CCM Workshop on the mise en pratique of the new definition of the kilogram 21 November 2012 The BIPM Watt Balance Hao FANG Mass Department
Outline Motivations Main features Current status Future improvement Conclusion 2
Why at the BIPM The BIPM watt balance will provide long-term sustainable operation for a primary realization of the kilogram on a cost-shared basis. will be available for participation in key comparisons with other primary realizations of the kilogram. 3
Main features: operation mode Simultaneous (weighing & moving) mode in addition to conventional two-phase operation mode v Less sensitive to changes in thermal environment and alignment Better understanding of the limiting factors of each approach B I F g F e L U = (LB)v + R(T)I undesirable resistive voltage 4
Main features: system design Table-size experiment at room temperature Permanent magnet (symmetric and closed circuit) Mass comparator & magnet kept fixed; vertical motion of the coil by an electrostatic motor Mechanism to dynamically correct imperfect vertical motion of the coil 5
Current apparatus: milestones operated in air inside a temporary laboratory 2005: Beginning construction 2009: Operational optical table 2010: First h determination 2011: Improved repeatability 6
Current apparatus: experimental set-up Mass comparator (not visible) Electrostatic motor B = 0,5 T L: = 250 mm; N = 1160 Test mass 3-axis interferometer Magnetic assembly m = 100 g I = 1 ma v = 0,2 mm/s U = 0,1 V Moving coil (not visible) 7
Current apparatus: results Relative difference against the CODATA 2010 value 3.E-05 2.E-05 1.E-05 0.E+00-1.E-05-2.E-05-3.E-05 1 x 10 5 Spring 2010 Relative standard deviation of 5.7 x 10 6 Summer 2011 Relative standard deviation of 1.4 x 10 6 offset due to limited alignment capabilities CODATA 2010 Planck constant h Type B: 5 x 10-5 Type A: 2 x 10-6 8
Removal of resistive voltage Coil 1 Coil 2 Bifilar coil Current (weighing) Velocity (moving) Simultaneous operation by using a bifilar coil Superconducting coil (R = 0) Cryogenic moving coil system Superconducting coil behaves differently in magnetic field as normal coil Three ways to determine h 2 simultaneous single coil & 1 bifilar Comparison with lower uncertainty Measurement campaign early 2012 Simultaneous and bifilar coil approaches equivalent at 10 7 level Bifilar coil seems to be promising 9
New laboratory: infrastructure base for watt balance (64t) base for gravimetry (10t) Temperature control Filtered power supply Low noise lighting (LEDs) One of the sites for ICAG 2009 10
y hillside New laboratory: surveys Temporary laboratory: optical table watt balance base N gravimetry base rms velocity 3.2 µm/s New laboratory: vacuum enclosure x Spatial gravity transfer Experimental relative gravity map Fitting of 120 measurement points Uncertainty < 5 µgal rms velocity 0.7 µm/s Vibration investigation Reduction by a factor of five in the new laboratory 11
Developments: new magnet Sm 2 Co 17 magnets Closed and symmetric circuit Good isolation against external electromagnetic perturbations Uniform field profile in gap Fabrication finished end of 2012 12
Developments: alignment tools 2.5 m initial alignment of the apparatus dynamic correction of coil trajectory Long term loan from NIST Align both B and L perpendicular to g 50 µrad on the test coil To be applied on the working coil and then the magnetic circuit Repeatability: position < 1 µm; angle < 1 µrad Improved version under fabrication 13
Developments: electrical measurements New current source Fully automated and programmable 1 ma, 2 ma, 5 ma, 10 ma (1 kg) std dev (80 ms) of 10 7 drift of 10 9 / min Two Josephson voltage standards 1 st Dedicated for the induced voltage Independent isolated current sources Ready in 2013 2 nd For current measurement Ready in 2014 14
What s going on 15
What s next In-situ calibration of weighing cell Improved apparatus operated in vacuum inside a dedicated laboratory Mass exchanger Dynamic alignment system vacuum: < 0,01 Pa no air convection; no air buoyancy no air refractive index correction Final magnet 16
Present staff 2 full-time staff daily operation Hao Fang Adrien Kiss Mass department Contributions from BIPM staff: Mass, Electricity, Time & Workshop 1 Fixed-term engineer (2-3 years) from 2013 Considering secondments & internships 17
Conclusion The BIPM watt balance will be operated in long-term for the realization of the new definition of the kilogram. Current apparatus: h determination 10-5 Upcoming improved apparatus inside a dedicated new laboratory 18