Zeina J. Kubarych

The New Kilogram Definition and its Implications for High-Precision Mass Tolerance Classes.

The SI unit of mass, the kilogram, is the only remaining artifact definition in the seven fundamental units of the SI system. It will be redefined in terms of the Planck constant as soon as certain experimental conditions, based on recommendations of the Consultative Committee for Mass and Related Quantities (CCM) are met. To better reflect reality, the redefinition will likely be accompanied by an increase in the uncertainties that National Metrology Institutes (NMIs) pass on to customers via artifact dissemination, which could have an impact on the reference standards that are used by secondary calibration laboratories if certain weight tolerances are adopted for use. This paper will compare the legal metrology requirements for precision mass calibration laboratories after the kilogram is redefined with the current capabilities based on the international prototype kilogram (IPK) realization of the kilogram.

Milligram mass metrology using an electrostatic force balance

Although mass is typically defined within the International System of Units (SI) at the kilogram level, the pending SI redefinition provides an opportunity to realize mass at any scale using electrical metrology. We propose the use of an electromechanical balance to realize mass at the milligram level using SI electrical units. An integrated concentric-cylinder vacuum gap capacitor allows us to leverage the highly precise references available for capacitance, voltage and length to generate an electrostatic reference force. Weighing experiments performed on 1 mg and 20 mg artifacts show the same or lower uncertainty than similar experiments performed by subdividing the kilogram. The measurement is currently limited by the stability of the materials that compose the mass artifacts and the changes in adsorbed layers on the artifact surfaces as they are transferred from vacuum to air.

Progress on vacuum-to-air mass calibration system using magnetic suspension to disseminate the Planck-constant realized kilogram

The kilogram is the unit of mass in the International System of units (SI) and has been defined as the mass of the International Prototype Kilogram (IPK) since 1889. In the future, a new definition of the kilogram will be based on precise measurements of the Planck constant. The new definition will occur in a vacuum environment by necessity, so the National Institute of Standards and Technology (NIST) is developing a mass calibration system in which a kilogram artifact in air can be directly compared with a kilogram realized in a vacuum environment. This apparatus uses magnetic levitation to couple the kilogram in air to a high precision mass balance in vacuum. Technical details of the levitation technique, the vacuum-to-air calibration system, and vehicles for transferring masses into and out of vacuum will be presented.

NIST vacuum-to-air mass calibration system as part of a Mise-en-Pratique for the new kilogram definition

The SI unit of mass, the kilogram, will be redefined in terms of the Planck constant, perhaps as soon as 2018. A methodology for the practical realization and dissemination of the new kilogram, or Mise-en-Pratique (MeP), is in progress within the Consultative Committee for Mass and Related Quantities (CCM). The new kilogram will be realized in a vacuum environment and will require transfer to air in order to maintain artifact mass metrology. NIST is developing a vacuum-to-air mass calibration system that uses magnetic suspension to directly compare a mass in vacuum to a mass in air or other gas. This method does not rely on empirical measurement of adsorption characteristics of water in order to correct for mass gain of a primary realization in air. This paper will discuss the realization of the new kilogram and propose a roadmap for dissemination using the NIST magnetic suspension calibration system.

The Planck constant, watt and vacuum balances, and an evolving Mise en pratique for the kilogram in North America

We report preparations underway at the National Institute of Standards and Technology (NIST) in the United States and at the National Research Council (NRC) in Canada to support the redefinition of the international system of units (SI) and the development of a Mise en pratique for the unit of mass. NIST and the NRC possess two of the worlds most precise moving coil watt balances and are developing a collaborative plan to reconcile differences between the respective measurement platforms by testing their ability to mutually realize the kilogram from the Planck constant. The important role to be played by mass metrology and new facilities, such as the NIST magnetic levitation balance and the NRC vacuum comparator, to establish the necessary link between a mass as measured in vacuum and its value in air will be considered.

Comparison of two methods for determining the sorption correction for a vacuum-realized kilogram

The International System of Units (SI) is expected to be revised in 2018, and in this new system, the unit of mass, the kilogram, will be realized in a vacuum environment. In order to transfer the vacuum realization to artifacts in atmospheric pressure air, the effects of the sorption of atmospheric contaminants must either be corrected for or eliminated. NIST has constructed a system that directly compares a mass in vacuum to a mass in air, thereby eliminating the need for sorption correction. We describe the operation of this system and compare results to those obtained from sorption corrections.

Results from the magnetic suspension mass comparator for vacuum-to-air mass dissemination

Experiments to realize the new definition of the kilogram will be carried out in vacuum. NIST has developed a magnetic suspension mass comparator to disseminate the new definition to air. This paper details the current status of the system. This includes results from recent work to characterize and improve the performance of the magnetic suspension portion of the system. A general overview of the system, as well as key results are given.

The NIST Mise en Pratique for the Realization and Dissemination of the Kilogram as Part of the “New SI”

Abstract: The International System of Units (SI) will be redefined in 2018 so that the present seven SI base units are realized by a set of defining constants having exact values. For the unit of mass, the kilogram, this means a change in realization from a physical artifact, the International Prototype Kilogram (IPK) to an experiment that uses the Planck constant to measure mass with an uncertainty. Although traditional artifact-based mass metrology will not change, National Measurement Institutes (NMIs) will change the way that they realize the unit of mass and disseminate it to working standards. Much of this change is due to the necessary vacuum environment of the experiments (Kibble balance and x-ray crystal density (XRCD)) that will use the Planck constant to measure mass. At the National Institute of Standards and Technology (NIST), mass measurements, artifact transfers, and storage of standard artifacts will be done in both vacuum and atmospheric pressure environments to produce and maintain SI-traceable mass standards. This process of realization and dissemination is known as a mise en Pratique and consists of four main components, each of which is described.

Constant pressure and vacuum transporter for 1 kg mass standards

This report describes the design and intended use of a mass transporter for transferring 1 kg mass standards between different locations and pressure environments without compromising the atmospheric conditions and, therefore, maintaining stability of the surface and mass values of the artifacts. This transporter is intended for transferring 1 kg artifacts between a vacuum mass comparator and the next upgrade of the National Institute of Standards & Technology (NIST) Watt balance.