OK, it's not original, but when somebody asks me about units
of measurement and how to manage them, I'm minded of the story
about that now infamous confusion between 'Newton seconds' and
'pound-force seconds', which saw the Mars Orbiter plunge heavily
into the Red Planet, a few years ago. Of less import is the apparent
'short' measure of a US 'pint' of beer, or the Mercedes built in
the USA with a fuel gauge calibrated in US gallons, giving the
impression that it was about 20% thirstier than its European counterpart!
Of course, all this does suggest that the international scientific
community has been a bit lax in pushing for a universal standard,
but whose fault is it? The eighteenth century French Academy of
Science gave us a lead with the SI (Systeme International d'Unites)
system, which is now very widely used. But our friends across the
Atlantic are quite happy with their version of the British Imperial
system, thank you very much, and are very reluctant to give it
up. Given that the USA is a formidable force in science and engineering,
there's not really very much we can do about that, at the moment.
So, how do we cope with the differences?
For a start, the Internet has made conversion a relatively simple
procedure, thanks to the many online interactive converters that
are freely available to browsers. I have one or two favourites;
in particular, FootRule.com (www.footrule.com), which, in addition
to the converters, has a wealth of fascinating information about
units and their origins. A couple of others worth trying are Science
Made Simple (www.sciencemadesimple.com) and OnlineConverters (http://onlineconverters.com).
However, while these may be fine if you dealing with a relatively
small number or parameters, what do you do when there are literally
thousands of metric and imperial measurements to keep track of?
Get out your pocket calculator and prepare for a long, hard slog
is one answer! You could use Excel if you're working with just
a few, basic units, but it's clumsy - first you have to install
and enable the Analysis ToolPak, then select the Convert function,
then refer to the Excel Help system to identify the codes you must
use ("uk_pt" for British pint, "ozm" for ounce,
etc) then drag or copy the command to all the relevant cells.
The only calculation software I've been able to find that is truly
units aware is Mathcad (http://mathcad.adeptscience.co.uk). Unlike
the more "heavyweight" maths packages, such as Maple
(http://maple.adeptscience.co.uk) and Mathematica (www.wolfram.com),
where you have to load a group of commands to specify and convert
units, Mathcad incorporates hundreds of built-in engineering and
scientific units so you can perform conversions on the fly.
Even better, Mathcad automatically checks that you are using units
consistently, thus helping you avoid those embarrassing and costly
units-based errors referred to earlier. Mathcad lets you associate
units with numbers, and carry and balance them throughout your
calculations. But it prevents you from adding dimensionally unequal
quantities. It won't let you, for example, add m^2 and m. If you
try to do this, it'll stop dead and won't numerically evaluate
the equation. It will even highlight the equation (or portion of
the equation) in which the error occurs - with an error message
("The units in this expression do not match") in plain
English! If the error occurs in a function that's the result of
a series of nested functions, there's an error-tracing tool that
locates the function or equation in which the error originally
occurs.
You can change the units of a result to your own required units
very easily - simply type your required units into a placeholder
and Mathcad does the conversion. In the latest version (Mathcad
12) you can add your own unit types and define which units you
want as default. For example, if you want to work in inches rather
than feet, just specify them as your default and all your subsequent
calculations will be in inches - you won't have to perform repeated
conversions at every step.
Dr Know's recommended download is the Mathcad 12 brochure - download yours today.