Dear friends
The definitions of SI units has been changed by BIPM: International Bureau of Weights and Measures.
Some of the highlights of the changes are presented here from BIPM's Press KIT. For detail study of the changes syou can go to the original site. The link of original site is on your left hand side on this blog in link list.
The definitions of SI units has been changed by BIPM: International Bureau of Weights and Measures.
Some of the highlights of the changes are presented here from BIPM's Press KIT. For detail study of the changes syou can go to the original site. The link of original site is on your left hand side on this blog in link list.
About the BIPM
The signing of the Metre Convention in 1875 created the BIPM and for the first time formalised international cooperation in metrology. In this way the foundations were laid for the worldwide uniformity of measurement in all aspects of our endeavours. Historically this focused on assisting industry and trade, but today it is just as vital as we tackle the grand challenges of the 21st century such as climate change, health and energy. The BIPM undertakes scientific work at the highest level on a selected set of physical and chemical quantities. The BIPM is the hub of a worldwide network of national metrology institutes (NMIs) which continue to realise and disseminate the chain of traceability to the SI into national accredited laboratories and industry.
The Measurement Community
The measurement community is made up of 250 institutes from 100 countries.
The International Committee for Weights and Measures (CIPM) Mutual Recognition Agreement (MRA) has been signed
by the representatives of 102 institutes – from 58 Member States, 42 Associates of the General Conference on Weights
and Measures (CGPM), and 4 international organizations – and covers a further 157 institutes.
The General Conference on Weights and Measures, established in 1875 under the terms of the Metre Convention, to
represent the interests of member states.
The Redefinition
The International System of Units (SI) is structured around seven base units, with at least another 22 (such as volume)
derived from these.
It is expected that four of the seven base units (the kilogram, ampere, kelvin and the mole) will be redefined according to
fixed values of natural constants. The new definitions will be based on fixed numerical values of the Planck constant (h),
the elementary charge (e), the Boltzmann constant (k) and the Avogadro constant (NA), as well as on other three
physical constants whose numerical values are already fixed in the present SI” or a similar formulation.
Since the second, metre, and candela are already defined by physical constants such as the speed of light, this revision
will make the definitions of all seven base units consistent. They will all be defined in terms of universal physical
constants.
Once the revision is implemented, the quantities for the physical constants will be fixed. The numerical values will be fixed
by definition. For example, the value for the speed of light will remain the same, therefore the units defined by the speed
of light – and other constants – will remain unchanged.
Practically, little will change; water will still freeze at 0.00 °C, your smartphone will function as normal. But this redefinition
of the SI lays strong foundations for the building blocks of future measurement and scientific research.
The SI units form a foundation for measurement across the world that ensures consistency and reliability. Just like
foundations are needed to keep a house from falling down, universal concepts of measurement are needed to support
innovation now and into the future.
Mass
Mass is measured in kilograms (kg).
At the International Bureau of Weights and Measures in France, there is a platinum-iridium alloy cylinder called the
International Prototype of the Kilogram (IPK). The mass of this artefact is what currently defines what we call a kilogram.
The mass of this artefact may have changed since it was produced in 1884, but we have no way of knowing.
Under the new definition, a kilogram will be defined instead by using the Planck and other constants. Using fixed, agreed
upon values, these constants, as invariable features of nature, will not change; an improvement upon the IPK, an artefact,
which could have changed in mass, been contaminated, or even lost.
Another key benefit of the redefined kilogram will be improved scalability for measurements. When you use physical
objects to measure things, accuracy decreases at sizes much smaller or larger than your standard.
A pharmaceutical company, for example, may need to measure the mass of chemicals for research on new drugs in
quantities that are a million times smaller than a standard kilogram. The mass of a litre bottle of soda, in contrast, is about
a kilogram. So, if you used equally sophisticated equipment today, the soda could be measured more accurately than the
tiny quantities in drug discovery. The new definition of the kilogram will allow much better measurements of milligram and
microgram masses.
The SI unit, mass, will be defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10– 34
when expressed in the unit joules per second, which is equal to kg m2 s–1, where the metre (m) and the second (s) are
defined in terms of the speed of light (c) and the hyperfine transition frequency of caesium (ΔνCs).
