World Metrology Day raises awareness of the importance of metrology and has been celebrated every year on the 20^th May since 2000. Organised by the BIPM (International Bureau of Weights and Measures) and the OIML (International Organisation of Legal Metrology), many national metrology institutes and regional metrology organisations around the globe participate in World Metrology Day[1]; for example, in 2021, thirty-six countries hosted World Metrology Day events.[2]

Quick Links:

• What is Metrology?
• What is World Metrology Day?
  • A (Very) Brief History of the Metre Convention
  • Development of SI Units
• Why Is World Metrology Day Important?
• Metrology in the Digital Era
• Further Information

What is Metrology?

The BIPM defines metrology as

“the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology.”[3]

Juan (Ada) Cai expands on this in the study The Case of the International Bureau of Weights and Measures, explaining that metrology is concerned with ensuring that measurements are accurate, stable, comparable, and coherent.[4]

What is World Metrology Day?

Designed to raise awareness of the importance of metrology, World Metrology Day celebrates the signing of the Metre Convention which took place on the 20^th May 1875.

A (Very) Brief History of the Metre Convention

Signed on the 20^th May 1875 by seventeen states, the Metre Convention is a diplomatic treaty that established the International Bureau of Weights and Measures, an inter-governmental organisation tasked with standardising systems of measurement across the world. To this day, the Metre Convention “remains the basis of international agreement on units of measurement and worldwide measurement systems.”[5]

It was born out of a need to tackle the confusion, mistrust, error, fraud, and stifling of scientific advancement brought about by a lack of standardised measurement. Prior to the metre convention, systems of measurement had been based on human morphology and included units such as the inch, hand, foot, and yard, which could all vary from human to human, town to town, and country to country. Growing trade, industry, and scientific discovery all demanded coherent, comparable measurement to be successful; to address this, politicians and scientists proposed using a unit of measurement derived from a standard evident in nature.

France’s metric system, which used a natural meter equivalent to the ten-millionth part of one-quarter of a terrestrial meridian (the distance between the North Pole and the equator), had been gaining popularity since the 1790s. Despite this, dependency on France’s original prototypes (such as the Metre Des Archives, which was eventually found to be 0.03% shorter than it should have been) to verify other countries’ national standards, together with a lack of consistency when it came to making copies of these prototypes, hindered the expansion of the metric system. In 1875 France hosted a conference in Paris to rectify these problems. At the meeting, it was established that:

• Three organisations would be set up to administer the treaty:
  • The International Bureau of Weights and Measures (BIPM)
  • The General Conference of Weight and Measures (CGPM)
  • The International Committee for Weights and Measures (CIPM)
• Multiple identical copies of the metre and kilogram would be created and distributed to member states; one copy of each would be selected as the international master copy
• The international master copies of the metre and kilogram would be kept in the laboratory
• The laboratory would be on neutral territory
• Members would compare their copies with the master copies at regular intervals
• The organisations would promote the use of the metric system

Seventeen countries, including Austria, Belgium, Denmark, France, Germany, Italy, Norway, Russia, Spain, Sweden, Switzerland, and Turkey, signed the treaty in 1875 agreeing to the above. The United Kingdom and the Netherlands attended the conference but did not sign the Metre Convention on the 20^th May 1875; however, they eventually joined in 1884 and 1929 respectively.[6]

Development of SI Units

The Metre Convention of 1875 was only concerned with standardising the kilogram and the metre. It was revised in both 1921 and 1933; firstly, to include physical measurements, and secondly, to merge electrical and photometry units into the metric system.

Many new units of measurement based on the metric system were developed during the late nineteenth and early twentieth centuries. However, measurement units used in electrical distribution systems and electrostatic and electromagnetic applications were incompatible with each other and the use of gravity in force and pressure definitions had resulted in the development of too many units. The creation of these problems meant that after the second world war the Union of Pure and Applied Physics and the French government asked the BIPM to investigate these problems.

In 1948, the 9^th CGPM commissioned the CIPM to investigate the units of measurement used by member states. Based on the CIPM’s report, in 1954 the 10^th CGPM decided to announce a new system of units that would include six base units:

• Metre
• Kilogram
• Second
• Ampere
• Kelvin
• Candela

Published in 1960, the 11^th CGPM named the new system the International System of Units, which is abbreviated to SI based on the French name Le Systèm international d’unités. The BIPM has described the SI as “the modern metric system”.[7]

Why is World Metrology Day Important?

Metrology is integral for ensuring our quality of life and applies to almost every aspect of our existence. We rely on accurate, stable, comparable, and coherent measurements for use in industry, trade, commerce, regulation, legislation, and science. For example:

• Parameters, such as size, weight, and output voltage, must be reliably comparable between countries to ensure the success of trade and commerce
• Consumers must trust that suppliers are delivering the quantities that they say they are, e.g., litres of petrol at the pump and metered gas and electricity
• A common international timescale is critical for ensuring accurate satellite navigation positioning, enabling electronic banking, and delivering functional internet and telecommunication services; a common international timescale also enables the coordination of meetings with colleagues in different locations around the world
• Reliable, accurate, comparable, coherent measurements are essential for ensuring accurate health diagnoses
• Effective implementation of regulations and specifications relies on coherent, comparable, accurate, and stable measurement[8]

Celebrating World Metrology Day acknowledges the vital role metrology, and specifically the Metre Convention, has in ensuring a worldwide coherent measurement system upon which scientific discovery/innovation, industrial manufacturing, and international trade can be based and advanced; thereby, improving quality of life and protecting the global environment. It also highlights and recognises “the contribution of all the people that work in intergovernmental and national metrology organisations and institutions throughout the year” to develop and validate new, sophisticated measurement techniques, systems, and regulations with the view to advancing scientific discovery and global trade.[9]

Metrology in the Digital Era

Metrology in the Digital Era has been chosen as the theme for 2022’s World Metrology Day. According to World Metrology Day’s press release

“This theme [Metrology in the Digital Era] was chosen because digital technology is revolutionising our community, and is one of the most exciting trends in society today.”[10]

Metrology is evolving with the digital era. For example, new clocks can measure the second more precisely than ever; therefore, the definition of the second must be more stringent. Currently, metrologists within the BIPM including Dr Noël C. Dimarcq, a physicist and president of the BIPM’s consultative committee for time and frequency, are finalising a list of criteria which must be fulfilled for the new definition to be confirmed. Dr Dimarcq anticipates “that most [criteria] would be fulfilled by 2026, and that formal approval would happen by 2030.”[11] As new definitions of the kilogram, ampere, kelvin, and metre (approved in 2018) are all determined in relation to time, it is vital that changes to the second’s definition are made carefully and that they do not alter its duration.[12]

Further Information

As a leading UK provider of test, measurement, and calibration equipment, and as a UKAS-accredited calibration lab and service centre, the importance of metrology is not lost on us.

Please visit our website to browse our extensive range of electrical, thermal, and scientific metrology equipment, including instruments by leading brands such as Druck, Fluke, FLIR, Mitutoyo, Norbar, Kern, and Sauter. Alternatively, view our full calibration scope here.

For help and advice regarding any of our products or calibration services, please contact our team on 01642 626144 or via our online form.

