Setting the standard for coffee brewing

Chemex, 30g, coffee

A Chemex, how much coffee do you need to make a good cup (or two?)

Serious coffee drinking requires a serious attention to preparation. Various online guides (such as this one from specify the ratio of water to coffee that you need and some will dictate the exact quantity of coffee that you should grind ready for your brew (30g for a standard, 500ml Chemex). Different brew methods require different amounts of coffee. Some will insist that the correct ratio of coffee to water is essential for a good coffee. So how can we ensure that 30g of coffee is really 30g? How do I know that what you measure as 30g is what I measure as 30g? It is a question that reveals a fascinating answer. The measurement of mass, the definition of the kilogram, is the only unit of measurement left for which we still use a physical standard as the reference.

This means that there is a physical lump of metal (it is actually a platinum-iridium alloy) that is sitting in a lab somewhere (Paris) against which all our definitions of mass are referenced. If you were to weigh out 1 Kg of coffee, your scales would, ultimately, be referencing this 1Kg lump of platinum-iridium in Paris. My scales reference the same standard and so we can be sure that, assuming our scales are accurate, your 30g is equivalent to my 30g. Many years ago (in 1884), forty replicas of this standard of measurement were made and distributed throughout the world. The idea was that rather than have to always refer to the Parisian standard, there would be a more local ‘standard’ that people could refer to. The problem of course is that the standards diverge, they have to be regularly re-calibrated so that the Kg in Paris weighs the same as the Kg in London (well, just outside London in Teddington, at the National Physical Laboratory).

gold weights, standard weights, not Kg

A set of gold weights from China in the British Museum collection. © Trustees of the British Museum

The reason appears to be because the standards get dirty. The surface of the metal adsorbs contaminants from the air which make the standard seem heavier. Admittedly, this may not be by much, only perhaps tens of μg, but over many tonnes, this small difference is going to add up. And if you trade in commodities (such as coffee beans) and are paying by weight of coffee then such differences, in large amounts, may be costly. So what is the solution? One method involves finding new ways to clean the standards so that they are contamination free. A more long term solution is to move away from measuring relative to a physical standard at all. After all, length is no longer measured with reference to a stick in a lab but with reference to the distance that light travels in a certain amount of time. Research is now being done into exactly this in metrology labs around the world. At some point in the not to distant future, it is very likely that the Kg will be defined with reference to an electrical measurement, for example, rather than with reference to a physical block of metal. For the meanwhile, we have to hope that the standards labs around the world keep their blocks of metal very clean otherwise, how would we ever expect to get the correct amount of coffee in our Chemex?

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