A measurement of the Planckconstant using pressure metrology                  C M Sutton Measurement Standards Laboratory ...
Contents     Introduction       The Planck constant & how it relates to the kilogram     Watt balance       How it works  ...
Introduction     Why measure the Planck constant?     Because the present artefact     kilogram is limiting     developmen...
What is the problem with the kilogram?       The present kilogram is the mass of the IPK       - International Prototype k...
Solution - re-define the kilogram      .. In terms of a fundamental constant         Planck constant h or Avogadro constan...
Current results for h & NA                             Lowest uR = 3.0 × 10-8                                          BUT...
A watt balance - how it works      Mechanical versus electrical power - two modesWeighing                                 ...
Concept - pressure balance watt balance       Weighing mode            Two pressure balances           Aim: Table-top     ...
What do other watt balances look like?     Quite different!     Traditional mass/force balance        Coil hanging from gi...
Research - Pressure balances 1      Weighing performance        MSL twin pressure balance 2           Repeatability vs loa...
Research - Pressure balances 2       Investigating                                                           0.75         ...
Research - Pressure balances 3      New pressure balance design        With IRL Mechanical workshop           Stationary p...
Research - Ground vibration at IRL      Noise rejection        Choose oscillation frequency - for dynamic mode         Whe...
Research - Dynamic coil position measurement      High-speed heterodyne laser interferometry                              ...
Research - Induced voltage measurement      Via ac Josephson voltage standard        Have purchased cryo-cooler           ...
Research - Magnetic field      Trial calculations on magnet design                                                        ...
Summary      Pressure balance watt balance       Concept established          Significantly different from existing watt b...
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14.00 o1 c sutton

  1. 1. A measurement of the Planckconstant using pressure metrology C M Sutton Measurement Standards Laboratory of New Zealand Industrial Research LimitedAcknowledgement: Many people - MSL, IRL, overseas collaborators … NZIP Conference, Wellington, 17 - 19 October 2011 v1014f
  2. 2. Contents Introduction The Planck constant & how it relates to the kilogram Watt balance How it works A pressure balance watt balance Concept How it compares with other watt balances Research activities Summary2
  3. 3. Introduction Why measure the Planck constant? Because the present artefact kilogram is limiting development of the SI The International System of Units and A new definition of the kilogram in terms of the Planck constant is a way forward International prototype kilogram ( IPK)3
  4. 4. What is the problem with the kilogram? The present kilogram is the mass of the IPK - International Prototype kilogram Only reliable to ~ 50 µg or 5 parts in 108 - or worse? Limiting other units Apparent variations in mass ampere with time for copies of the IPK mole candela In particular: The ampere could be defined in terms of quantum phenomena Josephson volt Quantum Hall effect and realised with a relative standard uncertainty uR ~ 10-94
  5. 5. Solution - re-define the kilogram .. In terms of a fundamental constant Planck constant h or Avogadro constant NA Only current options likely to achieve accuracy uR < 5 × 10-8 Link kg to h Link kg to NA Watt balance - Kibble 1975 Counting atoms in sphere USA, Canada of single crystal 28Si France (2), International collaboration Switzerland, - Australia, France, Germany, China, Italy,Japan, USA, … New Zealand … Budget several M€!5
  6. 6. Current results for h & NA Lowest uR = 3.0 × 10-8 BUT span of results is 31 × 10-8 cAr ( e ) M 0α 2 h= More measurements needed! 2 R∞ N A 6.022143 Target accuracy: Avogadro constant /(10 23 mol-1) uR ≈ 2 × 10-8 6.022142 USA watt balance 6.022141 Silicon Challenging: sphere 6.022140 ~2 s in 3 years or UK ~2 mm in 100 km! watt balance 6.022139 6.022138 CODATA 2006 NIST wb 2007 NPL wb 2007 IAC 20106
