The NIST Mass and Force Group disseminates the unit of mass, the kilogram, to the US measurement system through calibrations of customer weights and weight sets. NIST mass disseminations are traceable to the International System of Units (SI) through the Consensus Value (CV) of the kilogram as determined by the international community. NIST maintains traceability to the CV through the NIST-4 Kibble Balance which is used to transfer the CV to the NIST Pool of Mass Standards. This pool consists of platinum iridium and stainless-steel kilogram artifacts that are stored in either high vacuum or laboratory air. The pool of artifacts is also used to create working standards which form the beginning of the NIST mass scale.
Redefinition of the Kilogram and Consensus Value of the Kilogram
The unit of mass was redefined in May 2019 as part of a general revision to the International System of Units (SI). See SI Redefinition. The kilogram is now defined by the Planck constant. The International Prototype Kilogram (IPK) no longer realizes mass. As of February 1, 2021, the kilogram is realized through the Consensus Value (CV) of the kilogram as determined by international agreement. In relation to the IPK, the CV is:
CV = IPK – 0.002 mg
The uncertainty on the CV is ± 0.020 mg. The NIST mass scale has been updated to reflect traceability to the CV; see the traceability chain below.
Description of Acceptable Weights for Calibration at NIST
Calibrating a weight consists of establishing a mass value and an appropriate uncertainty. Metrological-grade weights are available from manufacturers, many of whom can directly furnish documentation suitable for meeting quality assurance contracts and requirements.
NIST calibrates individual weights or sets in the range of 1 mg to 50 kg in decimal subdivisions. Weights must be of one-piece construction, with no adjustment cavity. The material, and surface finish must be comparable to ASTM Type I, classes 00, 0, 1, 2, and OIML E1, E2, and F1. NIST also calibrates mass standards larger than 50 kg to 28 000 kg if the design, material, and surface finish are compatible with the intended usage. For these large mass standards, an adjustment with reference to a nominal or desired value can be included as a part of the calibration procedure. NIST does not perform “type-testing” or “proficiency testing” of mass standards.
Mass calibrations must be ordered on the NIST Calibration Storefront at www.shop.nist.gov. All NIST calibration services may be ordered through this site.
Abbott, Patrick J., “Redefined but Not Perfected: The On-going Saga of the Kilogram”. Cal Lab, The International Journal of Metrology, vol. 27, number 2, pp. 28 – 32.
Abbott, Patrick J. and Kubarych, Zeina, “Mass Calibration at NIST in the Revised SI”. Proceedings of the NCSLI International Workshop & Symposium, August 27-30, 2018, Portland, Oregon.