Saturday, December 08, 2007

Metrology

Earlier this year I went to a meeting at the OAS about metrology.

Metrology? Yaaaawn. Isn't the definition of units settled? Isn't that something you do in grade school?

Well, yes, and no. The study of units (metrology) mostly has to do with commerce - just as it did over two millennia ago when rulers wanted to ensure that merchants were measuring properly (and probably the motivation was not to protect the customer, but to ensure proper payment of taxes).

When you buy a thermometer, you'd like to know that it was accurate, right? Not taking your child to the hospital for an actual fever of 105 when it shows up as 103 on your thermometer could have serious repercussions. And these serious errors are out there: most thermometers in the world are made in (surprise, surprise) China. In a recent test carried out by the Uruguayan metrology lab, over 20% of the 120,000 annually imported thermometers for home use were found to be seriously defective. Similar failure rates were found for sphigmomanometers and other medical measuring equipment. (Source: Alexis Valquis, German Federal Technical and Physical Institute, PTB)

There are also cases where mismatches between standards can have large economic repercussions. The market for Canadian white paper is about $5 billion/year, with a great deal of this being in the European market. However, the North American and European 'standards' for paper 'brightness' differed by 0.5% to 1% on the same papers, and this implied an extra annual cost in bleach to Canadian mills of about $65 million to meet the European 'standard.' An intercomparison and recalibration removed the problem, which was completely artificial.

Even when you decide to use a standard from which to measure, you have to make sure you are using a common standard, since there are many different 'standards' out there. On building a bridge over the Rhine between Germany and Switzerland at Laufenberg, construction was almost finished when both sides realized there was a 54 cm height mismatch between the sides. They had known there was a 27 cm difference between the national standards, because the Swiss used the Trieste sealevel standard, while the Germans used the Amsterdam standard. However, since 54 = 27 x 2, someone forgot to check which one was actually higher than the other, and the corrections were applied in the wrong direction. You would think that two countries with such careful engineers would have caught this before it became a really expensive fix. (Source: Swiss Government website (in German))

Here's a good example of where you might be concerned (besides driving over a bridge where the sides didn't match): the amount of lead in wine. Samples from the same batch of wine were sent to labs all over the world, and the labs were asked to measure how much lead (Pb) was in the sample. Here's the spread in the reported results:

The stunning part of this is that the 10% spread is the narrow grey line, and the actual spread is well over 50%. The good news is that the national labs responsible for most of our safety got it right to within the 10% band. (Source: J. Anal. At. Spectrom., 2001, 16, 1091–1100, DOI: 10.1039/b103248h)

But what do you do if the 'standard' is actually changing? Incredibly, this is actually happening to the kilogram. As you might expect, for a long time the standard has been an actual physical object: a platinum and iridium cylinder cast in 1889 that is kept under high security at BIPM in Paris, along with six official copies (image below). Along with the original, many duplicates were made, which were shipped off to many countries existing at the time for them to use as their national references. The availability of many duplicates allows some sophisticated statistical studies, and they have allowed the rather odd conclusion to be drawn that, despite the security, it can be reliably demonstrated that this cylinder has lost about 50 micrograms over its lifetime. This change may seem small, but it has huge implications for the metric system, since there are many other derived units which depend on the base unit of the kilogram. There are all sorts of efforts underway to define the kilogram using physics rather than a physical object, as has been done with the meter (the meter is now how far light travels in 1/299,792,458 of a second, rather than the distance between two marks on a 'reference stick' kept in Paris). There is currently a struggle between two camps: one which wants to generate a new object - an ultraprecise sphere of ultrapure silicon, and the other which wants to simply agree on a specific number of Carbon-12 atoms (Source: Eurekalert article).

Another point about the metric system - there are three countries that have failed to convert: Liberia, Myanmar and the good old US of A. Good company to keep. And how do these countries define their own standards in these older units? They refer to the metric system standards, of course.

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