  7. 7. A watt balance - how it works Mechanical versus electrical power - two modesWeighing Dynamic mode mode Factor γ = U/v from dl ~ coil length dynamic mode ( ) uu r r r uu r uu r r r ( ) r r r r r F=I ∫ dl × B = I ⋅ γ = m g U =∫ ν × B ⋅ dl = − ∫ dl × B ⋅ v = −γ ⋅ v mg U UI f J2 Hence γ= = and m= =C h I v gv gv When I & U are measured – with quantum electrical standards » Josephson volt, quantum Hall resistance7
  8. 8. Concept - pressure balance watt balance Weighing mode Two pressure balances Aim: Table-top As force comparator size watt balance Coil fixed on piston Dynamic mode Oscillatory coil motion ~ 1 Hz, noise rejection Gas pressure balance: • No piston-cylinder contact - aerodynamic bearing - strong piston centring forces • Small piston-cylinder gap - < 1 µm8
  9. 9. What do other watt balances look like? Quite different! Traditional mass/force balance Coil hanging from gimbals USA watt balance Various means to move coil Beam, wheel, flexures … Un-wanted forces, motions Need to control coil position – all six degrees of freedom Constant coil velocity In dynamic mode dc induced voltage measurement Susceptible to noise9
  10. 10. Research - Pressure balances 1 Weighing performance MSL twin pressure balance 2 Repeatability vs load, (AB)4 or (AB)5 loading sequence – A ~ unloaded, B ~ loaded, Calculate dp and u(dp) » DHI pressure balances Near zero load u(dp) ~ 0.25 mPa or 2.5 × 10-9 of line pressure 0.8 0.6 u (dp ) /mPa Nearly good 0.4 enough 0.2 0.0 0 50 100 150 200 25010 Mass /g
  11. 11. Research - Pressure balances 2 Investigating 0.75 0.5 Damped resonant behaviour Amplitude /mm 0.25 To improve short-term repeatability 0 0 20 40 60 Damping depends on -0.25 -0.5 pressure, gas, geometry -0.75 Time /s Due to non-adiabatic behaviour of gas – With NIST Gaithersburg 20 & DH Instruments, USA CEC DHI 15 Aim to Q = π /λ reduce damping 10 & increase Q 5 0 0 1 2 311 Period t 0 /s
  12. 12. Research - Pressure balances 3 New pressure balance design With IRL Mechanical workshop Stationary piston – Allows wires to coil on piston Rotating cylinder Trial design for cylinder rotation – Axis defined by matched pair of angular contact ball bearings Prototype made – Currently being tested12
  13. 13. Research - Ground vibration at IRL Noise rejection Choose oscillation frequency - for dynamic mode Where background noise is low Fourier analysis of d & U - to give γ Rejects noise at other frequencies Ground vibration 1.E-04 Low from 1 Hz to 3 Hz 1.E-05 Acceleration /(m/s2) Matches preferred Level of vibration 1.E-06 frequency range 0.1 Hz to 5 Hz 1.E-07 –To avoid Velocity /(m/s) 1.E-08 mechanical resonances 1.E-09 Amplitude /m 1.E-10 0.1 1 10 10013 Frequency /Hz
  14. 14. Research - Dynamic coil position measurement High-speed heterodyne laser interferometry For measuring oscillatory coil motion ~ 1 MHz sample rate Developing processing electronics With Intelligent Machines & Devices Group 2.5 Apparent displacement /nm 1.5 0.5 -0.5 -1.5 -2.5 550 650 750 850 950 Time /s14
  15. 15. Research - Induced voltage measurement Via ac Josephson voltage standard Have purchased cryo-cooler New cryo-cooled cryostat Plan to purchase PJVS Programmable Josephson voltage standard With NIST Boulder, USA Investigating Differential sampling voltmeter To measure the difference between the induced coil voltage and the ac Josephson voltage With NIST Gaithersburg, USA15
  16. 16. Research - Magnetic field Trial calculations on magnet design 0.9532 0.9306 With HTS110 Radial induction B (MSL) /Tesla Radial induction B (LNE) /Tesla 0.9305 0.9530 LNE calculation Obtained similar variations 0.9528 MSL calculation 0.9304 0.9303 Of B with z to lab in France 0.9526 0.9302 0.9524 Rig to measure RTC 0.9522 0.9301 0.9300 Reversible temperature 0.9520 0 10 20 30 40 50 60 70 80 90 0.9299 coefficient Distance z in air gap /mm Of permanent magnets Want ~zero RTC With Electron Energy, USA16
  17. 17. Summary Pressure balance watt balance Concept established Significantly different from existing watt balances Research initiated on key factors Influencing feasibility and performance Aiming for operational watt balance In some form, mid-2013 Results in advance of Dec 2014 - CODATA Any questions?17

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