metrology

=Metrology=

toc Read here about the discipline of metrology, being the study of measurement, and how it relates to studies of scale.

=Overview= Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον (metron), "measure" + "λόγος" (logos), amongst others meaning "speech, oration, discourse, quote, study, calculation, reason". In Ancient Greek the term μετρολογία (metrologia) meant "theory of ratios". Metrology is defined by the International Bureau of Weights and Measures (BIPM) as "the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology." The ontology and international vocabulary of metrology (VIM) is maintained by the International Organisation for Standardisation. =History Of Metrology= Metrology has existed in some form or another since antiquity. The earliest forms of metrology were simply arbitrary standards set up by regional or local authorities, often based on practical measures such as the length of an arm. The earliest examples of these standardized measures are length, time, and weight. These standards were established in order to facilitate commerce and record human activity. Little progress was made with regard to proto-metrology until various scientists, chemists, and physicists started making headway during the scientific revolution. With the advances in the sciences, the comparison of experiment to theory required a rational system of units, and something more closely resembling modern metrology began to come into being. The discovery of atoms, electricity, thermodynamics, and other fundamental scientific principles could be applied to standards of measurement, and many inventions made it easier to quantitatively or qualitatively assess physical properties, using the defined units of measurement established by science. Metrology was thus one of the precursors to the Industrial Revolution, and was necessary for the implementation of mass production, equipment commonality, and assembly lines. Modern metrology has its roots in the French Revolution, with the political motivation to harmonize units all over France and the concept of establishing units of measurement based on constants of nature, and thus making measurement units available "for all people, for all time". In this case deriving a unit of length from the dimensions of the Earth, and a unit of mass from a cube of water. The result was platinum standards for the meter and the kilogram established as the basis of the metric system on June 22, 1799. This further led to the creation of the Système International d'Unités, or the International System of Units. This system has gained unprecedented worldwide acceptance as definitions and standards of modern measurement units. Though not the official system of units of all nations, the definitions and specifications of SI are globally accepted and recognized. The SI is maintained under the auspices of the Metre Convention and its institutions, the General Conference on Weights and Measures, or CGPM, its executive branch the International Committee for Weights and Measures, or CIPM, and its technical institution the International Bureau of Weights and Measures, or BIPM. =Basics of Metrology= Basics of metrology are described.

Margin Of Error
Mistakes can make measurements and counts incorrect. Even if there are no mistakes, nearly all measurements are still inexact. The term 'error' is reserved for that inexactness, also called measurement uncertainty. Among the few exact measurements are: All other measurements either have to be checked to be sufficiently correct or left to chance. Metrology is the science that establishes the correctness of specific measurement situations. This is done by anticipating and allowing for both mistakes and error. The precise distinction between measurement error and mistakes is not settled and varies by country. Repeatability and reproducibility studies help quantify the precision: one common method is an ANOVA gauge R&R study.
 * The absence of the quantity being measured, such as a voltmeter with its leads shorted together: the meter should read zero exactly.
 * Measurement of an accepted constant under qualifying conditions, such as the triple point of pure water: the thermometer should read 273.16 kelvin (0.01 degrees Celsius, 32.018 degrees Fahrenheit) when qualified equipment is used correctly.
 * Self-checking ratio metric measurements, such as a potentiometer: the ratio in between steps is independently adjusted and verified to be beyond influential inexactness.

Calibration
Calibration is the process where metrology is applied to measurement equipment and processes to ensure conformity with a known standard of measurement, usually traceable to a national standards board.

Factors
Factors are speed, completeness, reliability, assimilatability and cost.
 * The speed with which measurements can be accomplished on parts or surfaces in the process of manufacturing, which must match the TAKT Time of the production line.
 * The completeness with which the manufactured part can be measured such as described in high-definition metrology,
 * The ability of the measurement mechanism to operate reliably in a manufacturing plant environment considering temperature, vibration, dust, and a host of other potential hostile factors,
 * The ability of the measurement results, as they are presented, to be assimilated by the manufacturing operators or automation in time to effectively control the manufacturing process variables, and
 * The total financial cost of measuring each part.

Criteria
Accuracy is the degree of exactness which the final product corresponds to the measurement standard. Precision refers to the ability of a measurement to be consistently reproduced. Reliability refers to the consistency of accurate results over consecutive measurements over time. Traceability refers to the ongoing validations that the measurement of the final product conforms to the original standard of measurement =Issues In Metrology=

Issues in metrology are described

Error blindness
A measurement that is wrong feels like a measurement that is right. This is derived from the work of Kathryn Schulz, the first self-proclaimed wrongologist. She calls it error blindness, and recruits the Chuck Jones’ cartoon character “coyote” helps make this point. In the Loony Tunes world of coyote and road runner, road runner frequently runs straight off a cliff, a logical step for her since, being a bird, she can fly. Coyote runs off the cliff right after her. That’s when the laws of the cartoon world take over. Coyote can keep running in mid-air, until the moment that he becomes aware that he is running in mid-air. When that happens, and we share with him the fact that he has become suddenly, painfully aware that he is in mid-air, he falls. Measurement that has been repeated feels like any other measurement, so when we use it in a way that is incorrect, there is a phase where we behave like the coyote that has gone off the cliff but has not yet looked down. The person wielding the measurement is already wrong, he is already in trouble, but he feels like he’s on a reliable extension of previous work. Error blindness occurs because measurement values do not convey any cue about their applicable range. As a result, we may feel we are correct. Schultz also tells a story from the Beth Israel Deaconess medical center, where a woman was scheduled for operation on her right leg but instead, had her left leg operated upon. The vice president for health care quality at Beth Israel issued a statement and said, "For whatever reason, the surgeon simply felt that he was on the correct side of the patient." The point of this story is that trusting too much in the feeling of being on the correct side of anything can be very dangerous. This internal sense of rightness that we all experience so often is not a reliable guide to what is actually going on in a given act of measurement. The scalometer can give a sense of the number of OMs that a measurement departs from its best zone of applicability. Schulz goes on to define how we operate while are still feeling right, even though we are wrong, and she describes narcissistic tendencies. Since you are right, people that disagree with you must either be ignorant 9so we can enlighten them), stupid (so they are probably not amenable to enlightenment as they lack the conceptual tools that we have) or evil—being deliberately biased for their own malevolent purposes. Such narcissicstic “attachment to our own rightness keeps us from preventing mistakes when we absolutely need to and causes us to treat each other terribly.” Schulz urges people to be aware of this blindness. She quotes St. Augustine, "Fallor ergo sum" -- "I err therefore I am."