The BIPM Watt Balance

Transcription

1 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

2 Outline Motivations Main features Current status Future improvement Conclusion 2

3 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

4 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

5 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

6 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

7 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

8 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

9 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

10 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

11 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

12 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

13 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

14 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 nd For current measurement Ready in

15 What s going on 15

16 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

17 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

18 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