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New Assignment of Mass Values and Uncertainties to NIST Working Standards
1National Institute of Standards and Technology, Gaithersburg, MD 20899.
Summary
NIST mass standards previously showed a small, insignificant offset from International System of Units (SI) mass values. This discrepancy was corrected in 1990, ensuring improved accuracy in mass measurements.
Area of Science:
- Metrology
- Mass Measurement Science
Background:
- A suspected discrepancy existed between NIST working mass standards and International System of Units (SI) artifacts.
- This offset was approximately 0.17 mg/kg, posing minimal scientific impact.
Purpose of the Study:
- To document the history and resolution of a mass standard discrepancy at NIST.
- To detail studies enabling the correction and methods preventing recurrence.
Main Methods:
- Historical analysis of mass standard assignments.
- Comparative studies of NIST standards against SI-defined mass artifacts.
- Implementation of new calibration and verification protocols.
Main Results:
- Confirmed a 0.17 mg/kg offset in NIST mass standards.
- Successfully removed the discrepancy on January 1, 1990.
- Achieved long-term stability of 0.033 mg/kg for working standards relative to US national prototype kilograms.
Conclusions:
- The NIST mass discrepancy has been resolved, enhancing measurement accuracy.
- A provisional systematic uncertainty of 0.09 mg/kg is acknowledged, pending BIPM standard analysis.
- Ongoing verification of national prototype kilograms aims to further reduce systematic uncertainties.
