NIST-4 Watt Research Articles

First Measurements of the Flux Integral With the NIST-4 Watt Balance

In early 2014, construction of a new watt balance, named NIST-4, has started at the National Institute of Standards and Technology (NIST). In a watt balance, the gravitational force of an unknown mass is compensated by an electromagnetic force produced by a coil in a magnet system. The electromagnetic force depends on the current in the coil and the magnetic flux integral. Most watt balances feature an additional calibration mode, referred to as velocity mode, which allows one to measure the magnetic flux integral to high precision. In this article we describe first measurements of the flux integral in the new watt balance. We introduce measurement and data analysis techniques to assess the quality of the measurements and the adverse effects of vibrations on the instrument.

A Determination of the Local Acceleration of Gravity for the NIST-4 Watt Balance

A new watt balance is being constructed at the National Institute of Standards and Technology (NIST) in preparation for the redefinition of the International System of Units and the realization of mass through an exact value of the Planck constant. The total relative uncertainty goal for this instrument of a few parts in $10^{8}$ requires that the local acceleration due to gravity be known at the location of a test mass with a relative uncertainty on the order of only a few parts in $10^{9}$. To make this determination, both the horizontal and vertical gradients of gravity must be known such that gravity may be tied from an absolute reference in the laboratory to the precise mass location. We describe the procedures used to model and measure gravity variations throughout the laboratory and give our results.

Hysteresis and Related Error Mechanisms in the NIST Watt Balance Experiment.

The NIST watt balance experiment is being completely rebuilt after its 1998 determination of the Planck constant. That measurement yielded a result with an approximately 1×10−7 relative standard uncertainty. Because the goal of the new incarnation of the experiment is a ten-fold decrease in uncertainty, it has been necessary to reexamine many sources of systematic error. Hysteresis effects account for a substantial portion of the projected uncertainty budget. They arise from mechanical, magnetic, and thermal sources. The new experiment incorporates several improvements in the apparatus to address these issues, including stiffer components for transferring the mass standard on and off the balance, better servo control of the balance, better pivot materials, and the incorporation of erasing techniques into the mass transfer servo system. We have carried out a series of tests of hysteresis sources on a separate system, and apply their results to the watt apparatus. The studies presented here suggest that our improvements can be expected to reduce hysteresis signals by at least a factor of 10—perhaps as much as a factor of 50—over the 1998 experiment.

A result from the NIST watt balance and an analysis of uncertainties

An improved determination of the ratio of power, measured in terms of the Josephson and quantum Hall effects, and also the meter, kilogram, and second, has been completed. The result is expressed as: W/sub 90//W=1+(8/spl plusmn/87)/spl times/10/sup -9/. This is an order of magnitude improvement from the last NIST (formerly NBS) determination. The Type A relative standard uncertainty (statistical) is 30 nW/W, and the Type B relative standard uncertainty is 82 nW/W. Type B uncertainty components are listed and discussed within the context of experimental procedure descriptions. Improvements for the next version of this experiment are also mentioned.

Alignment uncertainties of the NIST watt experiment

The effects of alignment uncertainties of the NIST watt balance with respect to local gravity and the magnetic flux density of the balance have been analyzed. Techniques for measuring all quantities relevant to misalignment have been developed. The components of the relative combined standard uncertainty of the measured value of the watt due to alignment uncertainties have been reduced to 20 nW/W, and potential improvements in the balance design have been identified which could ultimately lead to a reduction of that uncertainty to below 10 nW/W.