Poor knowledge of SI Units

Every day, one is confronted with errors, omissions and plain laziness in their use

As a professional engineer, researcher and lecturer I am constantly annoyed by the poor use of SI Units, or to give it its full name 'The International System of Units', derived from the French Système International d'Unités.

SI Units are the backbone of all Engineering, Science, Technology and Mathematics; however, they are not well understood by students of the subject, many professional engineers or even publishers such as PE (check out last week's edition).

Every day, one is confronted with errors, omissions and plain laziness in their use!

Let's look at a simple example taken from a typical engineering website:

Size: 68mm x 35mm x 11mm
Weight: 0.055KGS
Voltage: 7.4v
Current: 16amp
Torque: 19g-cms

Can you spot the 17 errors?

As we should all know the SI Unit system is built upon seven base units: length (m), mass (kg), time (s), electric current (A), thermodynamic temperature (K), amount of substance (Mol) and luminous intensity (Cd), a number of coherent derived units, for example velocity (m/s) and 22 specially named derived units, for example frequency (Hertz), named after the German physicist Heinrich Rudolf Hertz (1857-1894).

Of course, there are perhaps more than 3,000 specific units in use in all areas of engineering and the physical sciences, so I would be the first to state that the area is complicated, with many rules and regulations. However, the fundamental definitions and use should be well understood and correctly applied.

As a challenge to all self-respecting professional engineers try this free online test: http://www.sporcle.com/games/si_units.php

If you score less than 70% you really need to do some revision.

Dr Stephen Prior