The hierarchy of measurement

* 3 min read
One of the things that people constantly identify as important when looking at a food label, and before making a decision to purchase, is what the weight of the food is. It isn’t unreasonable for a consumer to be interested in how much of something they are getting for their money.

Likewise people attach similar importance when buying items that are sold by their length. What is perhaps less likely is that the consumer will consider what the origins of the weight or length indications are, how the weights and measures are validated and what the checking weights and measures are validated against. In short, why is it that they can rely on the quantity stated? In this article, the origins of weight will be looked at with length being the subject of a future article.

In practical terms, the Weights and Measures Act 1985 provides the hierarchy of both key weights and measures in the UK and the Weights and Measures (Local and Working Standard Weights and Testing Equipment) Regulations 1986 provide for the tolerances of working and local standards of weight with other similar regulations accommodating the same principles for measure. The certainty therefore comes from traders’ weights and measures being tested against the ‘working standards’ of local authority weights and measures inspectors. These working standard weights and measures are in turn tested against a more accurate set of weights held by the local authority in controlled conditions; these weights are known as ‘local standards’. The local standards are in turn tested against the ‘UK secondary standards’ which are even more accurate and are held by the National Weights and Measures Laboratory. The ‘UK secondary standards’ are tested against the ‘UK primary standards’, held by the National Physical Laboratory and these standards are in turn bench-marked against the relevant International Standards. For weight, this is the ‘international prototype kilogram’ [Held at the International Bureau of Weights and Measures (BIPM) near Paris].

Originally, weight was based on the cereal grain (usually wheat) as it was a universal entity and varied little. The Egyptians devised stone weights based on a validated number of grains and by the time of the Romans such weights facilitated commerce relating to millions of tonnes of trade on a daily basis.

By the end of the 13th century the world had hundreds of different weights and nearly all of them were based on the grain. In England, the pound had been inherited from the Roman Empire and originally contained 12 ounces which were equivalent to 437 grains of barley. By 1875 the metric system, established in France, was being widely adopted. English engineer Johson Matthey cast the ‘grande k’ and it has been housed in the ‘Bureau International Des Poids et Measures’ ever since and is still the master weight, ‘the’ kilogram, and the only one existing as a physical object, though it is believed to have decreased in weight by the equivalent of a grain of sand in its lifetime.

More recently, recognising the need to have a unit of measure defined precisely against an unchanging standard, two teams have been trying to redefine the kilogram against more fundamental physical constants. In the US, ‘Team Watt Balance’ uses electromagnetism rather than gravity to balance a mass. Their dream is to redefine the kilo based on the fundamental link between mass and energy (Planck constant). In Germany ‘Team Silicon Sphere’ is trying to count every single atom in a ball of perfect silicon-28 (Avogadro constant). Presently, Team Watt Balance has measured Planck’s constant to within 13 parts per billion which is deemed accurate enough, hence, the definition of kilogram is expected to be redefined in these terms, a vote of finality is expected from the General Conference on Weights and Measures in November 2018. So, when consumers examine the weight on their shopping items they are part of a continuing search for certainty and consistency in a that has a history the length and time of civilisation itself.

 


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