The Government Laboratory (GL), Hong
Kong Council for Testing and Certification (HKCTC), Open University of Hong
Kong (OUHK) and Standards and Calibration Laboratory of Innovation and
Technology Commission (SCL) jointly organized the Metrology Symposium 2017 on
27 Oct 2017. The theme is “New
Development in Metrology – Redefinition of the SI”. Hong Kong Society for Quality (HKSQ) was one
of supporting organizations. The
background was that the 26th General Conference on Weights and
Measures (CGPM) would review the International System of Unit (SI) of four of
the seven base units namely kilogram, ampere, kelvin and mole by fixing the
values of the Planck constant (h), the elementary charge (e), the Boltzmann
constant (k) and the Avogadro constant (NA). This symposium attempted to unfold the latest
development of this important event to the local testing and certification
industry.
In the beginning, Professor K.C. Ho (Dean, School of Science and Technology, OUHK) gave welcoming address. He said metrology was for benchmarking the different measurements which were so important for living in the world.
Then Mr. Vincent Tang (Assistant Commissioner, ITC) gave opening remarks. He said there would have a big change of metrology world.
Group photo
The first speaker was Dr. Della Sin (Government Chemist, Government Laboratory) and her topic entitled “How Metrology Supports our Daily Life”. In the beginning, Dr. Sin briefed the measurement is everywhere in our life such as catching a train on time, flight safe operation and smartphone using GPS, etc.
Then she said metrology was not a
new things and it could be found in ancient civilizations such as Cubit using
the length of the Pharaoh’s forearm in Egypt for building the Pyramid and
Jia-liang (嘉量) measuring device for several volumetric standard in China. Then she introduced the metrology
infrastructure from Products to SI Unit.
She reviewed the base units which set since 1960 and 7 base units were
metre, kilogram, second, ampere, kelvin, mole and candela.
After that Dr. Sin introduced 9
technical fields in metrology areas and they were “Acoustics, Ultrasound &
Vibration”, “Electricity & Magnetism”, “Length”, “Mass & related
quantities”, “Photometry & Radiometry”, “Time & Frequency”, “Thermometry”,
“Ionizing Radiation” and “Chemistry”.
She also mentioned the legislation in Hong Kong related to metrology
such as Chapter 68 – Weight and Measures Ordinance and Chapter 214 –
Metrication Ordinance. Finally, she
briefed the metrology infrastructure of Hong Kong that HKSAR of China became an
associate of CGPM in April 2000. SCL
became the signatory of CIPM MRA in May 2000 and GL had been included as a
Designated Institute (DI) in the field of metrology in chemistry for Hong Kong,
China under CIPM MRA since May 2005.
Lastly, she mentioned that the SI provided a universal language for
measurement and “The changes to the SI will provide a springboard for future
innovation”.
The second speaker was Ir.
C.M.Tsui (Senior Electronics Engineer, SCL) and his presentation named “Time
frame for revision of the SI”. He
introduced the Metre Convention which was established on 20 May 1875 with 17
founding member states. As of 2016,
there were 58 member states and 41 associate states. In 1960, the General Conference of Weight and
Measures (CGPM) adopted the name International System of Units (SI) for the
system of units of measurement.
Then Ir. Tsui reviewed the seven
SI base units and CGPM resolutions since 2007.
The schedule of CODATA meeting and other important dates of metrology
were also stated.
Dr. Alex Lam (Chemist, Government
Laboratory) was the third speaker and his presentation title was “Redefinition
of the mold”. He said chemist used mole
(mol) to describe the quantity of particles in a substance (amount of
substance). Then he briefed the history
of the mole form atomic theory in 1808 to gas law in 1811 (Avogadro) to the
concept of the “mole” or “gram-molecule” by Wilhelm Ostwald in 1902, etc.
Dr. Lam briefed the current
definition of the mole that 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 follow diagram showed
the relationship of mole in Avogadro constant (NA) and molar mass.
i)
n(X) = N(X) / NA (a
defined number of entities (atoms/molecules))
ii)
n(X) = mass / molar mass (a practical quantity of material)
Redefine the SI aimed to have a
system based on invariant constants of nature to fully meet essential
requirements. Moreover, the prototype kilogram
(last physical artefact standard) drifting was observed. So that redefine the unit (kg) using the
Planck constant (h) (6.62x10-34 m2kgs-1) that
would be determining Avogadro constant or Watt balance experiment. The Avogadro Project started in the 1970s
that involved the determination of the number of 28Si atoms (N) in a
single crystal of Si enriched in 28Si (99.995%). The volume of the unit cell (Vcell)
is determined from the lattice distance (a) of the crystal (d220)
with the aid of an X-ray interferometer.
Therefore, the new definition of the Mole is that defined by taking the
fixed numerical value of the Avogadro constant NA to be 6.02214(X) x
1023 when expressed in the unit mol-1.
Dr. Steven Yang (Electronic
Engineer, SCL) was the fourth speaker and his presentation topic named “Redefinition
of the ampere”. Dr. Yang briefed the existing
SI Definition of ampere in 1948 that constant current which, if maintained in
two straight parallel conductors of infinite length, of negligible circular
cross-section, and placed 1 meter apart in vacuum, would produce between these
conductors a force equal to 2x10-7 newton per meter of length.