Footnotes

[1] Juan (Ada) Cai, OECD/BIPM (2020), International Regulatory Co-Operation and International Organisations: The Case of the International Bureau of Weights and Measures (BIPM), (OECD and BIPM, 2020), p.18, last accessed 20 May 2022

[2] World Metrology Day, World Metrology Day Events, last accessed 20 May 2022

[3] Internet Archive, What is Metrology?, last accessed 20 May 2022

[4] Juan (Ada) Cai, OECD/BIPM (2020), pp. 12-13

[5] Juan (Ada) Cai, OECD/BIPM (2020), p. 17

[6] Information for this section was gathered using the following sources:

• Juan (Ada) Cai, OECD/BIPM (2020), p. 17
• Kiddle, Metre Convention facts for kids, last accessed 20 May 2022
• Wikipedia, Metre Convention: Membership, last accessed 20 May 2022

[7] Information for this section was gathered using the following sources:

• Kiddle, Metre Convention facts for kids: Activities, last accessed 20 May 2022
• Kiddle, Metre Convention facts for kids: Development of SI, last accessed 20 May 2022

[8] Juan (Ada) Cai, OECD/BIPM (2020), p. 13, and, Stefanie Reichert, ‘The guardians of metrology’, Nature Physics, 18.222 (2022), last accessed 20 May 2022

[9] World Metrology Day, Press Release: World Metrology Day 20 May 2022, last accessed 20 May 2022

[10] World Metrology Day, Press Release: World Metrology Day 20 May 2022

[11] Alana Mitchell, ‘Get Ready for the new, improved second’, The New York Times, last accessed 04 May 2022

[12] Alana Mitchell, ‘Get Ready for the new, improved second’

The post What is World Metrology Day & Why Should You Care? appeared first on Calibrate.

The new accreditation is an expansion of PASS’s existing scope covering both Electrical and Pressure ranges. The extension of their UKAS scope will now allow PASS to offer a greater choice of calibration services to it’s growing customer base.

Recent investment has seen the company develop new laboratories and purchase specialist equipment for pressure and test chambers and ovens for their temperature service. This is all housed within their Head Office in North East England.

Catering for many industry sectors, PASS offers a wide range of products and services to cater for commercial, domestic and industrial sectors including utilities, electrical, manufacturing, facilities management, education, healthcare and more.

With a massive commitment to quality as well as it’s calibration accreditation (ISO/IEC 17025) PASS is a BS EN ISO 9001 accredited company who are also registered and audited by independent supplier databases UVDB, RISQS and ACHILLES as suppliers to the Utilities, Rail and Energy sectors.

With a constantly growing range of products and services PASS can calibrate a wide variety of test and measurement devices, providing customers with full traceability to UKAS and International Standards.

To see the PASS’s full UKAS scope visit the UKAS page here.

For more information on how PASS can service you and your business contact one of our Calibration Service Advisors on 01642 626 140.

The post UKAS Scope Expanded to Include Temperature & Frequency. appeared first on Calibrate.

There are a variety of tools used in industry, worksites or even in the army and aviation industry. Production and quality control are highly dependant on the correct performance of the tools we are using. But how can we be sure that all these tools are functioning properly? The reply is a single word: calibration! The only way to ensure that our tools are inside their correct specifications is to regularly calibrate them and monitor their performance.

The next question that arises is “which tools need calibration”? Of cours, simple mechanical tools such as hammers and mechanical screwdrivers don’t, as they don’t rely on any measurements. Depending on the process, we can see below some examples of equipment that is considered to be a tool and needs to be calibrated.

When a high pressure gas station is manufactured, most of the connections of pipes and critical equipment (valves, gas meters, etc.) are flanged. For the flange connections various nuts and bolts are used, which must be tightened by using torque wrenches. The torque that must be applied to each nut is specified by the manufacturer. If the torque is less than the nominal value, leakages or even disconnections may occur. If it is of a higher value, injuries or even damage to the critical equipment are possible to happen. The only way to be sure that the proper torque is applied is to have the torque wrench being used calibrated.

The pipes used for the high pressure gas station are manufactured according to international specifications. These pipes are also coated and painted according to these specifications. In order to test the coating and the painting, coating thickness testers and holiday detectors are used. A coating thickness tester measures the thickness of the painting while a holiday detector generates a high voltage and by this way detects any film discontinuities. Both of these measurements are very important in order to avoid corrosion of the pipes. Thus, both coating thickness testers and holiday detectors are deemed as tools that need to be properly calibrated.

In a worksite, many tools are used for construction. Some of them are laser distance meters, level meters and measuring tapes. All of them are considered to be critical equipment that needs calibration.

In the aluminium industry, aluminium must be heated up to a certain temperature in order to melt and produce aluminium profiles. This is performed in high temperature ovens which are programmed at a temperature setting and have an indicator that shows the oven’s temperature. But how can we be sure that the oven’s reading is correctly indicating the temperature inside the oven? We need to periodically calibrate the oven as a system with its indicator.

Another example of tools that need calibration is Electrical Testers. These testers are used for measuring voltage, current, continuity, etc. Since they are used mainly for safety applications, it is very important that they measure correctly. Earth ground and Insulating testers belong also to the category of tools used for safety measurements. All kinds of testers need calibration.

So, depending on the application, there are many tools that need to be calibrated.

Some of them are listed below:

• Torque Wrenches
• Torque screwdrivers
• Laser distance meters
• Coating thickness testers
• Holiday detectors
• Level meters
• Tapes and steel rulers
• Electrical Testers
• PAT Testers
• Cable Testers
• Insulation Testers
• Socket Testers
• Gas detectors
• Hardness Testers
• Vernier Callipers
• Feeler gages
• Electrofusion welding machines

These are just some examples of tools that need to be calibrated. Many more tools, depending on their application and the importance of their usage, need to be regularly checked.

Of course, a simple tool calibration every now and then, is not enough. The industry or the organization must have a well established quality system in order to control and document all the tools. Such a quality system must contain at least the following in order to be complete:

A master list of all the tools that need calibration.

This list must include all the tools that must be calibrated. For each tool, the number, the manufacturer, the serial number, the location, the calibration date, the calibration interval, the next calibration date and any other relevant information must be mentioned. This list allows for the maintenance responsible personnel to manage the tools and their calibration.

The calibration procedure for each type of tool

In some cases, in industry for example, many tools are calibrated internally by transfer standards. These standards are sent for calibration in external laboratories. Transfer standards can be reference thermometers for ovens calibration, gauge blocks for dimensional calibrations such as callipers, rulers and tapes, etc. The internal calibration procedures must be documented and validated within the quality system.

Calibration records

Either internal or external, calibration records for each tool must be kept. By having this historical data, the user can export useful information which can help him define the recalibration interval.

Procedure for the usage, handling and storage of the tools

Every tool must be used and handled as specified by the manufacturer. Also it must be stored in certain environmental conditions. It is very important to have procedures that describe all the above and also to train the personnel for the correct usage, handling and storage, because this will definitely increase the lifetime of the tools.

Determine the identification of the tools

There must be a procedure defining how the tools are identified according their calibration requirements. For tools that do not need calibration at all, a sticker “No Calibration Required” can be placed on them. Other stickers can be “Calibrate before Use”, “Not Calibrated” or “Do not Use”. The tools that are properly calibrated and functioning correctly can be distinguished by their “Calibration Label”.

All the aforementioned can help us in the control of the tools. We can be sure by this way, that we will not forget to calibrate them, and that the tools we are using are within specifications. By studying the calibration reports of each tool over time, we can understand its performance and decide whether to increase or reduce its calibration interval.

Even the best of the tools can go off-specification over time; this is why regular calibration is required. Calibration is of course time consuming and has a financial cost. But imagine having to recall a whole batch of vehicles in the automotive industry, or suffering an airplane crash because of a non-properly calibrated tool!

Written by Sofia

The post Tools, Do They Need Calibration? appeared first on Calibrate.

