A king, a law, a weight and a measurement

A little bit of history

Until the late eighteenth century, measures were extremely diverse. Measures of the same nature and similar values had different names by provinces or cities or villages of the same region. On the contrary, physical content of similarly titled measures usually differed from place to place and also according to the applicant corporation or the measured object.

Where did it all begin?

The need to measure has been focus around two needs:

  • movements, to identify its position
  • trade, to ensure fair trade

In 1789, during the French revolution, most of grievances ask for an uniformity of weight and measures: the inconsistency and multiple legacy systems are the work of feudalism. Un roi, une loi, un poids et une mesure ! (One king, one law, one weight and one measurement). It was therefore necessary to invent different units (meter and kilogram in particular) helpful in trade. Bishop deputy Talleyrand proposes, so that these new units were accepted by all, that they respect the following principle: The units should present nothing arbitrary nor particular to the situation of any people on earth.

Some definitions

Upon reflection, it was decided that the meter would have this definition: The meter is equal to the ten millionth part of a quarter of the earth's meridian. It took nearly 7 years to two astronomers of the time, Delambre and Méchain, to measure a part of earth's meridian between Dunkerque and Barcelona, which allowed to create a "standard meter" to define the meter. Since then the meter is in its fourth definition: The meter is equal to the distance traveled in vacuum by light in a period of 1/299 792 458 seconds. Obviously, all other international system units are based on definitions of this type except the kilogram which remains to date, based on an international prototype (the K2) manufactured in 1872 and copies of which were given to the constituent countries of the International Committee for Weights and Measures.

Legal metrology

Very quickly, and to meet the aspirations of the people - trust in the measurement results - the state has set up a service, legal metrology, responsible of guaranteeing the honesty of trade and the equality of all citizens before the measurement results. Thus, all the involved instruments in the context of commercial transactions are subject to periodic audits performed according to applicable decrees throughout France. These audits aim to ensure that the instruments (only the instruments!) remain within the limits set by their respective decree. No one today would doubt the measurement results. In our collective unconscious, the measured value has become the true value. Legal metrology has accomplished its mission successfully. But are goals of legal and industrial metrologies the same?

Metrology as productivity lever

Contrary to what we were instilled, often implicitly, by our civic culture, the measured value is never the true value. If this has been proved beneficial in our daily life, it is not the case in the industry. While the legal metrology seeks honesty and equality, industrial metrology must ensure the FUNCTIONALITY!

The challenges for industrial metrology

All of the factors involved in the measurement process contributes to degrade the result of the measurement:

  • The measuring means
  • The operator
  • The environment in which the measurement is taken
  • The measurement technique used
  • The measured object

Therefore, the measurement result can't be written in the form of a single numerical value Measured diameter = 20.012 mm but should be, to give an idea of what may be the true value of the measured quantity, expressed as the form Measured diameter = (20.012 ± 0.008) mm

Thus, the true value of the considered diameter is in the range:

Intervalle
Set of values that can be reasonably
attributed to the value of the considered diameter

Measurement uncertainty and capabilities

This is why the different quality standards require, one way or another, taking into account the measurement uncertainty in the declaration of conformity of a product, as stated in Chapter 4.11 of the standard ISO 9001 (1994):

[ … ] the amount of measurement uncertainty should be known and should be reasonable given
the degree of precision required to establish that the product meets requirements. [ … ]

To meet this requirement, everyone today knows the obligation to calibrate their measuring instruments, in other words evaluate, by comparison with the standard measure, errors of measuring instruments. This operation, if it is inevitable in the context of the research of measurement uncertainties, is not the only provision to take. Just as the instrument has its own errors and we must know them, it's also necessary to consider the errors from all other players in the measurement process: the operator, the measurement technique used, the environment, etc.

The ISO (International Organization for Standardization) published a standard, internationally recognized today, which proposes a method for evaluating the measurement uncertainty: GUM : ISO/CEI Guide 98-3 : Guide to the expression of uncertainty in measurement. There is also (particularly applicable to testing laboratories) a set of standards in which a more experimental approach is described: the ISO 5725 series.

Associated with these calculation standards, ISO publishes other standards which allow manufacturers to develop strategies for taking into account the measurement uncertainty in the declaration of conformity: the standard ISO 14253-1 (2000): Geometrical product specifications (GPS) — Inspection by measurement of workpieces and measuring equipment.

Moreover, car manufacturers have also developed experimental methods, applicable in other sectors, based on the concept of the capability of the measurement process. The capability of a measurement process is determined by the ratio of tolerance to check on the measurement uncertainty (or the opposite).

The future

Today, the metrologist has to be part of a progressive approach which will aim to improve productivity, and so the company's competitiveness. Indeed, the awareness of measurement uncertainties must lead to the questioning of tolerances. It should not be, by denying the actual reality, an additional constraint.

In metrology, every dime spent must be a dime invested!

Indicators must be more ambitious than today:

  • Waste reduction
  • Adjustment reduction
  • Cycle time reduction