Distance
Distance is measured in metres (m)
In 1791, a metre was defined as one ten-millionth of the distance from the equator to the North Pole. You can probably
guess how difficult this would have been to check things against.
In 1875, a bar made of a platinum/iridium alloy was designated as the international prototype of the metre. While more
practical, this was still not as accurate as we can now achieve. One of the issues with a prototype like this is that, over
time, the length can potentially change.
Since 1983, we have precisely defined a metre as the length of the path travelled by light in a vacuum in 1/299792458 of
a second. This accurate definition of the metre helps us measure the size and shape of things that are large, like bridges
and buildings, as well as tiny things, like proteins and viruses.
The SI unit, metre, will be defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458
when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency ΔνCs.
Time
The unit for measuring time is seconds (s)
Since 1967, the second has been defined by a number of cycles of the radiation from a particular caesium-133 transition –
9 192 631 770 to be precise. Before this, it was defined by the movement of the earth around the sun, which was
recognised as not predictable enough for highly accurate timekeeping.
Accurate timekeeping underpins the functioning of important technologies, like GPS or the internet, as well as having an
important role timestamping trades in the financial sector.
Ancient Egyptians used sundials and the Chinese used water clocks to tell the time, which was not very precise. Now, a
caesium-beam atomic clock is accurate to within 30 billionths of a second per year (and it works in all weather).
The SI unit, second, is defined by taking the fixed numerical value of the caesium frequency ΔνCs, the unperturbed
ground-state hyperfine transition frequency of the caesium-133 atom, to be 9 192 631 770 when expressed in the unit Hz,
which is equal to s–1.
Electric current
Electric current is measured in amperes (A)
Amperes, or ‘amps’ for short, measure electric current, which is a flow of electric charge. Charge is commonly carried by
electrons moving inside a wire, or, as in batteries, by ions in an electrolyte.
Accurate measurement of current is important for electrical devices like smartphones and laptops, as well as for
developing sustainable batteries with longer lives.
Today’s SI definition of the ampere is impractical and is rarely implemented because there are alternative approaches for
measuring electrical currents that are easier and more accurate. The current SI definition of the ampere describes the
amount of current within two infinitely parallel wires that would produce a specific force between the wires.
For over 30 years, this has meant that the best electrical measurements have been tied to quantum-based voltage
standards and quantum resistance standards instead of to the official SI approach. Nature-based approaches to precision
electrical measurements have been common practice since the latter part of the 20th century, and the redefinition of the SI
ampere in terms of natural constants is effectively catching the definitions up to reality in the lab.
The new definition of the ampere is expected to be more intuitive and easier to realise. It is tied to natural quantum
phenomena that has been used to create highly accurate voltage and resistance standards since the latter part of the 20th
century.
The SI unit, ampere, will be defined by taking the fixed numerical value of the elementary charge e to be
1.602 176 634 × 10–19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ΔνCs.
Temperature
Temperature is measured in kelvin (K)
It is possible for water to exist as a solid (ice), liquid and as a gas (water vapour). These three states coexist in stable
equilibrium at a certain temperature, +0.01 °C, or 273.16 K. One kelvin is currently defined as 1/273.16 of this
temperature.
However, this definition is unsuitable for scaling measurements and accurately measuring very cold temperatures (below
20 K) and extremely hot ones (over 1300 K). When you use physical objects to measure things, accuracy decreases at
sizes much smaller or larger than your standard.
It is expected that the kelvin will soon be defined using the Boltzmann constant, a constant of nature. This will link the
kelvin to the other SI units in a stable manner, independent of any particular substance.
Accurate temperature measurements are important in a wide range of areas, including manufacturing materials with
specific desired qualities.
The Si unit, kelvin, will be defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10–23
when expressed in the unit J K–1, which is equal to kg m2 s–2 K–1, where the kilogram, metre and second are defined in
terms of h, c and ΔνCs.
Amount of a substance
The amount of a substance is measured in moles (mol)
Currently, the definition of the mole is the amount of substance of a system which contains as many elementary entities
as there are atoms in 0.012 kilogram of carbon 12.
The new definition will remove the present link between the mole and the kilogram.
The word ‘mole’ comes from the word ‘molecule’.
Accurate measurements of moles are used to calculate how much of a drug goes into your body, and for producing
solutions of a precise concentration.
