The Seven SI Base Units and the 2019 Revision

Calibration establishes, under specified conditions, a relationship between standard quantity values and instrument indications, including their uncertainties, and then permits a measurement result to be obtained from an indication. Calibration and adjustment are different operations. The International System of Units (SI) supplies a common reference for measurements that can be expressed in SI units.

The CGPM adopted the revised SI in November 2018, with implementation on May 20, 2019. Fixing seven defining constants ended the international prototype kilogram’s role as the definition of mass. Physical weights, cells, resistors, and other artifacts remain essential working and transfer standards; their values and stability still require calibration.

The Seven Base SI Units

The SI is structured around seven mutually independent base units, each corresponding to a distinct physical dimension. All other physical quantities are mathematically derived from these seven foundational building blocks.

The Historical Artifact Dilemma and the Need for Redefinition

To understand why the 2019 redefinitions were critical, calibration technicians must examine the metrological vulnerability of artifact-based standards.

The International Prototype of the Kilogram (IPK)

From 1889 until May 19, 2019, the kilogram was uniquely defined as the exact mass of a single physical object: the International Prototype of the Kilogram (IPK), affectionately known as "Le Grand K". The IPK is a right-circular cylinder (height equal to diameter $\approx 39.17\text{ mm}$) composed of $90\%$ platinum and $10\%$ iridium by mass, manufactured in London by Johnson Matthey and stored under three nested glass bell jars in an environmentally controlled vault at the International Bureau of Weights and Measures (BIPM) in Sèvres, France.

Official copies were distributed to national metrology institutes and compared periodically with the IPK. Differences of tens of micrograms among prototypes exposed the limitations of defining a unit using one artifact: comparisons revealed relative change but could not independently identify a change of the defining prototype itself.

The mass of the official national copies had systematically drifted relative to the IPK by approximately $50\ \mu\text{g}$ (parts in $10^8$). Because the IPK was defined as exactly one kilogram by international treaty, metrologists could not establish with absolute certainty whether the national copies were gaining mass (via surface absorption of environmental hydrocarbons and atmospheric mercury) or whether the IPK was losing mass (via microscopic surface wear during cleaning protocols and outgassing). Under an artifact standard:

• A change of the defining artifact would change the physical reference for the unit; it would not physically change other objects.
• An artifact standard cannot be independently realized by NMIs; national standards had to be periodically hand-carried across international borders back to France for recalibration.
• Loss of the IPK would have required an international recovery of the reference using comparison evidence and surviving copies. It would not erase all existing calibration records or automatically invalidate every past mass result.

Historical Transitions of Other Base Units

The kilogram was simply the last artifact to fall. Metrology had already transitioned several base units away from artifacts:

1. The Meter Bar (1889–1960): Replaced a platinum-iridium meter bar with a wavelength of light from a krypton-86 discharge lamp ($1,650,763.73$ wavelengths in vacuum).
2. The Speed of Light Definition of the Meter (1983): In 1983, the 17th CGPM redefined the meter as the path length traveled by light in a vacuum in $1 / 299,792,458$ of a second, fixing $c = 299,792,458\text{ m/s}$.
3. The Ampere's Classical Definition (1948–2019): Formerly defined as that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to $2 \times 10^{-7}\text{ newtons per meter}$ of length. This definition was an unachievable theoretical thought experiment that could never be realized directly in a laboratory without substantial systematic uncertainty.
4. The Kelvin's Triple Point of Water Definition (1954–2019): Formerly defined as the fraction $1 / 273.16$ of the thermodynamic temperature of the triple point of water. This tied temperature to an isotopic blend called Vienna Standard Mean Ocean Water (VSMOW). Minute isotopic variations in hydrogen and oxygen ratios ($^2\text{H}/^1\text{H}$, $^{17}\text{O}/^{16}\text{O}$, $^{18}\text{O}/^{16}\text{O}$) in water samples created irreproducible temperature shifts on the order of tens of microkelvins.