The standard that needs some cleaning

One of the last “arbitrary” standards in physics is securely stored in a lab in Paris. I say “arbitrary” as it is not based on a physical constant. Instead it is “just” a chunk of platinum, or more correctly, a precisely manufactured piece of platinum and iridium that defines by itself the “kilogram”.

It’s called the International Prototype Kilogram, and was made in 1879. At the time, 40 exact copies were made and distributed across the world. This helped to keep track of the standard and of the calibration of instruments, without having to handle the standard itself too much. Handling means contamination, because even under very controlled circumstances and with the necessary precautions, stuff will stick to it during the process.

It was already known that the copies, with one exception, were gaining weight in respect to the “real” standard, presumably because they are handled more than the original. However, after careful investigation of the surface of the standard itself, it too was gaining weight due to contamination. It amounted to just a couple of grains of sand (50 micrograms per century), but that is enough difference so that action needs to be taken.

But how do you clean such a thing, in a standardized way? Just wiping it with baby wipes will likely a) leave trace amounts and b) might take off some of the original material. Now, after 20 years of study, Peter Cumpson, with the incredibly cool job as a metrologist, and others have devised a standardized method that can be applied to the standard and all copies with the same reproducible effect. As described in Metrologia and reported by Scientific American, the metal gets exposed to ozone and ultraviolet light, removing all carbon contamination. Not your average cleaning procedure …

It’s a start nevertheless, but it does not remove for instance mercury contamination (previously used in lab thermometers) which binds with the platinum. A better solution to avoid making and safeguarding an arbitrary chunk of metal would be to define it based on physical constants, as the second is now defined. For a good overview of current attempts, please check Wikipedia. Candidates include a watt balance (measuring the electric power needed to oppose a given weight) or counting a certain number of atoms (Avogadro Constant) but they are as of yet not precise enough. To put that in perspective, the error on the standard kilogram is 20 parts per billion (or 20 micrograms), and proposed methods do not yet improve on that.

So it seems that the cleaning method is a nice stopgap for a certain time to come. It has not yet been applied to the standard, and neither to one of its copies. The standard itself comes out only once every 50 years, and there are three independently kept keys necessary to get access to it in the vaults of the International Bureau for Weights and Measures. Just to show that our scientific measurements are maybe as important and valued as our gold reserves…

Leave a Reply

  1. I thought a Kilogram was based on a given quantity of water (1 litre). I suppose that doesn’t give you the necessary precision, though.

    • It used to be the inverse, ie the litre (or in SI terms the cubic decimetre) was defined by the kilogram standard for water at 4°. But apart from the temperature, there is also pressure and isotopic content to consider. So too complicated…

  2. But surely the act of handling the copies would see some of the surface wear away at a molecular level (which is all we are concerned about) so that would mean that the copies, which are being both handled and contaminated are more likely to be closer to the original weight (or do I mean mass?) than the original that is contaminated but not handled? Interesting conundrum.

    • Interesting. As this graph on Wikipedia http://en.wikipedia.org/wiki/File:Prototype_mass_drifts.jpg shows, prototypes 34 and 35 did indeed decrease. Perhaps some professor used them as paperweight …

      I actually think these prototypes were not handled as much either, serving only a couple of times to calibrate other copies. But as the contamination with mercury shows, just being “in the lab” was probably enough.

  3. I think that’s the whole point of the standardised cleaning method; to avoid handling the weights as much as possible.