Kongressbeiträge

The 2019 redefinition of the kilogram not only changes the way mass is defined but also broadens the horizon for a direct realization of other standards. The True Becquerel project at the National Institute of Standards and Technology (NIST) is creating a new paradigm for realization and dissemination of radionuclide activity. Standard Reference Materials for radioactivity are supplied as aqueous solutions of specific radionuclides which are characterized by massic activity in the units becquerel per gram of solution, Bq/g. The new method requires measuring the mass of a few milligrams of dispensed radionuclide liquid. An electrostatic force balance is used, due to its suitability for a milligram mass range. The goal is to measure the mass of dispensed fluid of 1 mg to 5 mg with a relative uncertainty of less than 0.05 %. A description of the balance operation is presented. Results of preliminary measurements with a reference mass indicate relative standard deviations less than 0.5 % for tens of tests and differ 0.54 % or less from an independent measurement of the reference mass.

The new Kibble balance at the National Institute of Standards and Technology (NIST) is part of the Quantum Electro-Mechanical Metrology Suite (QEMMS). Two quantum standards are incorporated directly in the electrical circuit of the Kibble balance for the realization of the unit of mass. This eliminates the need for external calibration in the Kibble balance experiment. The targeted uncertainty is 2 μg on a 100 g mass and a range from 10 g to 200 g will be covered. We introduce the measurement concept of the QEMMS, show the current state of development and publish first measurements proving the performance of the newly designed balance mechanics.

Weighing cells with electromagnetic force compensation are frequently used in precision balances and mass comparators. The kinematic structure is given by a compliant mechanism with concentrated compliances. Thin flexure hinges enable highly reproducible motion but limit the sensitivity to mass changes due to their rotational stiffness. To achieve the desired sensitivity, the stiffness of the mechanism must be further reduced by mechanical adjustments. To optimize the adjustment parameters, the initial stiffness of the mechanism needs to be characterized accurately. For this purpose, a novel self-testing method was developed. It allows accurate determination of the elastic stiffness of the weighing cell and the geometric stiffness caused by the masses of the linkages. The method uses static stiffness measurements in three orientations. The gravity vector must be orthogonal to the plane of motion to characterize the elastic stiffness. Determining the geometric stiffness requires the system to be in the working orientation. The upside-down orientation is used to confirm the results. This paper considers the novel method analytically and simulates using a rigid body model and the finite element method. The measurement of the stiffness of a weighing cell prototype is taken to validate the method.