  The entire world trade network would fall apart if not for metrology.  This same idea also applies to countries, states, and cities.  Everyone needs a solid system for communicating measurements.

So how do we pull this off?

On the global scale, the world measuring system — the International System of Units (SI) — is maintained by the General Conference of Weights and Measures (CGPM).  This assembly, based in France, the birthplace of the SI, is in charge of defining seven base units (Fig 1), which define every other unit in turn.  The CGPM ensures that these definitions are universally accessible and up to date with the latest scientific advances.

Today, all seven definitions published by the CGPM are based on natural constants, such as the speed of light.  This way, all countries have fair access to the unchanging unit definitions.  Furthermore, the definition process is not intuitive.  For example, the meter is defined as 1/299,792,458 the distance light travels in one second.  However, this comes from first officially and arbitrarily setting the speed of light at 299,792,458 meters per second, then deriving the definition of the meter.  It seems backwards, but truly is brilliant, as the speed of light never changes so neither does a meter.  The CGPM is in charge of setting these universal constants and their relationship to specific units.

To communicate with the CGPM, most countries have national metrological institutes which serve as middle men between the CGPM and the specific country’s needs.  In the U.S., this roll is filled by the National Institute for Standards and Technology (NIST) (Fig 2), which has the extra job of converting SI units into Customary units.  These institutes coordinate with their government to create national accuracy laws for industries and to train field metrologists to assist and oversee measurement within these industries.  The field metrologists help equipment makers calibrate their products and ensure other producers manufacture to the required precision.

In this way, it comes to pass that my meter stick was calibrated by the maker, with input from the metrologist, who was trained by the NIST, who talked with the CGPM, who defined the meter.  I can understand me friend from Beijing thanks to the work of the CGPM and hundreds of metrologists.