The SI unit, mole, will be defined as exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical
value of the Avogadro constant, NA, when expressed in the unit mol-1 and is called the Avogadro number. The amount of
substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may
be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
Luminous intensity
Luminous intensity is measured in candelas (cd)
A candela was once based on the brightness of a ‘standard’ candle. However, this ‘standard’ candle varied from country
to country, and from candle to candle, so this definition was not sufficiently accurate
In 1948 the 'new candle' became the candela and was adopted worldwide. It was defined as: “The magnitude of the
candela is such that the luminance of a full radiator at the temperature of solidification of platinum is 60 candelas per
square centimetre”. In 1979 the candela was redefined in terms of the watt at only one wavelength of light.
It measures luminous intensity, and can be used to calculate the brightness of lightbulbs or lasers.
The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of
frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W–1, which is equal to cd sr W–1, or
cd sr kg–1 m–2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.
For technical or scientific press you can include the Information for users about the proposed revision of the SI from the
Brand Book V2.
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• Since 1884, the kilogram has been defined by an artefact – from 2019, it will be defined by fundamental constants
of nature: URL #SIredefinition
• Constants like the speed of light are not going to change, but a physical artefact might. We’re redefining the
definition of SI units to enable future #measurement research: URL #SIredefinition
• Water will still freeze below 273.2 K, but the way we define the kelvin is changing – learn how: URL #SIredefinition
• #ThrowbackThursday - In 1324, an inch was defined as the length of three barleycorns. In 1983, the metre was
redefined by the distance light travels in vacuum in a tiny fraction of a second. : URL #SIredefinition
• The definition of the ampere, the unit of electric current, is becoming more intuitive and easier to realise: URL
#SIredefinition
• Under the new SI definition, if you have 6.022 140 76 ×1023 entities of a substance, you have one mole of it. Learn
more: URL #SIredefinition
• As a National Measurement Institute, we are proud to be part of the redefinition of SI units. Read more about the
future of #metrology: URL #SIredefinition
• Today is #WorldMetrologyDay – you can read more about the constant evolution of measurement: URL
#SIredefinition
• Being a fraction of a second off doesn’t make much difference when you’re boiling pasta, but it would to someone
like @UsainBolt. Learn how scientists are enhancing measurement: URL #SIredefinition
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Frequently asked questions
* What are measurements for?
Measurements are the quantitative way to compare one thing with another. The International System of Units, also known
as the SI, is the most widely used system of measurement in the world. It is a coherent system of seven base units
(kilogram, metre, second, ampere, kelvin, mole, candela) which allows science, industry and trade to measure all physical
objects and phenomena in the same way, using the same units and get the same number.
*How are the units of measurement defined?
Originally measurement units were defined by physical objects or properties of materials. For example, the metre was
originally defined by a metal bar exactly one metre in length and subsequently in terms of the wavelength of light.
However, these physical representations can change over time or in different environments, and are no longer accurate
enough for today’s research and technological applications. Over the last century scientists measured natural constants of
nature, such as the speed of light and the Planck constant, with increasing accuracy. They discovered that these were
more stable than physical objects, and fixed numerical values to these constants. Therefore, the SI base measurement
units are being redefined so that they are rely on these natural constants, the most stable things known in the physical
world. These natural constants do not vary, so are at least one million more times more stable.
*Why do we need more accurate definitions?
As science advances, ever more accurate measurements are both required and achievable. The standard and definition
must reflect this increasing accuracy. The kilogram has been based on a physical object certified in 1889, consisting of a
cylinder of platinum-iridium and it is the last unit to be based on an actual object. Its stability has been a matter of
significant concern, resulting in recent proposals to change the definition to one derived from constants of nature.
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By defining measurement units in terms of constants means that the definitions of the units do not need to be changed
over time. If we develop more accurate ways of measuring these constants, the definitions still apply, they can just be
realised with greater accuracy.
What is the SI?
The International System of Units (SI) is a globally agreed system of measurements. The SI has seven base units and a
number of derived units defined in terms of the base units. The SI units express measurements of any quantity like
physical size, temperature or time. This International System of Units is necessary to ensure that our everyday units of
measurement, whether of a metre, or a second, remain comparable and consistent worldwide. Being inaccurate by a
fraction of a second might not matter for cooking pasta, but it becomes very important for determining who won the 100
metres at the Olympics or in high frequency stock market trading.
