Mass and Pressure Standards
kilogram, kg: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
Mass is the last of the SI units to be based on a material artefact. Because there can be only one kilogram, it is immensely valuable; after all if the kilogram is damaged in any way then by definition the mass of the universe changes. Because of its value it has only been used a few times in the last 125 years. Instead it is used to calibrate six official platinum copies of the kilogram, which are then used to calibrate the platinum copies owned by national institutes and the BIPM, which are then used to calibrate stainless steel standards and so on, and so on. But that only calibrates the stainless steel kilograms. To calibrate a full reference set from 1 mg to 20 kg takes another couple of hundred weighings. The large number of links in this calibration chain is necessary in part to protect the standards because they are easily damaged in use, and in part to preserve accuracy during the build-up and build-down from the kilogram. Although the kilogram is stable to perhaps 1 part in 109, the accuracy available from most national laboratories is limited to a few parts in 107, except in unusual or particularly demanding applications. Because of the loss of accuracy with the long calibration chains and the advances in electronic balances over the last decade or so, mass standards are being stretched to the limit at almost every level. Calibration of the most accurate balances is especially problematic.
Currently a number of national laboratories are investing heavily in experiments to find an alternative to an artefact mass. Possible alternatives include counting a known number of silicon atoms, or replacing the combination of mass and gravitational attraction by a machine providing calculable electromagnetic forces. However the experiments are proving very difficult and so far have achieved an accuracy of only about 1 part in 107, not enough yet to replace the artefact kilogram.