Measurement of Mass under the SI Units

C.P. (Chuck) Ramani, P.E, CBO

April 4, 2019

Today, almost all nations of the world use the SI (metric) system of measurements that includes kilogram for mass, the meter for length, kilometer per hour for speed etc. The United States is the only advanced nation in the world that still relies on the old English system of measurements i.e. pounds for mass, feet for length etc. But, even here, all scientific pursuits are based on the SI system.

For humans to manage their affairs efficiently, an accurate world-wide measurement system must be available. Whether the activity being monitored is one’s health, business, the environment or food, a repeatable and reproducible measurement system must be in place. For instance, when you buy an ounce of gold in Dubai or in New Delhi or in Los Angeles, won’t you need assurance that the gold purchased is indeed 99.99% pure and that the ounce you paid for is indeed an ounce. The only way we could be assured that we received what we paid for is the existence o modern metrology. Metrological practice relates to measurement science which includes weighing, measuring etc., and ensuring that such activities are traceable to a single set of international standards through an unbroken chain of cross-comparisons.

In late 1700s, a group of scientists in France decided to do something about the differing methods of measurements of mass, length etc., by standardizing the basic units of measurements. While their initial thrust was to ensure greater consistency of measurements within the scientific sphere, they were well aware of the crying need for the same from the commercial and industrial sectors.

In 1795, the metric system was thus introduced to promote world-wide uniformity in units of measurement. In 1875 an organization called the Bureau International des Poids et Mesures (the International Bureau of Weights and Measures or BIPM for short) was established in Sevres near Paris, France. BIPM is an intergovernmental, sovereign entity through which Member States act together on matters related to measurement science and measurement standards. BIPM is part of the International Committee on Weights and Measures (CIPM).

From the outset, CIPM decided to keep the fundamental base metric units to a bare minimum with the understanding that other units would be derivatives of the basic units. The seven basic metric system units relate to mass (kilogram), length (meter), quantity of substance (mole), temperature (Kelvin), luminosity (candela), time (second) and current (ampere).

In this blog we will talk about the international practice of establishing traceability in mass measurements. Standardization of the unit of mass (kilogram) first required the production of an international kilogram artifact from which other national clones could be made. To ensure long-term stability of the international kilogram mass artifact, an alloy of platinum-iridium was chosen. A world kilogram standard called the International Prototype of the Kilogram (IPK) was then manufactured under extremely controlled conditions and held by BIPM.

The primary role of the BIPM’s activity in Mass is to provide traceability to the IPK. The term ‘traceability’ in this context means a series of unbroken cross-comparisons from the instrument or mass being calibrated to the IPK to determine the needed correction factor to be applied to ongoing measurements with the instrument. BIPM also maintains a number of prototypes of the IPK which are used to produce new national prototypes and also to calibrate existing prototype kilogram.

To understand mass calibrations better, let’s assume that a beam balance at a local industrial laboratory in California requires calibration. The company will typically seek this service from a local instrument calibration laboratory whose accredited scope includes mass calibrations. Using their set of certified reference weights called the ASTM E617 standard weight sets, they will perform the calibrations. Class 1 and 2 weight sets provide a high level of accuracy and are ideal for calibrating analytical and precision balances. Weight sets of Class 4 and above offer a lower level of accuracy, but are affordable for calibrating compact portable balances and bench scales. The reference weights are traceable to the IPK based on an unbroken chain of cross-comparisons via the national prototype kilogram held by NIST, the national metrology institute.

Given the worldwide consensus plan to redefine the basic units of measurement in the International System of Units in terms of invariants of nature, calibrations involving mass will, in the future, be done utilizing the watt balance which works on the principle that the weight of the test mass is proportional to the amount of electrical energy required to support it i.e. the watt. Thus, after a certain date in the future, the IPK and its national clones will be ‘retired’ to scientific archives (

The watt balance was renamed in 2016 as the Kibble balance after its inventor, Bryan Kibble of the National Physical Laboratory in England. Going forward, the amount of energy required to measure 1kg will depend on a value known as the Planck’s constant. Metrologists and scientists will gather on May 20, 2019 to fix the Planck’s constant for mass measurements, after which an accredited calibration lab with a Kibble balance will be able to determine the mass of an object without worrying about traceability to the IPK or its cloned national prototypes.


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