Standardising such measurements, not only helps to keep them consistent and accurate, but also helps society to build
confidence. For instance, the kilogram is used every day, and defining this unit helps to outline how much food a shop is
selling, and means that consumers can trust that the shop is really providing the amount they say they are. This
consistency is also relied on to ensure the correct dosage of medicine is taken even when measurements are very small.
In 1889 the international prototypes for the metre and the kilogram, together with the astronomical second as the unit of
time, were units constituted as the base units metre, kilogram, and second, the original MKS system. In 1946 the scope of
this was extended to adopt the ampere giving the four-dimensional system based on the metre, kilogram, second, and
ampere, the MKSA system.
Following an international inquiry by the BIPM, in 1954 the introduction of the ampere, the kelvin and the candela as base
units was approved joining the original MKSA system. The name International System of Units, with the abbreviation SI,
was given to the system in 1960.
The creation of the decimal metric system, the ancestor to the SI, was considered to be on 22 June 1799, when two
platinum standards representing the metre and the kilogram were deposited in the Archives de la République in Paris.
What are the seven base units?
• The kilogram (kg) – the SI base unit of mass
• The metre (m) – the SI base unit of length
• The second (s) – the SI base unit of time
• The ampere (A) – the SI base unit of electrical current
• The kelvin (K) – the SI base unit of thermodynamic temperature
• The mole (mol) – the SI base unit of an amount of substance
• The candela (cd) – the SI base unit of luminous intensity
For further information on how to use SI units, please visit: www.bipm.org/en/measurement-units/base-units.html
Why is the SI important?
The SI units form a foundation for measurement across the world to ensure consistency and reliability. Just like
foundations are needed to keep a house from falling down, universal concepts of measurement are needed to support
innovation now and in the future.
SI units can provide new opportunities for innovation. Some examples where greater accuracy is supporting better
methods and understanding with a positive impact on society include:
• The accurate measurement of temperature. This will support the ability to identify and measure reliably very
small changes across large time periods with greater accuracy. Therefore, it will allow for more precise monitoring
and better predictions for climate change.
• The more accurate administration of drugs. The pharmaceutical industry needs to use a standard for very small
amounts of mass in order to make dosages of medication even more appropriate for patients.
SI units can help us support innovation into the future. As our ability to measure properties improves, the standards we
have for measurement will need to keep up. SI units based on physical constants are important for developments to
come:
• The accuracy of services like Global Positioning System (GPS) are limited by our ability to use standard units, in
this case, the second to measure time We can track our locations effectively because we can establish time using
the SI definition of a second, which can be realised by an atomic clock. This advancement was made possible
because society had defined the second more accurately well before we had even discovered what it could be
used for. The atomic clock was made before computing really took off. Now accurate timing is a fundamental part
of the industry; without it the internet, mobile phones and other technologies could not work reliably.
What is the SI redefinition?
The global metrology community anticipates that a revision to the SI units will be agreed in 2018, when the General
Conference on Weights and Measures (CGPM) meets from the 13-16 November.
This decision is expected to mean a more practical definition of the SI. All of the units would be expressed in terms of
constants that can be observed in the natural world (for example, the speed of light, the Planck constant and the
Avogadro constant). Using these unchanging standards as the basis for measurement will mean that the definitions of the
units will remain reliable and unchanging into the future.
The new definitions will be based on fixed numerical values of the Planck constant (h), the elementary charge (e), the
Boltzmann constant (k) and the Avogadro constant (NA), as well as on other three physical constants whose numerical
values are already fixed in the present SI” or a similar formulation..
You can find more information on the constants the SI units will be based on here:
www.bipm.org/en/measurement-units/rev-si/
Why do we need this change?
For the first time we will see all base units in the SI dictated by constants of physical science we observe in nature. Using
the constants we have found in nature as our universal basis for measurement allows not only scientists, but also industry
and society, to have a measurement system that is more reliable, consistent, and scalable across quantities, from very
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large to very small. This change is needed to ensure that the definitions will not need to be modified to accommodate
innovation and even more accurate measurement techniques in future.