There is a close relationship between mathematics and measurements. All the quantities measured have two parts – the name of the unit and their numerical value. Moreover, the electrical units can be related to each other by using various mathematical equations. For example, the most commonly used equation for DC and LF measurements, Ohm’s Law (V = I x R), shows how the voltage, the current and the resistance in a DC circuit are strictly related to each other. If the values of any two of the above units are known, the value of the third can be calculated.

The SI Volt is a key unit in DC and LF metrology and is defined as power divided by current. Its definition is realized by experiments that compare electrical power against mechanical power via a force-balance. The results of these experiments are used to assign values to the voltage produced by inexpensive, stable, easily reproduced and maintained devices, such as electromechanical standard cells or electronic zener-stabilized voltage standards. These devices represent the SI Volt because their principles of operation do not involve a continuous comparison of electrical power with the power produced by realizations of the SI mechanical units.

The best available representation of the SI Volt is obtained from a Josephson Array. The voltage produced by this device is a function of the frequency of microwaves that irradiate it and the Josephson constant, a universal quantity independent of experimental variables.

The values of the national representations of SI units must be transferred to the representations of the units used by local calibration laboratories. The devices or artefacts which store or maintain all such representations, national or local, are conventionally called standards. The best local standards are characterized as primary standards. Their values are transferred to other local standards, usually called working or secondary standards. The values of the working or secondary standards are then transferred to devices, usually called calibrators, used to extend the value of the SI unit to a wide range of additional values.

Measurement Equipment

In order to perform the best possible measurement, the metrologist must select the appropriate equipment and use it correctly in effective test configurations. The selection of the suitable equipment requires an extensive knowledge of at least the following issues:

• Types of measurement equipment
• Their principles of operation
• Imperfections of equipment and test configurations
• Types of measurements that can be performed
• Purposes of calibration measurements

Some of the generic types of calibration equipment used for DC and Low Frequency measurements are presented below:

Secondary Standards

These standards usually include the devices that SI units from primary standards are transferred to. Secondary standards are used to calibrate calibrators or to increase the performance of calibrators when extremely accurate measurements are required (i.e. calibration of a laboratory DMM).

Null Detectors

Null Detectors are meters which measure voltage differences. When the difference is zero Volts, the meter’s reading is at null. A typical application of a null detector is the measurement of the difference in the output of two resistive voltage dividers whose inputs are in parallel across a voltage source. There are digital and analog null detectors. Typical analog null detectors are actually galvanometers and contain a pointer resting at the middle of a deflection scale when there is zero input voltage. The pointer can move in both directions, indicating in this way the polarity of the difference as well. Digital Null detectors are much more sensitive due to their ability to amplify the voltage difference. They can measure accurately a null between voltage amplitudes from < 1μV up to 1000V.

Calibrators

Calibrators are the most common equipment found in an electrical calibration laboratory. They are widely used to calibrate general purpose test equipment such as DMMs and oscilloscopes. Most calibrators are multifunctional. These means that they can provide a wide range of values for a number of different SI units such as DC voltage and current, resistance, low frequency (and sometimes even RF) AC voltages and AC current. Multifunction calibrators can be used to calibrate even 5 1/2 digit multimeters. Nowadays calibrators can be operated either manually or controlled by a computer by using the appropriate software. These automated measurements help in reducing the time needed for calibration and are more reliable since there is no user interference.

Test Leads

Test leads comprise of the wires, cables and connectors that are used to connect the measurement equipment and the instrument under test. Test leads must be compatible with the measurement that is being performed. For example, when measuring a very low level DC voltage, the low thermal emf error can be avoided by using connectors manufactured by the same material as the instrument’s connectors.

Types of Measurements

When performing DC and LF measurements, several types of measurements can be applied:

Direct Measurements

Direct measurements are performed when placing an instrument in direct contact with the phenomenon that needs to be measured. For example, when setting a calibrator to provide a DC voltage and then connect a handheld DMM to its output, then the DMM is conducting a direct measurement. The DMM’s display will indicate directly the DC voltage provided by the calibrator.

Differential Measurements

Differential measurements are performed when using an instrument, such as a null detector, to measure the difference between a known and an unknown quantity of the same value. This method is sometimes more accurate and provides higher resolution than performing a direct measurement.

Transfer Measurements

By conducting a transfer measurement, we actually transfer the value of a known quantity to the quantity under test. An example of a transfer measurement is when providing a voltage across two in series connected resistors. The one is the standard resistor (with a known value) and the other is the resistor under test. The value of the standard resistor is transferred to the resistor under test, by applying Ohm’s Law:

Vstd x Rstd = Vtest x Rtest =>

[math]Rtest=Rstd\frac{Vstd}{Vtest}[/math]

Ratio Measurements

Ratio measurements are commonly used in DC and LF metrology. The transfer measurement described above is also a ratio measurement since the value of the standard resistor is transferred to the resistor under test via a voltage ratio.

Indirect Measurements

By using indirect measurements, someone can find the value of interest from other values. For example DC current can be calculated (by using Ohm’s Law) when measuring the voltage drop across a known resistance, when the current is passing through that resistance.

Types of Calibration Measurements

By the term Calibration, any one of the following types of calibration may be suggested:

• Verify the performance of the instrument under test
• Adjust the response of the instrument under test
• Provide correction factors for the instrument under test

Depending on the customer’s requirements, any or all of the aforementioned methods can be applied. For example when a DMM is being calibrated for DC voltage, we are setting the calibrator to provide 10 V DC to its input. We observe the reading of the DMM’s display which is 9.95 V. We have just verified the performance of the DMM. If we find it to be out of the specifications, we can perform adjustment (either by hardware i.e. a potentiometer, or by software for newer models). This is the adjustment of the response of the DMM, which now reads 10.00 V. If an adjustment is not possible or the customer has requested to provide only the correction factors, we can state that the reading of the DMM is 9.95 V and the correction factor is 0.05 V.

Of course, all the above information must be properly mentioned and documented in a Calibration Report, providing the measurement uncertainty for each measured value.

Written by Sofia

The post Basic Information About DC and Low Frequency Measurements appeared first on Calibrate.

According to the Vocabulary in Metrology (VIM), the following definitions are given:

Measurement Standard: A material measure, measuring instrument, reference material or measuring system intended to define, realize, conserve or reproduce a unit or one or more values of a quantity to serve as a reference.

Measurement Traceability: The property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, all having stated uncertainties.

These two metrology concepts, standards and traceability, are closely related to each other. For a defined standard to be universally accepted, everybody must agree to trace their measurements back to it. For a calibration laboratory to feel secure that its measurement of a quantity is compatible with that of another calibration laboratory, it must accept the defined standard and establish traceability to it.

Standards

Every unit of measurement has a definition, a realization and a representation. The definition is the ideal and it is usually a member of the SI (International System of units). The realization is achieved by means of an experiment whose result matches the definition as closely as possible. The experiment is most often performed by a national laboratory and is usually time consuming and expensive. When the realization is obtained, the national laboratory stores its value as a representation of the unit. The national laboratory uses its representation as a master standard to which other representations are compared. Examples of modern measurement standards are the platinum-iridium kilogram for mass, the Cesium 133 atomic clock for time and frequency and the Josephson junction for voltage.

Ideally, everyone adopting a defined standard for their measurements of a quantity would expect the following:

–       Everyone considers the definition of the standard to be fixed and correct.

–       It is embodied in an artifact, or can be created through a well-defined scientific experiment.

–       If the standard is embodied in an artifact, there is a controlled way to propagate the embodied value from the master to the standards and measuring devices of all users in their various locations.

–       If the standard is re-created experimentally, the experimental results must be identical at all times and at all places, under all environmental conditions.

–       There must be a controlled way to scale the standard value upward and downward.

–       There is no loss of precision or accuracy as a result of the scaling process.

