Daniel Winester, Geodesist NOAA - NOS - National Geodetic Survey, performing a gravity transfer to the level of the reference mass, in the small cage hanging on the left side of the watt balance wheel.
Realize the electrical unit of voltage and provide an alternative definition of the unit of mass that is based on measured quantities determined by fundamental physical constants of nature. Realize the unit of Force in the International System of Units (SI) through the electrical units at the nanonewton level; provide calibrated force artifacts for research groups and industry; and establish intrinsic force standards of nature at the nanonewton level traceable to the SI.
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Daniel Winester, Geodesist NOAA - NOS - National Geodetic Survey, performing a gravity transfer to the level of the reference mass, in the small cage hanging on the left side of the watt balance wheel. |
The kilogram is the only remaining base unit in the SI whose definition is based on a physical artifact rather than on fundamental properties of nature. Environmental contamination or material loss from surface cleaning, or other unknown mechanisms, are causing the mass of the kilogram to vary by about 3 parts in 108 per century relative to sister prototypes. This observed drift highlights a significant shortcoming of the SI system. The measured values of many physical constants are based on mass, and these constants are regularly used in quantum-based measurement systems, such as the Josephson effect, which are becoming more significant to the growth of international technology and trade accreditation. Thus, with a time-drifting mass standard, adjustments to the value of physical constants must be made periodically to maintain the consistency of the SI system. Moreover, each future change will adversely affect a continuously growing technology base that relies increasingly on electronic testing, quality control, and environmental monitoring. The adoption of the electronic kilogram as the mass standard will improve the consistency of the SI and will also provide better determinations of many fundamental physical constants, such as the charge and mass of the electron, that serve the general scientific and technological communities.
Commercial and custom mechanical test instruments, including instrumented indentation machines and atomic force microscopes, have been developed with force detection capabilities extending to the nanonewton regime. Correspondingly, a desire for accurate and traceable force measurement is emerging within the International Organization for Standardization (ISO) task groups and American Society for Testing and Materials (ASTM) committees. However no methods for establishing force measurement traceability at levels below 10-5 N are currently available. It is within this context that the Microforce Realization and Measurement project has developed facilities and instruments capable of providing a viable primary force standard below 10-5 N; the ultimate goal is to realize force in this range at a relative uncertainty of as little as 10 pN/µN.
The equivalence of electrical and mechanical power provides a convenient route to the measurement of mass in terms of other quantum-mechanically defined measurement units. The apparatus at the Electronic Kilogram facility is a force balance connected to an induction coil in a large magnetic field. Configured as either an electric generator or motor, both kinds of power are measured in a way that is unaffected by the dissipative forces of friction and electromagnetic heating. The experimental observables are time, length, voltage, and resistance, measured with respect to fundamental and invariant quantum phenomena, respectively: atomic clocks, lasers, the Josephson effect, and the quantum Hall effect.
The entire experiment is a complex mesh of mechanical and electronic components controlled by computer. The systems are not only for watt data acquisition but also include reference standards, servo control, and environmental monitoring. All systems need to perform accurately and consistently 24 hours a day. Some peripheral systems include a programmable Josephson array voltage standard, a gravimeter, an ac resistance temperature bridge, a temperature controller for the building, and a current controller for the magnetic field solenoid. Even though the apparatus has been rebuilt and improved since 1998, a number of additional improvements are recognized as necessary for better instrument performance, easier operational use, and preventative maintenance. This is expected to compose much of the work in 2005.
Using a carefully designed Electrostatic Force Balance (EFB), measurements of length, capacitance, and voltage provide a viable primary small force standard consistent with the SI. The NIST EFB has been used to measure the force sensitivity of transfer artifacts such as piezoresistive microcantilevers and load cells. These transfer standards will be used to compare the EFB and the primary standards from the emerging small force programs at the NPL and PTB. Ultimately the EFB and transfer artifacts will be used to investigate and provide SI traceability to intrinsic standards such as capillary forces, atomic bond ruptures, and molecular structure reordering.
Publish a value for the Planck constant at about 0.1 ppm uncertainty or better to compare a NIST 2004 value with the international Avogadro and other Watt groups’ values, including NIST 1998
Upgrade electronics for regular monitoring of the kilogram at an uncertainty level of 0.01 ppm
Complete a thorough phase II intercomparison transfer of small force standards between NIST, NPL, and PTB
Electronic Kilogram — For the electronic-kilogram before fiscal year 2004, the relative standard deviation of the acquired watt data was 0.25 ppm. At this time the third version of the induction coil was finally put into use and noise in the watt data was immediately reduced by a factor of 5. With enhanced resolution, smaller drifts in day-to-day values were more easily identified and more metrological problems solved. In May 2004, an electric circuit reconfiguration reduced a nagging electrical problem, and the long-term (monthly) drift was further reduced. By September 2004, the relative standard deviation of the daily averaged watt values stood at 0.008 ppm.
