Metrological traceability

Administrativeprograms provide standardization of the metrology and calibration work andmake it possible to independently verify that the work was performed. Generally, theadministrative program is specific to the organization performing the work and atjdressescustomer requirements. General administrative program specifications created by industrygroups, such as the ANS (ANSI) Z540 series may also be covered in the administrative program. Other specifications created by the US Food and Drug Administration, US Federal Aviation Administration or other agencies would supplementor replace ANS Z540 for work performed in their domains. Often administrativeprograms can be as complicated and detailed as the measurement work itself.

Anadministrative program that has insufficient actual metrology or calibration capability isderisively referred to as a "lick and stick" program.

Standards

Standardsare objects or ideas that are designated as being authoritative for some accepted reason.Whatever value they possess is useful for comparison to unknowns for the purposeof establishing or confirming an assigned value based on the standard. The design ofthis comparison process for measurements is metrology. The execution of measurementcomparisons for the purpose of establishing the relationship between a standardand some other measuring device is calibration.

Theideal standard is independently reproducible without uncertainty. This is what the creatorsof the "meter" length standard were attempting to do in the 19th century when they defined ameter as one ten-millionth of the distance from the equator to one of the Earth'spoles. Later, it was learned that the Earth's surface is an unreliable basis for a standard.The Earth is not spherical and it is constantly changing in shape. But the special alloymeter bars that were created and accepted in that time period standardized internationallength measurement until the 1950s. Careful calibrations allowed tolerancesas small as 10 parts per million to be distributed and reproduced in metrology laboratoriesworldwide, regardless of whether the rest of the metric system was implementedand in spite of the shortfalls of the meter's original basis.

Metrological traceability

A coreconcept in metrology is metrological traceability, defuv^d by the BIPM as "the propertyof the result of a measurement or the value of a standard whereby it can be related tostated references, usually national or international standards, through an unbrokenchain of comparisons, all having stated uncertainties." The level of traceabilityestablishes the level of comparability of the measurement: whether the result ofa measurement can be compared to the previous one, a measurement result a year ago, or tothe result of a measurement performed anywhere else in the world.

Traceabilityis most often obtained by calibration, establishing the relation between the indication ofa measuring instrument and the value of a measurement standard. These standards areusually coordinated by national metrological institutes: National Institute ofStandards and Technology, National Physical Laboratory, UK, Physikalisch-Technisc'heBundesanstalt, etc.

Traceability isused to extend measurement from a method that works in one regime to a differentmethod that works in a different regime, by calibrating the two using an overlappingrange where both work. An example would be the measurement of the spacing ofatomic planes in the same crystal specimen using both X-rays and an electron beam.Traceability also refers to the methodology used to calibrate various instruments by relating them back to a primary standard.

Tracebility,accuracy, precision, systematic bias, evaluation of measurement uncertainty are criticalparts of a quality management system.

Basics Mistakes canmake measurements and counts incorrect. Even if there are no mistakes,

nearly allmeasurements are still inexact. The term 'error' is reserved for that

inexactness,also called measurement uncertainty. Among the few exact measurements

are:

The absence ofthe quantity being measured, such as a voltmeter with its leads shorted

together:the meter should read zero exactly.

Measurementof an accepted constant under qualifying conditions, such as the triple

point of purewater: the thermometer should read 273.16 kelvin (0.01 degrees Celsius,

32,018 degreesFahrenheit) when qualified equipment is used correctly.

Self-checkingratio metric measurements, such as a potentiomecer: the ratio in between

steps isindependently adjusted and verified to be beyond influential inexactness

All othermeasurements either have to be checked to be sufficiently correct or left to chance. Metrologyis the science that establishes the correctness of specific measurementsituations. 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 bycountry. Repeatability and reproducibility studies help quantify the precision: one commonmethod is an ANOVA gauge R&R study.

Calibrationis the process where metrology is applied to measurement equipment and processesto ensure conformity with a known standard of measurement, usually traceableto a national standards board.

Society

Sufficientlycorrect measurements are essential to commerce. About nine out of every ten peopleworking in metrology specialize in commercial measurement, most at the technicianlevel. Correct measurements are beneficial to manufacturing, but other methodsare available and sometimes are more appropriate.

Metrologyhas thrived at the interface between science and manufacturing. Aerospace, commercial nuclear power, medicine, medical devices and semiconductors rely on metrology totranslate theoretical science into mass produced reality.

The basicconcepts of metrology appear simple on the surface, and metrology is rarely taught ina systematic manner above the technician level. Within most businesses, metrologycore beliefs such as recording all setups and observations for possible future reference areopposed to the general business practice of minimizing recordkeeping to limit litigationeffects.

Applied metrology

Metrologylaboratories are places where both metrology and calibration work are performed.Calibration laboratories generally specialize in calibration work only.

Bothmetrology and calibration laboratories must isolate the work performed from influencesthat might affect the work. Temperature, humidity, vibration, electrical powersupply, radiated energy and other influences are often controlled. Generally, it is the rate ofchange or instability that is more detrimental than whatever value prevails.

Calibrationtechnicians execute calibration work. In large organizations, the work is further divided into three groups:

Ј. Group Definition

Set-up people arrange the equipment needed for calibration and verify that it

workscorrectly.

Operators execute the calibration procedures and collect data.

Tear-down people dismantle set-ups, check the components for damage and then

put thecomponents into a stored state. This is the entry-level position for people who didn't start inthe equipment warehouse or transportation functions

Alternatively, the technicians can be divided by major discipline areas:physical ""■ dimensional electrical, RF, microwave and so on. But the principles are the same regardless ofthe equipment.

Date: 2016-04-22; view: 1043