NIST maintains the national standard for mass in the form of the prototype kilograms (K4, K20, K79, K92, and K102) and provides services to support the parts of the national measurement system that rely directly or indirectly on mass measurements. These services include the calibration of suitable weights or weight sets. A calibration consists of establishing a mass value and the appropriate uncertainty for that value for each weight. 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, material, and surface finish comparable to ASTM Type I, classes 00, 0, 1, 2, and OIML E1, E2, and F1. NIST also calibrates large 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.
All mass calibrations are traceable to the SI through the following traceability chain: