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.

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

The post International System of Units and Common Metrology Terms appeared first on Calibrate.

• 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.

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.

But how do we know we can trust this measurement? Outside factors such as drift, knocks, bumps and even dirt can affect the overall accuracy of a piece of test equipment, and that’s exactly why we define a measurement standard to make sure that the tester you are using is able to generate results within a specific standard.

This basically means there are two parts to every measurement – the actual measurement taken, and the standard that this measurement is matched up against.  Without this standard, the measurement taken has no point of reference, and the chance of the measurement being completely incorrect increases dramatically.

The device you’re using to make measurements basically relies on a calculation that is integrated into its internal components – by using this calculation the device generates a result, and without the standard, there can be no calculation and no results.

Standards and Calibration

Whenever a piece of test equipment is constructed by a manufacturer, it will be calibrated to the standard (using the appropriate calculation) that the manufacturer has set for the product.

This means that each new product should, as standard, generate as accurate a result as possible. There are instances where this isn’t the case and the initial calibration of a fault inside the tester might render the measurement taken a bit dodgy, but these are generally few and far between.

This delicate balance is maintained by the state of the components inside the device and how well they are functioning in relation to the calculation. When the tester is used, the overall efficiency of a tester is then compromised somewhat, and the overall measurements taken by the device can be affected by outside parameters.

This effect can be negligible, but it can also be completely catastrophic to the readings taken by your device. In the case of something like a piece of electrical test equipment, generating incorrect readings as a result of the degradation compromises the safety of whatever you’re testing and the people who might be working around it or directly with it.

This is why a regular period of calibration, where your equipment is matched up to an existing standard and adjusted accordingly if necessary, is required.  The calibration process basically refers to a manufacturer’s standard, and a piece of equipment many times more accurate than the product you’re testing with is used to perform the calibration check and adjusts are made to the tester as necessary to ensure that the meter is generating measurements that are as accurate as possible.

It is highly recommended that the period between calibrations is around a year, although more specialist, high accuracy testers may require more regular calibration.

Written by Sofia

The post Measurement Standards: Why Are They Important? appeared first on Calibrate.

This is why Metrology and Calibration are needed. But what do these terms mean?

Metrology is 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. We could distinguish three major categories of Metrology:

–       Scientific Metrology deals with the organization and development of measurement standards and with their maintenance (highest level).

–       Industrial Metrology ensures the adequate functioning of measurement instruments used in industry, either in production or in testing processes.

–       Legal Metrology has to do with measurement involving economic transactions, safety and health.

Calibration is usually defined as a set of operations, performed in accordance with a definite, 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, errors in the instrument tested.

Having defined the term of calibration, the question that arises is “Can anyone perform calibrations?” The answer is no. Calibrations must be done by dedicated laboratories which have the appropriate equipment, trained personnel, documented procedures and can monitor and control the environmental conditions (temperature, humidity, vibration etc.) that affect the measurements. These laboratories can be separated into the following categories depending on their level and function:

–       Primary Laboratories: These are the highest level calibration laboratories which deal with research and development of measurement methods and procedures. They also maintain and calibrate the primary and secondary standards.

–       Secondary Laboratories: They are involved with secondary and working standards’ calibration. These laboratories can also perform lower accuracy calibrations where specialized techniques are required.

–       Research Laboratories: They mainly support research activities. Depending on the situations some Research Laboratories may have the most accurate standards since they might be involved with more abstract projects.

–       Calibration Laboratories: These laboratories are the most common ones. They are handling large volume calibrations and they use measurement standards which are calibrated by secondary level laboratories. Calibration Laboratories are focused on serving the customer the most quickly the possible, without affecting quality.

–       Mobile Laboratories: These are actually Calibration Laboratories “on wheels”. Sometimes there are instruments that cannot be sent to a laboratory. In these cases the easiest way to serve the customer is to send the Calibration Laboratory on site.

So why should we calibrate instruments? Is it necessary? Let’s see some fields and industries where calibration is very important:

–       Pharmaceutical: each medicine must contain a specific quantity and quality of certain substance.

