GR Metrology

The Kilogram will be Redefined in 2018: Here’s What You Need to Know


“It’s not obvious that it’s a big deal, but it’s a big deal,” Chief of Quantum Measurement at the National Institute of Standards and Technology (NIST), Jon Pratt, said. His comment regarding a platinum and iridium cylinder that sits under lock and key in Serves, France. This cylinder is no ordinary mass, it’s THE kilogram, commonly referred to as “le grand k,” the weight that weighs exactly one kilogram, and the single standard by which all other kilogram’s are measured against. THE kilogram was crafted in 1889 as the golden standard for the weight and now resides at the International Bureau of Weights & Measure (BIPM).

The kilogram is the only SI unit defined by a manufactured object and as heavily revered as it is, there’s a problem. Scientist all over the world have recognized it. As a result, the kilogram is being redefined and we’ve got everything you need to about it.


NIST oversees weight and measurement standards in the United States and has spent the past few years diligently working in part with international effort to redefine “le grand k” based on a fundamental universal constant. They are seeking to use a physical quantity in nature, similar to the speed of light or electric charge of a proton, opposed to a manufactured mass of metal. A unit that never changes regardless of location or time. Essentially, changing this standard from an object to an idea.

But what’s wrong with the kilogram as is? Well, the standard kilogram at BIPM is losing weight as we speak. It was found to be lighter than it’s once identical “witnesses”, by the approximate weight of one eyelash.  The BIPM prototype may be losing mass due to the impurities in it’s metal or the witnesses are gaining mass by accumulating contaminants. Either way, this method of calibration has caused problems all around the world, but NIST’s solution could take this 19th century practice to a 21st or even 22nd century process.


After discovering the kilogram’s imperfections, the 2014 General Conference on Weights and Measures was held to discuss the redefining of the kilogram based on the Planck’s constant concept. This is a value from quantum mechanics that describes the packets energy comes in. The value of this constant was unknown during 2014 and no further decisions could be made regarding the kilogram. The team at NIST has since spent the past few years preparing for the 2018 General Conference by trying to find a number accurate enough to define Planck’s constant.

They have been using a Kibble balance, formerly known as a Watt balance, for this task. Instead of balancing scales with weights, the Kibble balance uses electromagnetism. If scientists know the current, velocity and voltage of the Kibble’s coil as it is pulled, the Planck constant can be determined with extreme precision.

After 16 months of work, NIST had finally announced their findings. Planck’s constant was 6.626069934 x 10−34 kg∙m2/s with an uncertainty of 0.0000013%. In 2018, the next General Conference on Weights and Measure will take place where the scientific community will draft a resolution to redefine the kilogram based on this new found constant.


The bottom line is that this massive and innovative discovery won’t affect you much at all. Your scales and metrology equipment will continue to be calibrated by Grand Rapids Metrology’s set of standards which are calibrated by the State of Michigan which are calibrated by NIST and A2LA standards which are calibrated by the kilogram at BIPM in France. The only aspect of that equation that will change 2018 and beyond is the kilogram will transform from a physical object to an very precise idea.

This modernization will prevent any error in the standard of the kilogram. Deteriorating metal or added contaminants will no longer be a problem. Rest assured your equipment will remain calibrated to the highest standards and your piece of mind will stay intact during this transition.

For even more about the redefining of the kilogram, check out this article from NIST’s website.

Reference: The Washington Post: Speaking of Science



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