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Is the kilo losing weight?

14 January, 2016

The kilo is one of the last seven basic units of the metric system which is still defined by a physic object – unlike time, distance or temperature units. For decades now, meteorology specialists have been trying to replace the definition of kilo for another one.


The question is: Why?

Since 1889, the kilo has been defined by an object that is almost a museum piece of art. Known by the English IPK – International Prototype Kilogram -, it is a platinum-iridium cylinder that has been residing for 126 years below three glass cases, inside a safe at the International Committee for Weights and Measures in Sèvres – Paris.
The scientists have been complaining about the fact that the cylinder has been losing weight, or its rigor is not what they expected.


The question now is: is the kilo losing weight?

In the last 122 years, the prototype has been placed outside its storage just three times to be calibrated. Cleaning, polishing or touching on the international prototype are strictly controlled actions because any of those can change its weight. When compared to other test prototypes in laboratories of other countries, the cylinder presents a difference of 50 micrograms. The loss of weight, in the amount of 50 millionth of grams, is the equivalent of a grain of sand, but for the metrologists, who are explaining, the change represents a preoccupying disparity.

A theory for the loss of weight is that, over the years, the platinum-iridium alloy may have emitted some sort of gas that was incorporated to the metal bloc, made in London in 1879.


Will kilo really cease to be kilo?

More recently, in 2011, the International Committee for Weights and Measures (CIPM) came to a formal decision of underlining a new definition of kilogram in an essential constant of quantum physics: called the Constant of Planck. In theory, its definition is a little complex but, in the end what matters is that it should lead to a new definition, stable and precise, of kilogram.

As a national and international leading reference in the weighing sector, Cachapuz is attentive to this “heavy weight” subject, awaiting for new developments in this changing process.