# Calibration: Why is the standard kilogram losing weight?

October 18, 2007

If you think you have calibration problems, consider this: The world standard kilogram, a cylinder of platinum and iridium alloy which was cast in 1889, is apparently losing weight. There are reports that its weight has decreased by about 50 micrograms when compared with other copies kept around the world. The standard kilogram is kept in a vault at the International Bureau of Weights and Measures in Sevres, France, southwest of Paris. Regardless of what is or isn’t happening with the other references around the world, this one is THE kilogram and it’s the one that seems to be changing.

Scientists are baffled. They can’t really tell if the others are getting heavier or the master one is lighter. Nor can they figure out why the weight has changed at all. Of course, let’s be practical: 50 micrograms is described as the weight of a fingerprint.

The larger issue is the nature of standards. The kilogram is one of the last to depend on an actual artifact. Most others have been redefined. For example, the idea of defining a meter with a marked stick went out in 1983 when it was changed to the distance that light travels in 1/299,792,458 of a second. Keep that in mind the next time you have to calibrate a tank level sensor. Ask your boss if you can borrow a stopwatch.

Along the same lines, Ron Fox, a retired physicist, and Ted Hill, a mathematician, both from Georgia Tech, have suggested the gram be redefined as the mass of exactly 18 x 14074481 (cubed) carbon-12 atoms. I find that much more helpful. Go ahead and start counting. Let me know when you’re done.

—Peter Welander, process industries editor, PWelander@cfemedia.com , Control Engineering Weekly News