The SI in 2019: Traceability Questions for SIM A Presentation to Encourage Discussion and Understanding Alan Steele SIM General Assembly Montevideo,

Transcription

1 The SI in 2019: Traceability Questions for SIM A Presentation to Encourage Discussion and Understanding Alan Steele SIM General Assembly Montevideo, Uruguay November 2016

2 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 ( ν Cs, c, h, e, k B, N A, K cd ) How will SIM achieve traceability to the re-defined SI come 2019?

3 s m cd : No Change to Traceability For the second, metre, and candelathe proposed redefinition makes no change to the unit or to the traceability We will still use the existing defined value for ν Cs the transition frequency between the two hyperfine levels of the ground state of the caesium133 atom We will still use the existing defined value for cthe speed of light We will still use the existing defined value for K cd the luminous efficacy for monochromatic radiation of frequency 540 THz

4 mol: Traceability through N A The change in definition from a mass basis (12 g of 12 C) to a counting basis (N A 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

5 A : The quantum Ohm s Law The present SI definition of the ampereallows 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

6 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 σchange 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 σchange at the highest level of precision

7 K : What about the Triple Point of Water?

8 K : What about the Triple Point of Water? Fixing the value for k B (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 Lawon the thermodynamic scale Contact thermometry will take a little longer but in the meantime T TPW = Kand triple point cells may need to be calibrated

9 kg : What about the kilogram?

10 kg : Mass Scales Won t Change, BUT Once hthe 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

11 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 PtIrNational 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

12 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