The SI in 2019:
Traceability Questions for SIM
A Presentation to Encourage Discussion and Understanding
Alan Steele
SIM General Assembly – Montevideo, Uruguay – November 2016
Defining the SI Via Fundamental Constants
• The proposed structure of the International System of Units is quite
different from the one we have now
• We will move away from defined Base Units (s, m, kg, A, K, mol, cd)
• We will shift to defined constants (∆νCs, c, h, e, kB, NA, Kcd)
• How will SIM achieve traceability to the re-defined SI come 2019?
s m cd : No Change to Traceability
• For the second, metre, and candela the proposed redefinition makes
no change to the unit or to the traceability
• We will still use the existing defined value for ∆νCs the transition frequency
between the two hyperfine levels of the ground state of the caesium 133
atom
• We will still use the existing defined value for c the speed of light
• We will still use the existing defined value for Kcd the luminous efficacy for
monochromatic radiation of frequency 540 THz
mol : Traceability through NA
• The change in definition from a mass basis (12 g of 12C) to a counting
basis (NA entities in a mole) has little (or no?!?) practical consequence
• We can leave this one aside from the perspective of dissemination
• There may be interesting areas of research in chemistry that arise, but
that is beyond the scope of today’s discussion
A : The quantum Ohm’s Law
• The present SI definition of the ampere allows no practical realization
since it relies on infinitely long wires of vanishing cross section able to
carry a current large enough to produce quite a strong mechanical
force!
• With the fixing of e (elementary charge) and h (Planck constant), the
macroscopic quantum phenomena for voltage (Josephson effect) and
resistance (Quantum Hall effect), can use Ohm’s Law for a primary
realization of current and to provide traceability for electrical units
A : The quantum Ohm’s Law
• Redefinition: Josephson voltage standard will increase by 98 ppb
• Best voltage calibrations have uncertainty (k=2) of 1.5 ppb
This represents a 13 σ change at the highest level of precision
• Redefinition: QHR resistance standard will increase by 18 ppb
• Best resistance calibrations have uncertainty (k=2) of 12 ppb
This represents a 1.5 σ change at the highest level of precision
K : What about the Triple Point of Water?
K : What about the Triple Point of Water?
• Fixing the value for kB (Boltzmann constant) enables thermodynamic
temperature to be used directly with temperature scales, rather than
just at the triple point of water as is the case in ITS-90
• Radiation thermometry can be disseminated from Planck’s Law on the
thermodynamic scale
• Contact thermometry will take a little longer but in the meantime
TTPW = 273.16 K and triple point cells may need to be “calibrated”
kg : What about the kilogram?
kg : Mass Scales Won’t Change, BUT…
• Once h the value of the Planck constant is fixed, the International
Prototype of the kilogram will no longer define the unit of mass
• Other techniques – specifically the XRCD Si spheres and the Kibble
(Watt) balances – will become the way to realize the unit of mass
from its definition
• Can have realizations at any required mass value, not just at 1 kg
• SIM – through NIST and NRC – has two operational Kibble balances
that have sufficiently low uncertainty at the level of 1 kg masses
kg : Mass Scales Won’t Change, BUT…
• Traceability still rests on the calibration of the 1 kg national standard
that is at the top of each national mass scale
• Existing dissemination schemes from a primary national standard –
including PtIr National Prototypes or stainless steel OIML Class E1
weights – can still be used without modification
• NRC and NIST have discussed the idea of disseminating primary
calibrations from Kibble balances to all of the SIM member states
What Next?
• We should now discuss these issues and consider how to proceed
• Let’s have a period of questions and answers for clarification
• Suggested Next Steps:
• Hold a Workshop on the redefinitions in the SI to address scientific metrology
needs and quality management system implications
• Examine broader issues including ideas to raise public awareness of the new
definitions and the impact of metrology for industry and innovation