There are two key ways the SI will change to create a more stable and future-proof basis for measurement:
It will take physical artefacts out of the equation:
• Many of the SI units began life based on a physical artefact. Previously, in order to ensure that quantities like the
metre remained consistent, there was a physical representation of the measure (such as a metre stick) that was
used as a basis for all of the other metres globally. The kilogram is still defined by a physical object– equal to the
mass of the International Prototype of the Kilogram (IPK), a physical artefact stored at the International Bureau of
Weights and Measures in France.
• This technique has the limitation that the properties of the artefact may change over time and it could be damaged.
There can also be changes to size due to the process of translating measurements from one artefact to another. As
a replica is created from the original kilogram, small differences could occur that may not be noticed immediately,
but in time, and with improvements in technology and science, may become significant.
• It can also be extremely difficult to check measurements this way. Not only because the physical artefact will need
to undergo a process of cleaning to ensure nothing is affecting the measurement (apparently chamois leather will
do the trick for the IPK) but if we want a universal standard for measurements we need a definition that can be
realised anywhere. As such, a change is necessary to democratise this process, so that definitions can be checked
anywhere in the world.
Separating the realisation of the unit from its definition for the first time ever:
• For the first time all the definitions will be separate from their realisations. Instead of definitions becoming outdated
as we find better ways to realise units, definitions will remain constant and future proof.
• For example, the ampere is currently defined as the magnetic force between two wires at a certain distance apart,
which means that it uses the realisation of measurement to define it. However, advancements like the development
of digital voltmeters and the advent of the Josephson Effect, have revealed better ways of realising the ampere,
making the original approach obsolete.
What does this mean in practice?
On the surface, it will appear not much has changed. In the same way that if you replaced the decaying foundations of a
house with new shiny ones, it may not be possible to identify the difference from the surface, but some substantial
changes have taken place to ensure the longevity of the property.
These changes will ensure that the SI definitions will never become outdated by advancements in technology, but will
continue to remain robust.
When will the proposed change come into effect?
Redefinition, if agreed, will come into practice on World Metrology Day, 20 May 2019.
What is World Metrology Day?
World Metrology Day is an annual event on 20 May during which more than 80 countries celebrate the impact
measurement has on our daily lives.
On this day, the international metrology community, which works to ensure that accurate measurements can be made
across the world, raises awareness of the impact and importance of having reliable measurements. The theme for World
Metrology Day 2018 is Constant Evolution of the International System of Units (SI).
This year’s theme was chosen due to the proposed SI redefinition, expected to be agreed in November 2018, when the
General Conference on Weights and Measures meets. This will be one of the largest changes to the International System
of Units (the SI) since its inception.
The date marks the beginning of a formal international collaboration in metrology in 1875, when the first international
measurement treaty, the Metre Convention, was signed by representatives from 17 nations to agree on the coordination
of measurement.
This treaty saw the creation of organisations to oversee the running of the institute, including the General Conference on
Weights and Measures, CGPM.
In 1921, at the 6th meeting of the CGPM, the mandate of the body was extended from mass and length to all physical
measurements. In 1960, at the 11th meeting, the ‘International System of Units’ was established.
Who agrees to the SI?
The signing of the Metre Convention in 1875 saw the creation of the BIPM. It operates under the supervision of the CIPM
which is itself set under the authority of the CGPM.
The CGPM is the General Conference on Weights and Measures that meets every four to six years. With delegates from
all of the 59 member states, this body discusses and chooses to endorse changes to the SI, after taking on board advice
from the CIPM.
The CIPM is the International Committee for Weights and Measures. It is a committee made up of eighteen individuals,
each of a different nationality, nominated by the CGPM for their high level of understanding in the field. The CIPM is still to
this day responsible for decisions about the SI with the goal of creating a reliable basis for measurements that can be
used now and into the future. The international community now includes 59 Member States, and 42 Associate States and
Economies.
The BIPM is the International Bureau of Weights and Measures based in Sèvres where it has laboratories that provide
metrology services for the member states. It also carries out coordination and liaison activities and houses the secretariat
for the CIPM and its consultative committees. The BIPM’s original purpose was to house the international prototypes
defining units of the SI, as such it is where the international prototype of the kilogram resides.
