Category :
Railways
Metrology and Inspection
- Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement.
- 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.
- Metrology is a very broad field and may be divided into three basic activities, though there is considerable overlap between the activities:
- Realization of these units of measurement in practice
- Application of chains of traceability linking measurements made in practice to reference standards.
- Metrology also has three basic subfields, all of which make use of the three basic activities, though in varying proportions:
\[-\]Scientific or fundamental metrology
\[-\]Applied, technical or industrial metrology
\[-\]Legal metrology
- Scientific or fundamental metrology concerns the establishment of quantity systems, unit systems, units of measurement, the development of new measurement methods, realisation of measurement standards and the transfer of traceability from these standards to users in society.
- The BIPM maintains a database of the metrological calibration and measurement capabilities of various institutes around the world. These institutes, whose activities are peer-reviewed, provide the top-level reference points for metrological traceability.
- In the area of measurement the BIPM has identified nine metrology areas including length, mass and time.
- Applied, technical or industrial metrology concerns the application of measurement science to manufacturing and other processes and their use in society, ensuring the suitability of measurement instruments, their calibration and quality control of measurements.
- Although the emphasis in this area of metrology is on the measurements themselves, traceability of the calibration of the measurement devices is necessary to ensure confidence in the measurements.
- Legal metrology "concerns activities which result from statutory requirements and concern measurement, units of measurement, measuring instruments and methods of measurement and which are performed by competent
- Such statutory requirements might arise from, amongst others, the needs for protection of health, public safety, the environment, enabling taxation, protection of consumers and fair trade.
- The OIML was set up to assist in harmonising such regulations across national boundaries to ensure that legal requirements do not inhibit trade. In Europe WELMEC was established to promote cooperation on the field of legal metrology.
- A core concept in metrology is metrological traceability defined by the Joint Committee for Guides in Metrology as "property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty".
- Metrological traceability permits comparison of measurements, whether the result is compared to the previous result in the same laboratory, a measurement result a year ago, or to the result of a measurement performed anywhere else in the world.
- Traceability is most often obtained by calibration, establishing the relation between the indication of a measuring instrument and the value of a measurement standard. These standards are usually coordinated by national metrological institutes.
- Traceability is used to extend measurement from a method that works in one regime to a different method that works in a different regime, by calibrating the two using an overlapping range where both work.
- Traceability, accuracy, precision, systematic bias, evaluation of measurement uncertainty are critical parts of a quality management system.
- 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:
- 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.
- 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.
- 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.
- Metrology laboratories are places where both metrology and calibration work are performed. Calibration laboratories generally specialize in calibration work only.
- Both metrology and calibration laboratories must isolate the work performed from influences that might affect the work. Temperature, humidity, vibration, electrical power supply, radiated energy and other influences are often controlled.
- Generally, it is the rate of change or instability that is more detrimental than whatever value prevails.
- Calibration technicians execute calibration work. In large organizations, the work is further divided into three groups.
- Metrology technicians perform investigation work in addition to calibrations. They also apply proven principles to known situations and evaluate unexpected or contradictory results.
- Specific education in metrology was formerly limited to sub-professional work. Most of the branches of the US Military train 'enlisted-grade' technicians to meet their specific needs.
- Large industrial organizations also develop people who demonstrate aptitude in testing functions.
- Mythologists are people who perform metrology work at and above the technician levels.
- The metrology and calibration work described above is always accompanied by documentation. The documentation can be divided into two types: one related to the task, and the other related the administrative program.
- Task documentation includes calibration procedures and the collected data. Administrative program documentation includes equipment identification data, 'calibration certificates', calibration time interval information and 'as-found' or 'out-of-tolerance' notifications.
- Standards are 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 purpose of establishing or confirming an assigned value based on the standard.
- The design of this comparison process for measurements is metrology. The execution of measurement comparisons for the purpose of establishing the relationship between a standard and some other measuring device is calibration.
- The ideal standard is independently reproducible without uncertainty. This is what the creators of the "metre" length standard were attempting to do in the 19th century when they defined a metre as one ten-millionth of the distance from the equator to one of the Earth’s poles.
- It was later learned that the Earth's surface is an unreliable basis for a standard, as the Earth is not spherical and it is constantly changing in shape. But the special alloy metre bars that were created and accepted in that time period standardized international length measurement until the 1950s.
- Careful calibrations allowed tolerances as small as 10 parts per million to be distributed and reproduced in metrology laboratories worldwide, regardless of whether the rest of the metric system was implemented and in spite of the shortfalls of the metre's original basis.
- Currently, five independent units of measure are internationally recognized:temperature interval, linear distance, electrical current, frequency and mass. Any measurement can be based on one or more of these measurement units.
- To supplement these five, two units of angle measurement that are also independent are recognized. For example, Ohm's law is a widely known concept in electrical study. Of the three units of measure involved, only current (ampere) is an independent unit. Voltage and resistance units are dependent on current units, as defined by ohm’s law.
- In industrial metrology, several issues beyond accuracy constrain the usability of metrology methods. These include:
- 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 the manufacturing process variables.