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The graph shows the level of improvement in the resolution and stability of the watt results. The watt values are the deviation from identity in ppm for force * velocity relative to voltage * current. The actual values do not contain all corrections, including a control uncertainty of up to 0.2 ppm. The difference between the NIST 1998 number and the CODATA value is approximately 0.0 ppm. |
Also by May 2004, the watt balance had a stable alignment, an improved electrical measurement circuit, and all reference standards maintained at a high level of accuracy; so highly sensitive systematic error checking at the 0.01 ppm level began. Tests included using different reference mass materials, reversing the magnetic field, checking hysteresis in the superconducting solenoid’s magnetic field, and seeking correlation between watt variations and environmental parameters. Even the value of gravity at the level of the mass had to be checked, given the changes around the lab from rebuilding the apparatus (see photo on page 23). Systematic corrections have been found, but are less than 0.04 ppm.
MicroForce Realization — The MicroForce Realization Project EFB has achieved agreement between an electrostatic and gravitational force of 10-5 N to within a few hundred pico/micro Newtons. The EFB has subsequently been used to measure the force sensitivity of a piezoresistive micro-cantilever by directly probing the EFB. The calibrated cantilever was used as a secondary force standard to transfer the unit of force to an optical lever-based sensor mounted in an atomic force microscope. This experiment was perhaps the first ever force calibration of an atomic force microscope to preserve an unbroken traceability chain to an appropriate national standard. Recently a calibrated piezoresistive cantilever was shipped to a university researcher to verify a thermal and dimensional calibration technique for AFM cantilevers. The agreement between the three methods was within 10 %. This was the first time an SI traceable small force standard was provided to an independent research effort by a national metrology institute. national metrology institute.
R. Steiner, E. Williams, and D. Newell are collaborating via periodic workshops with NPL-UK, METAS-Switzerland, and BNM-LCIE-France as part of the international CCEM group on the watt balance realization of the kilogram.
Edwin Williams and David Newell are collaborating with the Automated Production Technology, Precision Engineering, and Ceramics Divisions at NIST in a competence project for Microforce Realization and Measurement.
Collaboration with Physics Professor Nancy Burnham at Worcester Polytechnic Institute has yielded a comparison of three separate calibration techniques of AFM cantilevers showing agreement to within 10 %.
Collaboration with Peter Cumpson of NPL has yielded an agreement to compare NIST and NPL small force transfer artifacts in November 2004.
R. L. Steiner, D. B. Newell, E. R. Williams, R. Liu, and P. Gournay, “The NIST project for the electronic realization of the kilogram,” IEEE Trans. Instrum. Meas. (Conference on Precision Electromagnetic Measurements 2004), to be published (2005).
R. L. Steiner, E. R. Williams, and D. B. Newell, “Details of the 1998 watt balance experiment determining the Planck constant,” NIST J. Res., submitted (2004).
R. L. Steiner, D. B. Newell, E. R. Williams, R. Liu, and P. Gournay, “The NIST electronic realization of the kilogram project,” 2004 Conference on Precision Electromagnetic Measurements, June, 2004, London, England, pp. 52-3. (June 2004).
G. A. Matei, E. J. Thoreson, J. R. Pratt D. B. Newell, and N. A. Burnham, “Thermal method of cantilever calibration over a 200 kHz bandwidth,” Nanotechnology, submitted September 8, 2004.
J. R. Pratt, D. T. Smith, D. B. Newell, J. A. Kramar, and E. Whitenton, “Progress towards SI traceable force metrology for nanomechanics,” J. Mater. Res., 19 (1), pp. 366-379 (2004).
J. R. Pratt, D. B. Newell, J. A. Kramar, and E. Whitenton, “Calibration of piezoresistive cantilever force sensors using the NIST electrostatic force balance,” Proceedings of IMECE’03: 2003 ASME International Mechanical Engineering Congress & Exposition, Washington, DC, 16-21 November 2003.
J. R. Pratt, D. T. Smith, J. A. Kramar, and D. B. Newell, “Microforce and Instrumented Indentation Research at the National Institute of Standards and Technology,” Proc. 4th Inter, Symposium on MEMs and Nanotechnology, 2003 Society of Experimental and Applied Mechanics Annual Conference, Charlotte, NC, pp. 299-306, 2-4 June 2003.
J. R. Pratt, D. Newell, J. Kramar, J. Mulholland, and E. Whitenton, “Probe-force calibration experiments using the NIST electrostatic force balance,” Proc. of the American Society for Precision Engineering 2003 Winter Topical Meeting, University of Florida, pp. 64-69 (2003).
D. B. Newell, J. A. Kramar, J. R. Pratt, D. T. Smith, and E. R. Williams, “The NIST microforce realization and measurement project,” IEEE Trans. on Instrum. Meas., Special issue of Conference on Precision Electromagnetic Measurements 2002 52(2), pp. 508-511 (2003).
J. P. Schwarz, R. Liu, D. B. Newell, R. L. Steiner, E. R. Williams, D. Smith, A. Erdemir, and J. Woodford, “Hysteresis and related error mechanisms in the NIST watt balance experiment,” J. Res. Natl. Inst. Stand. Technol., 106 (2), pp. 381-389 (2001).