Not all the measurements performed in a calibration laboratory trace back directly to national standards. Depending on the quantity being measured, standards can be any of the following:

–       National Standards

–       Intrinsic Standards

–       Ratio type calibrations

–       Consensus Standards

–       Indirectly-derived quantities

National Standards: These are the standards maintained by each country usually by its national laboratory. Local laboratories maintain local standards or other standards that are used to transfer a quantity between a national laboratory and a local laboratory. Transfer standards are usually physically transported to the national laboratory for comparison with the national standards. National standards usually have years, or even decades of history associated with them. They have a proven and accepted stability and uncertainty.

Intrinsic Standards: These standards provide a standard quantity considered to be without error (always within a stated uncertainty). For each and any standard of this kind a procedure exists which, if properly followed, will produce a quantity with an uncertainty that does not exceed stated limits. Examples of intrinsic standards associated with electrical calibration are the Josephson Array for DC voltage, the Quantum Hall Effect standard for resistance and the Cesium standard for time. The intrinsic standards exist in order to allow calibration laboratories to be less dependent from national laboratories.

Ratio Standards: These standards are used to obtain other values of a unit from a traceable artifact or intrinsic standard. For example, the 10V DC output of a Fluke 732B artifact standard can be extended upward or downward by using proper ratio techniques. Devices that are used to determine exact ratios are considered to be laboratory standards and require periodic check or calibration in order to ensure the traceability of measurements.

Consensus Standards: These standards can be an artifact or process that is mutually acceptable to a supplier and a customer whenever there is no national or intrinsic standard.

Indirectly-derived Quantities: They are calculated by measuring another quantity. An example is direct current, which is one of the base units in the SI system of units, but it is not directly traceable to national standards. A common practice is to pass the current through a known resistor, measure the voltage drop across the resistor and calculate the current by using Ohm’s Law.

Traceability

Traceability refers to the procedures and records that are used and maintained to demonstrate that calibrations made by a local calibration laboratory accurately represent the quantities of interest. Three major facts are mutually agreed in order to maintain traceability:

–       World-wide adoption of the International System of Units (SI) as the basic system of units of weights and measure.

–       The establishment of national laboratories which are responsible for the maintenance of the representations of the SI units and their transfer to calibration laboratories.

–       Definition, implementation and use of methods and procedures that allow individual calibration laboratories to compare their local standards with those of the national laboratories.

In order for a calibration laboratory to be confident for the quality of its own standards, a good practice is to often participate in interlaboratory comparisons. To support the claim of traceability, the calibration laboratory must have documented measurement procedures and provide a description of the chain of comparisons that were used to establish a connection to a particular stated reference. The following requirements must be fulfilled in order to provide valid statements of traceability:

–       A very well defined quantity that has been measured.

–       A complete description of the measurement system used to perform the measurement.

–       A stated measurement result accompanied by a documented uncertainty.

–       A complete specification of the stated reference at the time the measurement system was compared to it.

–       An internal measurement assurance program for establishing the status of the measurement system at the time relevant to the claim of traceability.

–       An internal measurement assurance program for establishing the status of the stated reference at the time that the measurement was performed.

The traceability of measurements needs not only to be valid but also to be presented to the customer. Calibration reports, besides presenting the calibration results, also provide evidence about the traceability of the measurements. Calibration reports must always contain a statement in order to prove the unbreakable chain of comparisons to national standards. For ISO 17025 accredited laboratories the traceability statement is a requirement. But even when the calibration is performed by a non-accredited laboratory, the customer must always require the traceability of measurements to be proven.

Written by Sofia

The post The Importance of Measurement Standards and Traceability appeared first on Calibrate.

The International System of Units (SI) is the foundation of modern metrology. The abbreviation SI is taken from the French name, Système International d’Unités, and is actually the modern form of the metric system. SI was established in 1960 by the General Conference of Weights and Measures.

The SI units are used internationally and they are the basis of all modern measurements. There are also Customary units (inch, foot, pound, and yard) which are defined in accordance with the SI units. For example, an inch is defined as being2,54 centimetresin length.

National Laboratories perform experiments to realize the SI units as defined. Some of these experiments lead to a representation of the unit. For example the representation of the “Volt” unit is realized with a Josephson array.

The System of Units

The SI consists of 29 units:

•  7 Base Units
• 2 Supplementary Units
• 20 Derivative Units

Base Units

The seven base units, from which all other measurement parameters are traced, are: length, mass, time, electric current, thermodynamic temperature, luminous intensity and amount of substance.

Length

The meter (m) is the SI unit of measurement for length. It is defined as the distance travelled by light in vacuum during a time interval of 1/299792458 second.

Mass

The kilogram (kg) is the SI unit of measurement for mass. Kg is the only unit which is still defined as a physical artefact, the mass of the International Prototype Kilogram which is a cylinder of platinum iridium alloy kept by BIPM inParis,France.

Time

The SI unit of time is the second (s). It is defined as the duration of 9 192 631 770 cycles of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

Electric Current

The SI unit of measurement for electric current is the ampere (A). It is defined as the constant electric current producing a force of 2×10^-7 newtons per meter of length, if maintained in two straight parallel conductors of infinite length, one meter apart in vacuum.

Thermodynamic Temperature

The Kelvin (K) is the SI unit of measurement for thermodynamic temperature. It is defined as the 1/273.16 of the thermodynamic temperature of the triple point of water.

Luminous Intensity

The Candela (cd) is the SI unit of measurement for luminous intensity. It is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation at a frequency of 540×10¹² Hertz, with a radiant intensity in that direction of 1/683 Watts per steradian.

Amount of Substance

The SI unit of measurement for an amount of substance is the mole (mol). It is defined as the amount of substance of a system that contains as many elementary entities as there are atoms in0.012 kilogram of carbon 12.

Supplementary Units

Plane angles and solid angles are the two supplementary units in the SI system. They are dimensionless quantities and are defined as follows:

Plane Angles

The SI unit of measurement for plane angles is the radian (rad). It is defined as a plane angle with vertex at the centre of a circle that is subtended by an arc equal in length to the radius.

Solid Angles

The steradian (sr) is the SI unit of measurement for solid angles. It is defined as the solid angle with vertex at the centre of a sphere that is subtended by an area of a spherical circle equal to that of a square with sides equal in the length to the radius.

Derived Units

There are 20 derived units which are obtained by combining the seven SI base units with each other and with other derived or supplementary units. The table below presents the derived units and their relation to the base units. As time goes on, more derived units may be added in order to cover the needs of science.

Common Metrology Terms

Everybody who is involved in metrology, from calibration technicians to customers in search of the suitable calibration laboratory, must have a good knowledge of the most commonly used metrology terms. Some of these terms are defined below in simple words.