–       Automotive: The dimensions of each vehicle must be specifically defined and produced. Each component of the vehicle (motor, breaks, etc.) must operate under strict specifications.

–       Army: There are special Army Standards defining specific tests and calibrations.

–       Economical Transactions: How do we know that our gas meter measures correctly the consumed gas? How do we know that the10 gallonsof oil, provided by the gas station, are truly10 gallons?

All the above are just some of applications where Metrology and Calibration are necessary or even mandatory. Health, Safety, Industry, Military applications, Billing – all these activities rely and depend on accurate measurements.

Written by Sofia

The post Metrology and Calibration – What Are They? appeared first on Calibrate.

Technicians performing a calibration service typically have received training, usually in an apprentice status, in highly specialist subjects such as electronic repair. A calibration technician does not need a university degree as employers and manufacturers will provide on-the-job training that can lead to both an ONC (Ordinary National Certificate) and HNC (Higher National Certificate) in instrumentation and engineering. The technician will require an aptitude in solving technical problems and in thinking logically and a serious attention to detail and accuracy. But with the increasing sophistication of instrumentation, some specialist calibration technicians will need university degrees in subjects such as mechanical, electrical or systems engineering.

The technician has to be able to solve a problem with an instrument by starting with an initial diagnosis of a problem and follow this through with corrective action.

In addition to understanding the mechanics of any instruments such as a pressure gauge, flow meter, gas detector or temperature sensor, the calibration technician should understand the process that any particular system is measuring. It is one matter to calibrate and adjust an instrument in accordance with UKAS standards, but quite another to understand how it may or may not perform within a larger industrial process loop. The technician must also understand the consequences of incorrect calibration of a gauge or instrument.

Written by Barry Atkins at www.calibrate.co.uk

The post The Technician’s Role in Calibration appeared first on Calibrate.

This is why a responsible person in charge of these projects and companies must ensure that all the instruments they use in their new enterprise are well calibrated.  With calibrated instruments you can rely on your measurement results and negate the risk of inaccurate measurements being taken during the production process.

To make sure your equipment is in in good working order you should always keep two things in mind; supplier and calibration.  You should buy equipment from a well-respected name whenever you can, as more times than not cheap products on sale means they are cheap for a reason.

Not only is it vital to find reliable suppliers of measuring devices, but you also need an independent company that will provide accurate calibration services. It is crucial to have a good relationship with the calibration service supplier in any innovative experiment. You never know when something may go wrong and over-reach the capacity of existing or planned measuring instruments. Additional advice on the use and capacity of new instrumentation may provide the drive to continue with an innovative experiment that could lead to an important new discovery.

The calibration service will ensure the reliability of your instrumentation and measurements, allowing you to remain confident that you can adhere to both national and international standards of measurement during the entirety of the project.

If you’re running a constant project over an extended period of time it is also extremely important that you understand the concept of ‘drift’ and how this can affect equipment. Although generally products shipped from manufacturers will be as accurate as possible when they first arrive, the effect of time and other unexpected influences can cause the overall accuracy of the equipment to ‘drift’, thus affecting the overall results generated by the device.

Similarly, using older equipment that might not have been calibrated for a long time could also be similar and produce incorrect results.

As we previously mentioned, the slightest difference in measurement could have a disastrous effect on the overall result of your project.  If the vital equipment you’re using in your project has begun to drift and is producing wrong results, chances are the overall final product your project produces will be affected by the change.

This is why calibration regularly is extremely important as it negates the effect of drift on your equipment and makes sure you are always working with optimum efficiency.

Written by Barry Atkins at www.calibrate.co.uk

The post Experimental Projects and Calibration: Why Is It Important? appeared first on Calibrate.

Remove Your Tester’s Batteries!

One of the most simple things that can cause damage to a tester is often neglected – removing batteries when storing it. Although we do generally tend to trust batteries placed inside things if they are left for long period of time, there is a significant change that that the battery could be damaged, which ultimately leads to some pretty nasty times for your tester.

