Mon Jan 7, 2013, 09:32 PM
UnrepentantLiberal (11,700 posts)
Why the kilogram is getting heavier
According to researchers at Newcastle University in the United Kingdom, the kilogram is very likely getting heavier. How can this be? Mainly because we’re talking about the definitive kilogram, the International Prototype Kilogram (IPK) kept at the International Bureau of Weights and Measures in Paris. But because this is the kilo against which all kilos are defined, in a theoretical sense at least, all kilograms will technically be heavier too.
The IPK is a small cylinder of platinum-iridium alloy, about 39 mm (or an inch and a half) in both height and diameter. Using a unique Theta-probe XPS machine, Professor Peter Cumpson and Doctor Naoko Sano of Newcastle University have analyzed surfaces similar to those of the IPK to quantify the build-up of hydrocarbon contaminants. Their research indicates that the IPK is likely to have gained tens of micrograms in mass since the standard was introduced in 1875.
XPS stands for X-ray Photoelectron Spectroscopy, a process which involves irradiating a material with X-rays, and analyzing the quantity and energy of the electrons that are emitted, granting insights into the surface chemistry of the material, and the differences in it before and after some process (such as cleaning). What makes the researcher’s XPS machine unique is its argon cluster ion gun which emits charged clusters of argon, each containing around a thousand atoms. It’s this component that allows analysis of the organic layer – the gunk – without damaging the underlying inorganic material.
“It doesn’t really matter what it weighs as long as we are all working to the same exact standard – the problem is there are slight differences,” said Cumpson, in a press release put out by the University. “Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original.” The 40 replicas Cumpson refers to were made in 1884, and distributed across the globe to help countries conform to the standard.
5 replies, 1238 views
Why the kilogram is getting heavier (Original post)
Response to Scuba (Reply #1)
Mon Jan 7, 2013, 10:24 PM
mindwalker_i (4,407 posts)
2. Technically, we will mass a little less
Kg is a unit of mass - the amount of stuff. It's only convertible to weight since we all hang out in a relatively constant gravity field.
Response to UnrepentantLiberal (Original post)
Tue Jan 8, 2013, 07:09 AM
Jim__ (10,593 posts)
4. The kilogram is ... the only SI unit that is still directly defined by an artifact.
Can the kilogram be defined by a fundamental physical property that can be reproduced in different labs? I'm not sure how difficult it is to reproduce distilled water with the correct isotopic composition. From wikipedia:
The kilogram is the only SI base unit with an SI prefix ("kilo", symbol "k") as part of its name. It is also the only SI unit that is still directly defined by an artifact rather than a fundamental physical property that can be reproduced in different laboratories.
Modern measurements of Vienna Standard Mean Ocean Water, which is pure distilled water with an isotopic composition representative of the average of the world’s oceans, show it has a density of 0.999975 ±0.000001 kg/L at its point of maximum density (3.984 °C) under one standard atmosphere (760 torr) of pressure. Thus, a cubic decimeter of water at its point of maximum density is only 25 parts per million less massive than the IPK; that is to say, the 25 milligram difference shows that the scientists over 214 years ago managed to make the mass of the Kilogram of the Archives equal that of a cubic decimeter of water at 4 °C to within the mass of a single excess grain of rice.