• Accuracy (Measurement Accuracy): A number which indicates the closeness of a measured value to the true value.
• Adjustment: An operation that is performed in order to initially establish or restore at a later point an instrument’s specified performance level.
• Calibration: A set of operations performed in accordance with a definite and documented procedure that compares the measurements performed by an instrument, to those made by a more accurate instrument or standard, for the purpose of detecting and reporting or eliminating by adjustment any errors in the instrument tested.
• Calibration Interval: A specified or designated period of time between calibrations of an instrument. During this interval the instrument should remain within specified performance levels.
• Calibration Label: A label affixed to an instrument to show its calibration status. Usually the label contains the instrument’s identification, the person who performed the last calibration, the date of the last calibration and the date of the next calibration.
• Calibration Laboratory: A work space equipped with the appropriate test instruments, controlled environment, trained personnel and documented calibration procedures. Cal Labs perform many routine calibrations.
• Calibration Report: A document which describes the calibration, provides the calibration results, mentions the calibration responsible, the conditions of measurements, the equipment used, the procedure of measurement and the measurement uncertainties.
• Error (Measurement Error): The difference between the measured value and the true value of a measurement. The actual value of an error can never be known exactly, only estimated.
• Measurement: A set of operations performed on a physical object or system according to an established, documented procedure, in order to determine the value of the object or system.
• Metrology: The science of measurement. It contains everything that has to do with measurement: Designing, performing, documenting the measurement, evaluating and analyzing the results, calculating the measurement uncertainties.
• Seal (Tamper Seal): A seal of appropriate design and material that is attached to an instrument to clearly indicate tampering. The purpose is to ensure warranty of calibration.
• Specification: A documented presentation of the parameters, including accuracy or uncertainty, describing the capability of an instrument.
• Standard (1) (Measurement Standard): An object, artifact, instrument, system or experiment that stores, represents or provides a physical quantity which serves as the basis for measuring the quantity.
• Standard (2) (Paper Standard): A document describing the operations and processes that must be performed in order for a particular goal to be achieved. A well known standard is ISO 17025 which describes the requirements for the competence of testing and calibration laboratories.
• Tolerance: The limits of the range of values that apply to a properly functioning measuring instrument. This term is strongly connected to the accuracy term.
• Traceability: A calibration is traceable when each instrument and standard, in hierarchy stretching back to the national standards, was itself properly calibrated and the results properly documented. The documentation provides the information needed to prove that all calibrations in the calibration chain were properly performed.
• Uncertainty: An estimate of the range of values, usually centered on the measurement value, which contains the true value of a measured quantity, with a stated probability.
• Verification: The set of operations that assures that specified requirements have been met, or leads to a decision to perform adjustments, repair, downgrade performance or remove from use.
• ppm (parts per million): a convenient way if expressing small fraction and percentages. For example, 15 ppm = 15 / 1000000 or 0.000015 or 0.0015%.
• Unit Under Test (UUT): The instrument that is being tested / calibrated.

Written by Sofia

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• Bell Prover: This is actually a calibrated vessel with well-known volume characteristics. It is often used as a primary standard. The Bell Prover provides a certain volume to the gas meter under test and thus a direct comparison between the meter’s reading and the volume’s value can be performed. This method is mainly used for calibrating small domestic / diaphragm meters.
• Gravimetric method: A very accurate weighing scale is used to define the amount of gas that actually flows through the meter during the calibration procedure. This method is considered to be very accurate.
• Test Bench: It is a complete system which uses master meters as reference standards. The same amount of volume flows through the master meter and the gas meter under test, and the two meter readings are compared to each other. This method is widely used by many calibration laboratories all over the world. Depending on the size of the master meters, high flowrates can be achieved and very large gas meters can be calibrated.

Before proceeding with the analysis of the Test Bench method of calibration, it is important to give some information about the metrological characteristics of the most common types of gas meters.

When a gas meter is calibrated, the result reported in the calibration report is the % Error of Indication which must be measured in specific flow points (depending on the meter’s type and rangeability). Error of Indication (f) is defined as follows:

[math]f=\frac{V_{MUT}-V_{TRUE}}{V_{TRUE}}-100 \%[/math]
Where:

V_MUT =   Volume Reading of the MUT

V_TRUE =   Actual Volume that flowed through the MUT

Based on these measurements, a calibration curve can be included in the calibration report. A typical calibration curve can be seen below:

The black curve indicates the Error of Indication of the gas meter in several flow points, while the red lines represent the error limits (maximum permissible error) for the specific type of meter. Error limits as well as measurement flow points are specifically defined within the corresponding European Standards for each type of gas meter.

Diaphragm Meters

According to EN 1359 the error limits of Diaphragm gas meters are shown in the following table:

The flow points for the calibration of a diaphragm meter are Qmax, 0,2Qmax, Qmin.

Rotary Displacement meters

For Rotary Displacement meters, the error limits are defined according to EN 12480:

Where Qt (Transitional Flowrate) is given by the following table:

EN 12480 also specifies that the meter must be calibrated at the following flow points depending on the meter’s rangeability (Qmin/Qmax):

Turbine meters

Turbine gas meters are calibrated according to EN 12261 which specifies the following error limits:

Where Qt (Transitional Flowrate) is given by the following table:

The test flow rates for turbine gas meters are defined as follows:

Calibrating gas meters with the Test Bench method

The Test Bench consists of:

• Two master meters of high accuracy
• Control valves for flow adjustment
• A fan working in suction mode
• Temperature transmitters
• Pressure Transmitters
• Software for communication and data processing

In the above figure the master meters used are one G16 and one G650 covering a total flow range of 0,5m³/h to 1000 m³/h. Usually rotary displacement meters with dual impellers are used in order to eliminate pulsation and resonance to the flow profile. In large test benches, where high flow rates must be achieved, turbine meters are also used as master meters.

In the case that the Calibration Medium is air at atmospheric pressure (which is an acceptable method for calibrating gas meters which operate at a pressure lower than 4 bar, as defined in EN 12261), the principal of operation of the Test Bench is described below.

The air enters through the filters (with the fan operating in suction mode) and flows through the master meter. The flow is adjusted via the control valves and the speed of the fan. The air passes through the meter under test and returns to the room exiting from the fan.

A temperature and a pressure sensor are placed on each meter (master meter and meter under test).

The test bench must have the capability to monitor the pressure drop at the meter under test as well as leakages at any part of the installation.

During the measurement (at each flow point) the following data are measured at the master meter and at the meter under test:

• The pulses
• The temperature
• The pressure
•  The measurement time
• The barometric pressure

The indication error of the meter under test results from the comparison of the readings of the meter under test and the master meter and is given from the following formula:

[math]f_{MUT}=(\frac{V_{MUT}-(1+f_{STD}/100)P_{MUT}-T_{STD}}{V_{STD}-P_{STD}-T_{MUT}}-1)-100 \%[/math]
Where:

[math]V_{MUT}=\frac {N_{MUT}}{F_{MUT}}[/math]

and

[math]V_{MUT}=\frac {N_{MUT}}{F_{MUT}}-\frac{t_{MUT}}{t_{STD}}[/math]

Where:

V_MUT : The volume of the meter under test

V_STD : The volume of the master meter

f_STD : The error of indication of the master meter (taken from the master meter’s calibration report)

P_MUT : The pressure measured at the meter under test

P_STD : The pressure measured at the master meter

T_MUT : The temperature measured at the meter under test

T_STD : The temperature measured at the master meter

N_MUT : The number of pulses of the meter under test during the measurement

N_STD : The number of pulses of the master meter during the measurement

F_MUT : The pulse value of the meter under test (written on the meter’s index)

F_STD : The pulse value of the master meter (taken from the calibration report)

t_MUT : The measurement time of the meter under test

t_STD : The measurement time of the master meter

Specifications regarding the pressure and temperature measurement points, the upstream and downstream piping and the pressure loss measurement are mentioned in the relevant European Standards (EN 12480, EN 12261, etc.).

Useful information regarding construction and operation of Test Benches are also given in PTB Band 29 Guide “Testing of volume gas meters with air at atmospheric pressure”.

