Secret Scotland

If it's secret, and in Scotland…

Today is Weights & Measures Day

20 May is Weights & Measures Day.

‘Weights & Measures’ covers a vast field in which standardised, legal, fiscal measurements are made, and standards are agreed nationally and internationally.

Even if you don’t realise it, there a hugely technical network maintaining references that are used to ensure your kilogram of potatoes, litre of petrol, and kilowatt of energy (gas or electricity) is delivered accurately and consistently wherever you are.

And I don’t mean by Trading Standards, by whatever name, I mean the actual value of that kilo or litre, or volt or ampere that determines a kilowatt (or I should really add time which needs an accurate second) to determine your energy consumption in kilowatt-hours. Gas is a little more complicated, as it has to be further analysed to determine how many kWh a given volume delivers as this can vary depending on the chemical make-up, and that needs yet more weight and measure.

This used to be fairly complex and exacting science (I know, I was one of only 300 or so people who specialised in this around the country).

But, as seems to be the case with all things these days, the addition of some computers and software dumbed this down, and there are many more people in this business today.

However, I think few of them understand things in the same way as we did in the past, and that aspect has become the province of a few better qualified experts in the field.

Ignoring the overly alarmist and dramatic headline, this article is still a reasonable look at just one ancient, venerable, and respected standard…

Hidden in a vault outside Paris, vacuum-sealed under three bell jars, sits a palm-sized metal cylinder known as the International Prototype Kilogram, or “Le Grand K.” Forged in 1879 from an alloy of platinum and iridium, it was hailed as the “perfect” kilogram—the gold standard by which other kilograms would be judged.

Why the Metric System Might Be Screwed

Le Grand K

Le Grand K

Sadly, Le Grand K has been losing weight, not a lot, but the accuracy demanded, the unexpected change has caused the odd raised eyebrow.

Without delving into detail, there has been a technological drive regarding physical standards in recent years, with the aim of replacing thing like pieces of metal (used to define length for example) with fundamental standard based on reproducible physical phenomena.

So the metre is no longer defined by a lump of metal that changes size with temperature, but is defined as the length of the path travelled by light in a vacuum in second.

Similarly, once a fraction of the 24-hour day (which can vary in length), the second has, since 1967, been defined as 9,192,631,770 cycles of the radiation that gets an atom of cesium-133 to vibrate between two energy states.

The important difference between the new and the old definitions is that they are considered to be invariant, and reproducible anywhere merely be following the same procedures.

And that where Le Grand K trips and stumbles, since it depends on variable things like mix, density, and purity of the elements used to create it, and the measurements of its dimensions. Not to mention contamination, and possible chemical effects on its exposed surface.

Last time I looked, there was at least one project (there are others though) intended to create the kilogram using other standards, such as the force generated by an electric current, and measuring that to create a kilogram equivalent that is bases on other precisely defined standards.

I’m not sure how close they are, as the method has to very accurate, and not subject to outside interference.


When I wrote the above, the work underway to change the definition of the kilogram was not quite complete, although it had all been done, it still had to be considered, agreed, and then implemented by all the relevant official institutes.

That was completed a few months ago, and the physical kilogram is no longer the master.

The kilogram is now defined by a method which is reproducible anywhere, should not vary when reproduced anywhere, so should not vary with time, like the old physical kilogram.

Note, I didn’t say it was EASY!

See the NPL’s explanation….

The kilogram is the SI base unit of mass

The impact of the SI redefinition

There’s a more reader-friendly explanation here…

It’s Official: The Definition of a Kilogram Has Changed

This time around, I’m in luck.

Back in 1990, metrologists changed some definitions. The result wasn’t any great difference in existing measurements (a volt changed by something like 1 part in 10 million), but for those if us working at the ppm (part per million) level, it was a real pain.

Anything which depended on electrical measurements and which had to have its uncertainty accurately reported had to have that reported accurately, and since 1 part in 10 million is 10% of 1 part in a million… we had to revise everything.

And this included temperature measurement too, since accurate temperature measurement is made using resistance sensors, that meant any change to voltage (or current) definitions changed too.

It all get very complex and inter-related, especially when clients want things explained and accounted for.

So, I’m rather glad I don’t have to do that this time.

20/05/2019 - Posted by | Civilian | ,

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