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Pet Peeves -- Abuses of the metric system prefixes and units
How often have you seen "mS," "Khz," or "Mg," and wondered what exactly it was that the poet meant?
As "beguiling" as the metric system appears to be, it's actually quite simple, and has rules that can be followed if a little thought
is applied.
In the above examples, we see "milli Siemens" which is a measure of electrical conductance (when the author most likely meant milliseconds,
"ms"), we see something that starts with a unit of temperature, Kelvin, followed by garbage (the author probably meant "kHz"), and then
we see megagrams (the author probably meant "mg" for milligrams).
Don't get me wrong on the last one -- I actually saw "Mg" on a crane indicating its carrying capacity. :-)
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Prefixes
Here are some of the official metric prefixes:
| Abbreviation | Full | Multiplier |
|---|
| a | atto | 10-18 |
| f | femto | 10-15 |
| p | pico | 10-12 |
| n | nano | 10-9 |
| u | micro | 10-6 |
| m | milli | 10-3 |
| c | centi | 10-2 |
| d | deci | 10-1 |
| D | deka | 101 |
| h | hecto | 102 |
| k | kilo | 103 |
| M | mega | 106 |
| G | giga | 109 |
| T | tera | 1012 |
| P | peta | 1015 |
| E | exa | 1018 |
Note that the "u" for "micro" is a bit of a compromise between being able to express the Greek symbol (µ) in plain text.
And why do electronics types use ridiculously small and ridiculously huge units of Farad to express capacitance? I've
seen "160,000 uF" (when 160 mF would be fine, or, even though I don't like leading "0." constructs, "0.16 F" would have worked) and "0.001 uF" (when 1 nF would do).
Or, as seen on "super bright LEDs", they'll tell you it has "5000 mcd" -- why not just 5 cd?
This would be like me telling you the odometer reading on my car is 76,000,000 meters, when 76 Mm would do :-)
Drug companies do this too -- 1000 milligrams! Wow, you mean 1 gram? :-)
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Units
Here are some of the common units that you might see day to day:
| Abbreviation | Full | Is a unit of... |
|---|
| Hz | Hertz | frequency |
| N | Newton | force |
| Pa | Pascal | pressure |
| J | Joule | energy |
| W | Watt | power |
| C | Coulomb | electrical charge |
| V | Volt | electrical potential |
| F | Farad | electrical capacitance |
| T | Tesla | magnetic flux density |
| H | Henry | inductance |
| cd | Candela | luminous intensity |
| K | Kelvin | temperature |
| m | Metre | length |
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Date and Time
When exactly is/was 07/06/05???
It's another one of my pet peeves -- some people assume that's July 6th, 2005, others assume it's the 7th of June, 2005, and still others will
swear up and down that it's 2007 Jun 5th. Great!
Luckily, there's a solution.
YYYY-MM-DD.
This format makes sense for a number of reasons:
- it's ISO-8601
- it's ordered from most significant to least significant, just like time (HHMMSS, you'd never even think of MMHH, even though you might say "5 minutes past noon").
- it sorts lexically
- it's unambiguous (nobody uses YYYY-DD-MM)
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