Measurement Uncertainty

When using a Test Bench to calibrate gas meters, similar to the one described above, there are several sources of measurement uncertainties:

• Repeatability of the meter under test
• Uncertainties resulting from the calibration of the temperature sensors, pressure sensors, barometer and master meters
• Fluctuation of pressure and temperature during measurement
• Drift of the sensors and the master meters
• Clock pulses are truncated due to the “A gated by B” method
• Limited resolution of the sensors, the master meters and the meter under test

Based on the formula mentioned above:

[math]f_{MUT}=(\frac{V_{MUT}-(1+f_{STD}/100)P_{MUT}-T_{STD}}{V_{STD}-P_{STD}-T_{MUT}}-1)-100\%[/math]

The standard uncertainty of measurement can be calculated by the following method:

[math]u(f_{MUT}=\sqrt{\sum(\frac{\partial f_{MUT}}{\partial xi})^2-(u(xi))^2}[/math]
Where:

x_i is each measurement component (pressure, temperature, etc)

u(x_i) is the standard uncertainty of each x_i

[math]\frac{\partial f_{MUT}}{\partial xi}[/math]is the partial derivative of f_MUT related to each x_i

A typical uncertainty value resulting from a measurement similar to the one described above, can be around 0,3%.

The same measurement philosophy can be used for high pressure measurements with natural gas as test medium. In this case the calculations are more complicated, since many more quantities affect the measurement (pressure, natural gas composition, etc).

In both cases, high or low pressure, air or natural gas, the calibration method must be well documented and validated, since gas meters are mainly used for billing processes. One way to achieve this is accreditation according to ISO 17025.

Written by Sofia

The post Technical Methods And Requirements For Gas Meter Calibration appeared first on Calibrate.

• Diaphragm meters
• Rotary displacement meters

On the other hand, flow meters measure volume indirectly. They use natural laws and flow principals, in order to determine the effect of the incoming gas. Turbine meters are the most common flow meters.

Diaphragm meters

Diaphragm meters have four measurement chambers. Two by two, the measurement chambers form a section which is separated by using a deformable wall, the diaphragm. Both diaphragms are connected to each other and they lead, via a rotating piston, to the counter.

By knowing beforehand the volume of each chamber, we can measure directly the gas volume that passes through the diaphragm meter.The main reasons for the increase of the measurement error of diaphragm meters are the leakage both at the moving parts and at the diaphragm leakage. This leakage usually influences the low flow range and results in a lower reading.

Diaphragm meters are mainly installed in domestic applications because they have especially large rangeability (Q_max/Q_min = 160).

Diaphragm meters, can be equipped with pulse generators in order to transmit the meter’s reading.

Rotary Displacement meters

Rotary displacement meters consist of two rotating impellers that move opposite to one another. The impellers are placed inside a housing. The cross section of the impellers, vertically

positioned to the rotating axis, is formed in such a way, that the gap between the impellers and the housing is very small, regardless of the impellers’ position.

The impellers are placed in such a way, that they don’t touch each other during the rotation. The gas volume can be measured directly since the volume of each measurement chamber is well known.Rotary displacement meters can also achieve high rangeabilities like Q_max/Q_min = 250, but the small gaps lead to high sensitivity to dust particles. In order to avoid the damage on the impellers, since they are manufactured from light metals, special care must be given to proper installation and precautions to be taken, such as filters placed upstream of the meters.

The error curve of a rotary displacement meter is determined by the losses between the impellers as well as between the impellers and the housing.

During the measurement of the gas volume, the gas runs through the meter permanently. This results in pulsation, which through inappropriate pipeline arrangement, can lead to peaks and cause significant measurement errors. In order to avoid the influence of the error curve due to pulsations during the calibration, silencers are often used as well as appropriate piping upstream of the meter.

Turbine meters

Turbine meters consist of a sealed housing, an extrusion section, the turbine wheel and a gearwheel system, which leads the meter’s index.

During gas flow, the turbine wheel rotates. The turbine meter is manufactured in such way that the turbine wheel’s rotation frequency is proportionate to the flow velocity. In this way, the gas volume is calculated based on the rotation of the turbine wheel.

Deviations to the error curve are mainly caused from mechanical losses due to friction at the bearings and the gearwheel system.Turbine meters are usually installed in applications where large gas quantities must be measured, in medium and high pressure stations. These meters feature one or more pulse generators which may provide high or low frequency pulses.

Turbine meters, as well as other flow meters, are sensitive to disturbed upstream flow profile.

Use and Calibration of gas meters

Nowadays, that all the physical quantities’ measurements are part of the production process as well as the billing process, it is very important to be precise and reliable.

Gas meters are no exception. During their usage period, gas meters must be in good operational condition and must have a valid calibration certificate. Proper operational condition depends on the correct use and maintenance. Nevertheless, in order for a gas meter calibration to be valid, the following criteria must be fulfilled:

• The calibration conditions must be equivalent to the gas meter’s installation conditions.

It is recommended that the gas meters should be calibrated under conditions as close as possible to the ones during the meter’s actual operation. Special attention must be given to the calibration medium (natural gas or atmospheric air), the pressure, the temperature and the upstream and downstream piping.

If possible, the required adjustments must be performed in order to have a predictable and reliable operation of the gas meter.

All gas meters require periodic calibration even though some commercial literature claims the opposite. Non-regularly calibrated gas meters, may produce significant errors.

• Calibration must be performed by accredited and traceable laboratories.

When the laboratory performing the calibration, is accredited according to ISO/IEC 17025, the following parameters are guaranteed:

• The reliability of the measurement results and the calibration certificates.
• The appropriate environmental conditions and laboratory facilities, during the calibration.
• The accuracy of the reference equipment used for the calibration.
• The validity of the calibration procedures and the measurement uncertainties calculation.
• The measurements’ traceability to national standards.
• The laboratory’s participation in Interlaboratory Comparisons.
• The Best Measurement Capabilities, which are the lowest measurement uncertainties that the laboratory can achieve within its scope of accreditation.

• The gas meter’s calibration must be valid during its period of operation.

In order to fulfill this requirement, the following question must be answered:

How often do we calibrate a gas meter?

This is a question that often arises. The answer to this question is not easy. All gas meters must be regularly calibrated, regardless of their type, size, construction or age. But how often?

Time has significant influence on the gas meter’s performance. As time passes, gas meters tend to drift due to gradual and sometimes imperceptible environmental effects. Variations may exist even if there is no obvious damage to their internal parts. This is why recalibration is necessary.

The most common practice is to define a short recalibration period for the new gas meters and the meters used in new applications or environments. Afterwards, as sequential calibrations are performed, the drift between each calibration can be evaluated. Based on these data, the user can decide whether to increase or decrease the recalibration interval.

The calibration laboratory can help its customers determine the appropriate calibration intervals. The gas meter’s calibration history can be studied by the laboratory and an evaluation of the drift can be performed, in order to define the suitable recalibration interval.

Factors affecting the performance of a gas meter

The most important factors that affect the performance of a gas meter are:

• Deposits on internal surfaces. Layers of salt, minerals, oxidation etc. can have measurable affects on a gas meter’s performance, even if it seems to operate correctly. All types of gas meters can be affected, even the ones without moving parts like Coriolis, Vortex, Ultrasonic, etc.
• Chemical attack, influences gas meters, especially those having moving parts. A change to the geometry of a turbine meter for example, will affect its performance.
• Abuse (shock, over speeding etc.) even when there is not any obvious damage, can cause variation of the gas meter’s performance characteristics.
• Aging of gas meters causes changes (sometimes even improvements) on their performance.

• Electrical changes usually occur as a result of aging.
• Mechanical changes to meter performance due to the bearings usually occur soon after the manufacture of the meters or the bearings replacement and continue with a much slower rate during the lifetime of the gas meter.
• Fluid property differences are a very important factor. If a gas meter has been calibrated in one type of fluid and used in another, significant differences may be observed.
• Improper installation is another major factor affecting the differences between the calibration results and the actual operation of the gas meter.
•  External influences affect all types of gas meters. Some meters are affected from vibration, others from temperature or pressure.

