Charlie P, a regular if occasionally critical contributor to MV, promised he would on his return from holiday give his thoughts on “the flip side of the SI as far as the average man in the street is concerned”. These now appear below. They are of course not endorsed by the Editors of MV, but may provide food for thought.

Lack of empathy with non-scientist human users

* Not optimised to accommodate human frailties and weaknesses.

* Not accessible to all strata of society.

* Commonly used units are not based on a human-friendly scale or with reference to readily and universally available cue objects.

* The units do not lend themselves to accurate approximation without the use of measuring tools.

* The strict base-10 system does not facilitate divisibility by multiple factors (whereas base-12 does).

* The strict base-10 system does not facilitate mental arithmetic, which is easier with fractions than with decimals.

* The size of the metre derives from a ten-millionth of the distance from the equator to the north pole through Paris! Hardly a human scale.

* The size of the gram derives from the weight of one-millionth of a cubic metre of water! Again, not a human scale.

* The rules of writing metric units are so complex and so obtuse that very few people outside of academia are competent in using them. Examples of incorrect usage can be found on government websites, in newspapers, on food packaging, on road signs, on measuring instruments, and even on the UKMA website.

Irregular and inconsistent

* The non-decimal traditional second, based on 1/86,400th of a day, is used as the base unit of time. For some reason(!) the redefinition of the second to be 1 day/100,000 was rejected.

* The base unit of mass (kg) already has a prefix, so breaks the prefix use norms, and adds a level of complexity to the teaching and understanding of the system.

* The symbol for the litre (L or l) is not case sensitive, unlike all the other symbols.

* Land area is measured in hectares (ha), an irregular use of the “hecto” prefix on the now non-existant “are” unit.

* Multiple names for exactly the same unit exist. Examples: 1 sievert = 1 gray, 1 tonne = 1 Gg, 1 litre = 1 dm[^3] (one cubic decimetre), 1 hectare = 1 hm[^2] (one square hectometre).

* The most common speed unit does not have a power-of-10 factor with the SI unit of speed: 90 km/h = 25 m/s.

* Time units (day, hour, minute, second ) and angle units (degree, minute, second) are not decimally power-of-ten factored.

Difficult to use

* Long and complex multi-syllable unit names, for which abbreviations are prohibited. Examples: “kilometres per hour”, “watt per square metre steradian” (unit of radiance).

* Case-sensitive unit symbols can make stencilling/Dymoing impossible. Try spray-stencilling “25 kg” on a sack of potatoes using a lowercase-free stencil.

* Duplicated symbols. Examples: d = deci, d = day. h = hecto, h = hour. m = milli, m = metre. T = tera, T = tesla.

* Reliance on the use of non-roman characters (mu, omega, degrees) and superscript/subscript text (for squared and cubed) can make typing/stencilling,Dymoing impossible. Try making a set of Dymo labels to stick on a rack of resistor storage drawers.

* Case sensitivity of symbols, coupled with the use of non-roman characters, introduces a massive risk of writing and transcription errors. Example: a drug dose of 1 mg (milligram) is one-thousand times larger than a drug dose of 1 [mu]g (microgram).

Too much room for ambiguity and confusion

* The symbol “m” is used for both metre and milli.

* The symbol “T” is used for both tesla and tera.

* The litre has two symbols: “L” and “l”.

* “Mm” “mm”, “da” “dA”, “Pa” “PA” “pA”.

* “Mil” can mean millimetre (mm), millilitre (ml or mL) or milligram ([mu]g).

* Milligram (mg) and microgram ([mu]g) are readily confused.

* Millimetre (mm) and micrometre ([mu]g) are readily confused.

* The SI is defined in French, and its English translation uses unidiomatic terms.

* The writing of metric measures is so fraught with the danger of ambiguity that a whole section of the “SI Brochure” is dedicated to trying to codify it.

Too bureaucratic

* Units are strictly categorised as: “SI base units”, “SI derived units”, “Non-SI units accepted for use with the SI” and “Other non-SI units not recommended for use”. To use SI correctly, users need to know which unit is in which category, and which category is permitted for their particular use.

* The use of the most natural and commonly used abbreviation for “kilometres per hour” (kph) is prohibited by the SI.

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