Then he introduced the practice
realization of ampere by 2 intrinsic standards and they were Josephson Array
Voltage Standard and Quantum Hall Resistance Standard. (Ohm’s law: A = V/Ω)
After that he mentioned the
re-definition of SI unit of electric current that the ampere (A) is defined by
taking the fixed numerical value of the elementary charge (e) to be 1.602 176
620 8 x10-19 when expressed in the unit C, which is equal to A s,
where the second is defined in terms of △vCs. After employed new definition, the voltage
values would be changed by 1.1x10-7 and resistance values would be
changed by 1.8x10-8.
Tea break
I met Ms. Brenda Lam (SCL) (Old
colleague in EPA Centre, CityU) and took a photo for memory.
I took a photo with Ir. SL Mak (OUHK)
and Ms. Maria Ng (Secretariat, HKCTC) for memory.
The fifth speaker was Ir. Raymond
Leung (Electrical and Mechanical Engineer, SCL) and his topic was “Redefinition
of the kilogram”. He briefed the current
definition of the kilogram that is the unit of mass; it is equal to the mass of
the international prototype of the kilogram (The International Prototype stored
under three bell jars at the BIPM).
Then Ir. Leung stated two methods
of realizing the definition of the kilogram that were Kibble balance and
comparing with a single atom of a specified isotope (e.g. single crystals of
silicon). The principle of the Kibble
balance was introduced that compared an electrical power (voltage x current)
with a mechanical power (force x velocity).
[U · I = mgv] They had two mode to operate that were
weighing mode and moving mode (reduced the magnetic field uncertainty).
Even though a primary kg-to-kg
measurement with a Kibble balance would yield a higher uncertainty ~20ug at the
kg level, it was confident that the value assigned to the Planck constant (h)
would be accurate and a suitable basis of the ongoing definition of the mass
scale. Moreover, the results of past
measurement need not be changed.
The sixth speaker was Ir. Julian
Cheung (Electronics Engineer, SCL) and his presentation title was “Redefinition
of the kelvin”. He introduced the
concept of thermodynamic temperature that an absolute measure of the average
total internal energy of an object or objects.
Then Ir. Cheung briefed the history development of temperature
measurement.
The bridge between total internal
energy and thermodynamic temperature is Boltzmann constant (kB). [E = kBT] Therefore, the relationships
between the SI units were different.
Finally, Ir Cheung briefed
different temperature fixed points of International Temperature Scale (triple
point of different elements such as neon, oxygen, argon, etc.). The kelvin would be defined by assigning an
exact numerical value to Boltzmann’s constant.
He concluded that in the longer term, the new definition would allow the
accuracy of temperature measurements to gradually improve without the
limitations associated with the manufacture and use of triple point of water
cells.
The last speaker was Ir. C.M.Tsui
(Senior Electronics Engineer, SCL) and he presented the topic named “Status of
revision of GUM”. He introduced the
Joint Committee for Guides in Metrology (JCGM) which was responsible for Guide
to the Expression of Uncertainty in Measurement (GUM) and International
Vocabulary of Metrology (VIM).
Then Ir. Tsui said there were two
new documents issued as follows:
-
JCGM 101:2008 Supplement 1: Propagation of distributions using a Monte
Carlo method (GUM-S1)
-
JCGM 102: 2011 Supplement 2: Extension to any number of output
quantities (GUM-S2)
However, it was found that the
GUM was not consistent with GUM-S1 and GUM-S2.
It was because of two main statistical schools of through that GUM
believed Frequentist but the GUM-S1 and GUM-S2 believed Bayesian which considered
the prior knowledge.
If used GUM-S1 & GUM-S2, the
type A uncertainty would be changed and the formula showed in the following
table.
In practical, the lesser the
number of sample, the larger the Type A uncertainty was obtained based on Bayesian
approach in probability density function.
Finally, Ir Tsui introduced the
most updated documents JCGM 108 – Bayesian methods would be developed. However, the existing GUM is continued until
all parties agreed to use the new one. For
the time being, both GUM and S1/S2 could be used.
Q&A session.
Some laboratories concerned if
they needed to calibrate all their primary / secondary standards after the
redefinition of SI units. SCL said they
were still used the existing one and certificates still quoted JCGM
100:2008.
Reference:
The WMD,
the WAD and the WSD Forum 2017 - https://qualityalchemist.blogspot.hk/2017/06/the-wmd-wad-and-wsd-forum-2017.html#more
Seminar
on the WMD, the WAD and the WSD 2016 (flyer) - http://www.itc.gov.hk/en/quality/hkas/images/Photos/World_Accreditation_Day/WAD_2016_flyer.pdf
Seminar
Celebrating the WMD, the WAD and the WSD 2015 - https://qualityalchemist.blogspot.hk/2015/05/seminar-celebrating-wmd-wad-and-wsd-2015.html
Seminar
Celebrating the WMD, the WAD and the WSD 2014 - http://qualityalchemist.blogspot.hk/2014/05/seminar-celebrating-wmd-wad-and-wsd-2014.html
World
Metrology Day 2013 Seminar - http://qualityalchemist.blogspot.hk/2013/05/world-metrology-day-2013-seminar.html
World
Metrology Day 2012 Seminar - http://qualityalchemist.blogspot.hk/2012/05/world-metrology-day-2012-seminar.html
World
Accreditation Day 2012 Seminar - http://qualityalchemist.blogspot.hk/2012/06/world-accreditation-day-2012-seminar.html
OUHK – http://www.ouhk.edu.hk/
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