The conclusion resulting from the aforementioned is that gas meters, depending on their type and their operation conditions, must be calibrated within appropriate time intervals and from a suitable calibration laboratory, in order to provide reliable results. Don’t forget that gas meters are mainly used for billing purposes.

Written by Sofia

The post Gas Meters – Principles of Operation appeared first on Calibrate.

According to GUM (Guide to the Expression of Uncertainty of Measurement) and VIM (International Vocabulary of Metrology) the Uncertainty of Measurement is a parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand.

Another definition of uncertainty could be: Measurement uncertainty is a range of values, usually centered on the measurement value, which contains the true value with a stated probability.

The term uncertainty is always followed by two more terms:

Confidence Interval: It is the range of values which corresponds with the stated uncertainty. “Confidence interval” is a technical term with a precise statistical meaning. It always applies to the mean of several measurements (or equivalent statistical measure) and never to an individual measurement result.

Confidence level: this is the probability associated with the confidence interval. We would say, “The true value can be expected to lie within +/-X units of the measured value with 95% confidence.” This means that we have evidence which leads us to believe that if the same measurement were to be performed many times, we would find the true value to be in this interval 95% of the time, which is the same as saying the probability of the true value being in the interval is 95%.

The uncertainty of the result of a measurement reflects the lack of exact knowledge of the value of the measurand. The result of a measurement after the correction for recognized systematic effects is still only an estimate of the true value of the measurand because of the uncertainty arising from random effects and from imperfect correction of the result for systematic errors.

In practice, there are many possible sources of uncertainty in a measurement, including:

• incomplete definition of the measurand
• imperfect realization of the definition of the measurand
• nonrepresentative sampling – the sample measured may not represent the defined measurand
• inadequate knowledge of the effects of environmental conditions of the measurement or imperfect measurement of environmental conditions
• personal bias in reading analogue instruments
• finite instrument resolution
• inexact values of measurement standards and reference materials
• inexact values of constants and other parameters obtained from external sources and used in the data-reduction algorithm
• approximations and assumptions incorporated in the measurement method and procedure
• variations in repeated observations of the measurand under apparently identical conditions

The ISO Guide to the Expression of Uncertainty in Measurement redefines uncertainty as the equivalent of a standard deviation and recommends that uncertainties be placed in two categories, “A” and “B”, based on the method by which they are evaluated. Uncertainties evaluated by method “A” are those that are based on a statistical evaluation of a series of measurements. Uncertainties evaluated by method “B” are those that are evaluated by any other method than a statistical evaluation of a series of measurements.

Having defined the term Uncertainty of Measurement, we can see below how we are able to evaluate it.

Suppose that Y is the true value of our measurand. Y, which is also stated as the output quantity, depends upon a number of input quantities Xi (i=1, 2, …, N) according to the following formula:

Y = f (X₁, X₂, …, X_N)

[math]Y=f(X_1, X_2, X_N)[/math]
The model function f represents the procedure of measurement and the method of evaluation. It describes how values of the output quantity Y are obtained from values of the input quantities Xi.

As indicated above, we can never know the true value of the measurand or the true value of the input quantities. We can only have an estimate of them. If we suppose that y is the estimate of the value Y and x_i are the estimates of the values of Xi, then the above formula becomes:

y = f (x₁, x₂, …, x_N)

[math]y=f(x_1, x_2, x_N)[/math]
The standard uncertainty of measurement associated with the output estimate y, is represented as u(y) and it is determined from the input estimates x_i and their associated standard uncertainties u(x_i).

Evaluation of Uncertainty of Measurement of input estimates

The uncertainty of measurement of an input quantity can be classified either as “Type A” or as “Type B”.

Type A uncertainties usually result from several independent observations of one of the input quantities, under the same conditions of measurement.

Suppose that we perform n statistically independent observations of the input quantity Xi and each time the individual observed value is q_j (j=1, 2, …, n). The estimate result would be the arithmetic mean or average [math]\small \bar q[/math]:

[math]\bar q=\frac{1}{n}\sum_{j=1}^n qj[/math]
The uncertainty of measurement associated with the estimate [math]\small \bar q[/math]  can be evaluated as follows:

The experimental variance s²(q) of the values q_j is given by:

[math]s^2(q)=\frac{1}{n-1} \sum_{j=1}^n(qj-\bar q)^2[/math]
The experimental variance of the mean s²([math]\small \bar q[/math]) is:

 [math]s^2(\bar q)=\frac{s^2(q)}{n}[/math]
The standard uncertainty [math]\small u(\bar q)[/math] of the input estimate [math]\bar q[/math] is the experimental standard deviation of the mean which is the positive square root of the experimental variance of the mean:

  [math]u(\bar q)=s(\bar q)[/math]
Type B uncertainties are usually evaluated by using any scientific knowledge we have regarding our input quantities. Such knowledge may come from:

–       previous measurement data

–       experience or knowledge of the behavior and properties of standard instruments

–       manufacturer’s specifications

–       data provided in calibration certificate

–       uncertainties assigned to reference data taken from handbooks

In order to determine the type B uncertainties, we usually use probability distributions. Below are some examples of the most common probability distributions used:

Rectangular distribution: It is used when only the upper and lower limits of the input quantity x_i are known (i.e. manufacturer’s specifications, resolution, a temperature range, etc). The true value may be anywhere inside these limits. If we assume that the upper and lower limit are equal to a then the standard uncertainty would be:

[math]u(x_i)=\frac{a}{\sqrt 3}[/math]
U-shaped distribution: In this case, again the upper and lower limits (a) are known, but it is more likely that the true value lies near the limits. The standard uncertainty is:

[math]u(x_i)=\frac{a}{\sqrt 2}[/math]
Triangular distribution: Unlike the U-shape, in this case (upper limit = lower limit = a) the true value is expected to be near the center. The standard uncertainty in this case is:

[math]u(x_i)=\frac{a}{\sqrt 6}[/math]
Normal distribution: The uncertainty of a calibration certificate is stated as an expanded uncertainty U with a coverage factor k. The standard uncertainty would then be:

[math]u(x_i)=\frac{U}{k}[/math]
Having analyzed the evaluation of the Uncertainty of Measurement of the input quantities, we can now proceed and describe how the Standard Uncertainty of our output estimate is calculated.

The standard uncertainty u(y) of our measurement result (or output estimate) y is given by:

 [math]u(y)=\sqrt{\sum_{i=1}^N ui^2(y)}(1)[/math]
The term u_i(y) is the contribution to the standard uncertainty associated with the output estimate y resulting from the standard uncertainty associated with the input estimate x_i:

[math]u_i(y) = c_iu(x_i)[/math]
Where c_i is the sensitivity coefficient and describes the extent to which the output estimate y is influenced by variations of the input quantity x_i. It can be evaluated from the model function f by using the partial derivative formula:

 [math]c_i=\frac {\partial f}{\partial x_i}[/math]
If we apply the last two formulas to the measurement result standard uncertainty formula (1) we can have the final equation for the calculation of our measurement uncertainty u(y):

[math]u(y)=\sqrt{\sum_{i=1}^N c^2 i-u^2(xi)}[/math]
A simple and common way to implement all the aforementioned (input quantities, standard uncertainties, sensitivity coefficients and uncertainty contributions) and perform our measurement uncertainty analysis is to use a table like the one shown below:

So, we have done all calculation and we estimated the standard uncertainty of our result! Are we done now?

No! We have to calculate the Expanded Uncertainty of Measurement.

The Expanded Uncertainty of Measurement U, is obtained by multiplying our standard uncertainty u(y) by a coverage factor k:

[math]U = k×u(y)[/math]
When a normal (Gaussian) distribution can be assumed for the measurand, the standard coverage factor k=2 is applied. Then the assigned expanded uncertainty corresponds to a coverage probability of approximately 95%.

