Advanced Mass Metrology Training with Guidance on the Kilogram Consensus Value

NIST OWM will welcome a full class of students to its Advanced Mass Metrology course at NIST’s Gaithersburg, Maryland campus at the end of this July. This nine-day course is held every other year at NIST.

What will be learned over the nine-day period?

The seminar focuses on the comprehension and application of the advanced mass dissemination process through the equations and calculations of weighing designs. It reviews the quality system for mass Echelon I calibrations from the receipt of the artifact to the generation of an ISO/IEC 17025 compliant calibration certificate. It includes the operation of laboratory equipment, review of documentary and reference standards, and specifications relevant to the measurements. Some of the documentary standards studied include the NISTIR 6969, NISTIR 5672, NIST Technical Note 952, and NIST/SEMATECH e-Handbook of Statistical Methods. The course consists of 50 % lectures and 50 % hands-on measurement activities completed in the OWM training laboratory. OWM receives assistance from experts at NIST, including lecturers from the Quantum Measurement Division (Mass and Force Group and Fundamental Electrical Measurement Group) and the Statistical Engineering Division. The Quantum Measurement Division also provides a tour of NIST’s Kibble Balance, which was built as part of the effort to redefine the SI unit of mass, the kilogram, and continues to be utilized for realizing the kilogram.

One of the developing topics to be reviewed and emphasized during the course is how all mass laboratories must account for the uncertainties and change in the mass scale due to having to realize the kilogram from a Consensus Value (CV). The CV is a result of an average of measurements taken during Key Comparisons between the National Measurement Institutes (NMIs) that have experiments that realize the kilogram through the Kibble Balance or Silicon Sphere (x-ray crystal density) methods (using a fixed Planck constant). The CV value and uncertainty after the latest Key Comparison in 2023 (key comparisons) is now a total of -7 µg with an uncertainty ± 20 µg (see BIPM derivation). NIST has implemented the CV to its mass scale and uncertainties (and related Calibration and Measurement Capabilities (CMCs) since the redefinition in 2019. All mass laboratories claiming traceability to the SI kilogram should ensure this uncertainty component has been accounted for by their calibration supplier.

The advanced mass metrology course will examine how to propagate the CV and uncertainties to your mass scale based on when your standards were last calibrated (meaning whether the CV and related uncertainty has been applied). This is especially true and is most impactful to mass Echelon I labs providing calibrations for other Echelon I or II laboratories. Improvements in the Kilogram realization processes are ongoing among the participating NMIs. The CV and the uncertainty component will continue to evolve and will need to be monitored as realization experiments (key comparisons) continue. Another key comparison report is due to be published this year which will further update the consensus value until the CCM deems sufficient agreement has been met between participant experiments (see CCM). Once sufficient agreement is reached, the CCM will proceed to Step 3 of the implementation which is independent realizations with no need for a CV. A basic training on this topic (Realization of the Kilogram and the Consensus Value) was also provided during the CRMAP technical session on Thursday, June 5, 2025.

NIST OWM provides essential professional training to support all state laboratory metrology programs in delivering traceable measurement results to their customers and stakeholders. Please see the training classes posted at OWM Training and Events listing classes through 2026. Please feel free to contact us directly at micheal.hicks [at] nist.gov (micheal[dot]hicks[at]nist[dot]gov) (Laboratory Metrology Program Leader) and elizabeth.koncki [at] nist.gov (elizabeth[dot]koncki[at]nist[dot]gov) (Laboratory Metrology Physical Scientist) for more information.