Metrology Symposium 2017 (Redefinition of the SI Unit)

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.  




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