Now we can state in the calibration certificate the result of our measurement y and the associated expanded uncertainty U:

y ± U

We must add the following explanatory note, in order to give to the customer/user all the necessary information on how to use this uncertainty:

“The reported expanded uncertainty of measurement is stated as the standard uncertainty of measurement multiplied by the coverage factor k=2, which for a normal distribution corresponds to a coverage probability of approximately 95%. The standard uncertainty of measurement has been determined in accordance with…”

(in the end we must state the guide which we used to perform our evaluation, i.e. EA-04/02, or GUM)

And what does our statement mean? It simply means that we estimated our measurement result to have a value y, and we can be 95% certain that the true value Y would be inside the values y-U to y+U.

Written by Sofia

The post Definition and Evaluation of the Uncertainty of Measurement appeared first on Calibrate.

UKAS accreditation services cover the following standards:

–       ISO 17025 (Calibration and Testing Laboratories)

–       ISO 15189 (Medical Laboratories)

–       ISO 15195 (Medical Reference Measurement Laboratories)

–       ISO 17021 (Certification Bodies)

–       EN 45011 (ISO Guide 65) (Certification Bodies)

–       ISO 17024 (Certification Bodies)

–       ISO 14065 (Certification Bodies)

–       EU Council Regulation (EC) 1221/2009 (Certification Bodies)

–       ISO 17020 (Inspection Bodies)

–       ISO 17043 (Proficiency testing schemes)

–       ISO Guide 34 (Reference Material Producers)

–

Since Accreditation is used internationally, most countries have their own accreditation bodies which are similar to UKAS. These accreditation bodies (including UKAS) are internationally recognized through European and world multilateral recognition agreements. In this way products and services tested in the UK can be accepted in Europe and worldwide without needing additional testing.

How is UKAS Capable of Providing Accreditation?

Besides technical expertise which is held within UKAS, there are also a number of Technical Advisory Committees. Their role is to provide advice on technical matters related to UKAS activities. Technical Advisory Committees can be comprised by various bodies and individuals such as professional bodies, accredited organizations, customers and regulatory bodies. A Technical Advisory Committee can be normally composed by invited members who are capable of providing technical advice and they may come from the following fields:

–       UKAS assessors

–       UKAS accredited bodies

–       Customers of UKAS accredited bodies

–       Independent specialists

–       Governmental specialists

–       Universities

The members of these committees can provide advice to UKAS for a number of specific technical matters, including:

–       Review and acceptance of special procedures as a basis for accreditation.

–       Identification of potential assessors

–       Formulation and review of specific technical criteria to facilitate effective and consistent application of UKAS’ activities

–       The rules and conduct of proficiency testing and inter-laboratory comparisons

Having defined UKAS’ role in accreditation the next critical question that comes to mind is:

How Can A Calibration Laboratory Attain UKAS Accreditation According To ISO 17025?

First of all the laboratory must fill out an application form with all necessary information. Most important is to define the scope of the laboratory’s accreditation. The scope must contain the following information:

–       The type of calibration

–       The range of measurements

–       The best measurement capability

–       The authorized signatories

The final accreditation scope of the laboratory will be agreed and finalized after the completion of UKAS assessment.

When UKAS receives the application, a member of UKAS staff is assigned as Assessment Manager for the specific project. This person must normally have knowledge of the field of calibrations concerned. Usually the Assessment Manager acts also as the Lead Assessor and selects the appropriate assessment team, consisting of as many Technical Assessors as necessary to provide technical expertise to assess the laboratory’s competence.

During the application procedure, the laboratory has to send to UKAS its quality manual and any basic information on its activities, procedures, equipment used, etc.

The Lead Assessor reviews the quality manual and all the relevant documentation and if everything is found to be satisfactory, a pre-assessment visit is arranged (in some cases, a pre-assessment visit is not necessary. In these cases an initial assessment visit is scheduled directly).

The pre-assessment visit is usually conducted by the Lead Assessor (and a Technical Assessor if it is necessary) and it is normally completed in one day. During this visit the Assessor discusses his/her first findings about the management system, the quality manual and the operating procedures. During the pre-assessment the proposed scope of accreditation is investigated. A first examination of the laboratory’s facilities and equipment is performed. Finally the Assessor will identify to the laboratory any points that require further attention in order to fulfill the ISO 17025 requirements.

After the successful completion of the pre-assessment visit, the Lead Assessor will report his/her findings to UKAS and the initial assessment will be scheduled as soon as the composition of the assessment team will be determined.

The laboratory will receive a detailed initial assessment visit plan which will mention the sections, activities and locations to be assessed by each assessor. The plan will also specify the calibrations that each assessor must witness during the visit.

After completing all the above, the most critical time has arrived: the time for the initial assessment.

This assessment consists of the following sections:

A) The introductory meeting between the assessment team and the representatives of the laboratory. In this meeting, the members of both sides become acquainted and the Lead Assessor explains briefly what is expected during the assessment. At least the following issues are covered:

–       Explanation of the purpose of the assessment and confirmation that the laboratory’s representatives understand the procedure.

–       The status of the Quality manual

–       Confirmation of the visit plan and the scheduled calibrations for witnessing

–       Confirmation of confidentiality

–       Clarifications (if needed) to the laboratory’s personnel

–       Confirmation of the range of calibrations performed by the laboratory

B) The Assessment. This is the main part of the initial assessment procedure. Each Assessor will focus to their respective area. Usually the Lead Assessor examines the Management System documentation and the Technical Assessor’s review, the technical procedures and their implementation, the ability to perform proper and traceable measurements, the uncertainty budgets, etc. At this stage, the Technical Assessors also witness the scheduled calibrations. A member of the laboratory must accompany each assessor. The assessors try to conduct the assessment in such a way as to be similar to an on-going working day. During the assessment, the assessors record their findings (such as possible nonconformities or opportunities for improvement). The object of the assessment is to observe whether the laboratory’s work meets the requirements of ISO 17025.

C) Private meeting between the Assessors. When the Assessment is completed, the Assessors meet by themselves in order to produce an overall view of the laboratory’s work. The Lead Assessor compiles the Assessment Report which will contain the assessors’ findings, the key areas that need improvement and the recommendations to UKAS. Usually, if there aren’t any major nonconformities, the recommendation would be to proceed with the laboratory’s accreditation as soon as there is a satisfactory clearance of nonconformities.

D) Final Meeting between the Assessors and the representatives of the laboratory. During this meeting the Lead Assessor presents the findings to the laboratory’s management and informs them about the recommendation that will be made to UKAS. The laboratory’s management fill up the Improvement Action Report forms to record the proposed improvement actions, and a date by which these actions will be implemented is mutually agreed (typically, a three month period is accepted).

By completing all the above sections, the initial assessment is over. Now the laboratory must provide to UKAS evidence of the improvement actions. The assessors inspect this evidence and if it is found to be satisfactory, they present it to UKAS. Rarely a follow-up visit might be necessary to assess the improvement actions taken.

Once the assessment team gives a positive reply about the improvement actions to UKAS, accreditation is granted. A letter is then published and sent to the laboratory with the Accreditation Certificate.  The laboratory’s schedule of accreditation is then published on UKAS website, mentioning details about the laboratory’s Calibration activities.

The laboratory can now issue ISO 17025 accredited calibration certificates which can carry the UKAS symbol: the Royal Crown.

Of course, in order to maintain the accreditation the laboratory is subject to periodic surveillance (once per year) and re-assessment (once every four years).

Written by Sofia

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