Battery acid is an extremely corrosive material, and when it leaks, the acid can come directly into contact with plastics and internal components inside the tester.   Battery degradation is much more likely to happen if a tester is left idle for a long period of time, and if the acid is leaking out of the batteries, there’s a chance that the battery cradle can be warped leaving it useless or – in the worse case scenario – the acid can leak onto the PCB or components and render them completely unusable.

This would then result in costly repairs having to be made to your device, so make sure you don’t get caught out and remove your tester’s batteries when it isn’t being used! It may seem like a hassle but we really can’t stress this enough – we’ve seen countless instances where not removing the batteries in a tester has caused massive damage.

Perform Regular Battery Checks

This again applies if your tester will be left for a long period of time, sitting idle and not doing a whole lot.  If your tester is going to be left for a long time, there’s a chance that the battery could degrade, ultimately leading to it not being able to generate a charge powerful enough to power a tester.

An example of this is found in the BATTPAT PAT tester – if these devices are left unused for a lengthy period of time the internal battery drops so low that it no longer has the charge to activate the charging circuit when a charger is plugged in. As a result, the meter won’t charge at all, rendering it entirely useless until a repair is made.

Lead Checks and Safety

Test device care is extremely important, but this care also extends to the various types of lead you might use with your equipment as well.

There are so many factors that might cause damage to your leads. If you don’t like regularly replacing your leads with expensive lead sets, we recommend you avoid them like they’ve got the plague.

When storing your leads, they should be loosely coiled, wrapped or folded to avoid kinks and knots becoming present in the leads. When testing it is essential you remove these knots and kinks as they can ultimately affect the readings that the tester generates. Excessive stretching or incorrect storage of cables can also affect the internal wiring inside, and when wiring is damaged it becomes both likely to not generate correct readings and also becomes a danger.

It may seem obvious as well but you really should take care not to stand on leads. It happens a lot more commonly than you might think and excessive weight on leads can also result in them becoming damaged.

Also make sure you don’t just tape up leads – this is extremely dangerous and you should always put health and safety first and purchase a suitable replacement.

General Use

This is perhaps the most obvious section in this guide, but you’d be amazed how often we see testers subjected to negligence that ends up with them being damaged.

First of all you should never, ever, lift your tester up by the mains lead, particularly if the lead is hard-wired into your tester. Leads that aren’t hardwired in could come loose and your tester could fall onto hard ground, while holding a hard-wired cable to lift up your potentially heavy testing device could ultimately result in the cable coming loose, rendering it ineffective.

Here’s another obvious one – try to avoid knocking or dropping your tester.  Some testers do have extra protection to stop them from being damaged when dropped from a height, but a lot of them also don’t! So make sure you aren’t careless with your tester; just remember how much you probably paid for it.

Finally, if a fuse is blown inside your tester’s plug, make sure you replace it with the right one. You should be able to easily determine the correct fuse rating and you should always remember that not replacing it with the correct one can be extremely dangerous.

Cleanliness

We hate to sound like your mother nagging you to tidy stuff, but the build up of dirt, general wear and tear and other forms of nasty grime in your tester can cause incorrect readings, affect internal components and can even cause internal shorts.

We recommend first of all that you regularly check that there are no loose bits of wire lying around – if so, they can work your way into your tester and short across multiple components, which can end up in a costly repair.

The build of general grime in your tester can also cause it to start generating incorrect readings, so make sure you take steps regularly to give your tester a thorough clean.  We actually do this as part of our calibration and repair service, as we know exactly how damaging it can be.

Tester Storage

When you’re storing your meter we recommend its kept somewhere safe and dry where it will not be exposed to severe or prolonged temperature changes.

For example, leaving your tester in the back of your van overnight in the winter might have an affect on the internal components, as as the temperature drops outside and warms towards the morning, the internal components of your testing device can be affected by the sudden changes in temperature.

Of course leaving your tester in a van isn’t a good idea either as it could be stolen, so just to be safe we recommend you keep it in a carry case inside your house when it isn’t in use!

The post Tester Maintenance Guide: Don’t Neglect These Simple Tips! appeared first on Calibrate.