16 February 2010

A short introduction to the metric system

We do use the metric system, or, more specifically, the Systeme Internationale, in doing science.  This causes no grief for any countries outside the US.  But about half my readers are in the US, so there is a constant issue about using the scientific units to talk about scientific topics.  (I'm sufficiently native that when discussing non-scientific topics, I do normally use feet/fathoms/rods/miles/....)

For the use of readers who tend to freeze when seeing metric units, here is a quick and dirty guide.  Emphasis on quick and dirty.  It will not be accurate, but it will give you the right general sense.  Good enough, at least, for Science Jabberwocky purposes.  It should also do for the non-US readers who are baffled when I use only the Imperial units.  (I try to avoid that, but probably lapse.)

1 inch = 2 cm (centimeters)
1 meter = 1 yard = 3 feet
1 kilometer  = 0.5 miles
1 cm = 0.5 inches
1 mile = 2 kilometers

0 C = 32 F = freezing
10 C = 50 F = jacket weather
20 C = 68 F = room temperature
30 C = 86 F = hot weather
37 C = 98.6 F = body temperature
40 C = 104 F = blistering heat
-40 C = -40 F

1 liter = 1 quart
4 liters = 1 gallon

1 kg = 2 pounds
5 g = 1 tsp (water)
1 kg = 1 liter (water)

1 meter/second (m/s) = 2 mph

I stress, these are not accurate figures (except for the temperatures).  If you want accurate, you should be working with scientific units in the first place.  Failing that, be using a resource that has the full precision conversions.  1 meter is actually 39.37 inches, not the 36 listed above.  The worst approximation is between mile and km.  1 mile = 1.60934 km, not 2.


Peter said...

Slightly surprised you didn't give 1 in = 2.5cm and 1 mile = 1.5 km. Both would be much more accurate and just as easy to remember as the versions given.

Dunc said...

This may also be useful: http://xkcd.com/526/

jg said...

Dunc, that's a great link; I intend to look at it some more.

Here's a site I've used to sort out various units I see on pressure that get used in climate studies: http://www.unit-conversion.info/
or Unit conversions

About this time last year I was reading Ray Pierrehumbert's online Principles of Planetary Climate and realized what I really needed was a study guide of temperatures (in Kelvin) and pressures most relevant to climate. I started one here and would welcome suggestions as to what to add:

or Planet temperatures


Anonymous said...

The thing that kills me is the use of short "tons", given that in Europe they now often spell metric tonnes as "tons" as well. I find myself needing to always double check every time I have to work with a ton or tonne... I've decided that from now on, I should just use Mg instead of tonnes. (and never short or long tons, of course!)


S2 said...

I was so glad when the UK decided to go metric. I spent far too much of my childhood trying to learn by rote such things as fathoms, grains, quarters, hundredweights, poles, chains, furlongs, leagues, stones, nautical miles, gills, fluid ounces, perches, roods, and so on.
Even our currency was a headache (shillings, florins, crowns and half crowns, farthings, guineas, etc.)

We went (sort of) metric in 1971 but, being British, we did it half heatedly - so it is still quite common in builders yards to hear someone asking for "three metres of 2 X 1 (inches)". Newborn babies are still generally announced in pounds and ounces, people still usually quote their height in feet and inches. And of course our road signs and speed limits are still in miles and mph.

We haven't really gone metric (outside of science & engineering) - we've just absorbed some new measurements into the system.

Robert Grumbine said...

So you'd appreciate the race I started in my running club -- on New Year's day, we run N rods, or N fathoms, where N is the year. I ran the 2010 rods race this year.

I've discovered that, although numbers people find 1.5 and 2.5 quite easy to remember, non-numbers people find them both much more difficult than 2.

Thanks for the xkcd link. I've been reading them off and on for some time, and with enjoyment.

jg: thanks for the links. Your planetary temperatures illustration is one I might want to borrow a simplified version of at some time.

marcus: tons, tonnes, long tons, short tons, ... it's a mess. But be glad you aren't trying to guess which ounce, or grain, or pint/peck/firkin/hogshead/... is being used at the moment :-)