Three Common Mistakes When Using Metric Units
A while back, I had someone point out a few errors I made concerning the use of metric units. This caused me to review the SI system to make sure I had it correct. (I am sure I’ll continue to screw up a few things but, hey, life is a journey.)
The International System of Units, universally abbreviated SI (from the French Le Système International d’Unités), is the modern metric system of measurement. For a thorough treatment of the topic, take a look at the National Institute of Standards and Technology (NIST) publication: Guide for the Use of the International System of Units (SI). A shorter and easier-to-read document was written by Charles Poynton: Writing SI units and symbols.
But more to the point, here are three common mistakes I often see occurring in ham radio literature:
- Using mHz instead of MHz to indicate megahertz (one million hertz). Upper case M indicates mega, while lower case m indicates milli (one thousandth). In ham radio usage, we rarely speak of frequencies in mHz. Note that I wrote mega with a lower case m even though the abbreviation has a upper case M. The unit of hertz is with a lower case h but when abbreviated as Hz, it should be upper case.
- Using KHz or khz instead of kHz to indicate kilohertz (one thousand hertz). Lower case k should be used for kilo but upper case H is used for hertz. It is common to see upper case K used to indicate 1024 in digital systems.
- Using M instead of m to indicate the unit of meter. The proper way to refer to the wavelength of the 144 MHz ham band is 2m, not 2M. Similarly, the abbreviation for kilometers is km, not kM or KM. The abbreviation for centimeter is cm.
Units that are based on a person’s name use an upper case letter in the abbreviation. For example, ampere, volt, watt and hertz are abbreviated as A, V, W and Hz respectively. When the unit is spelled out, it is left lower case (go figure).
While the world does keep on turning when we make these mistakes, accuracy and understanding is improved by proper usage. Did I miss any other common SI errors?
73, Bob K0NR
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Learned something new. If I did know this, it hasn’t been in my memory in over 30 years.
The proper way to write all metric units in English is with a non-breakable space between the number and the unit, so 2m is not correct and should be 2 m. In case of units having degree symbol, the space goes between number and degree symbol and there is no space between degree symbol and unit, like for example 20 °C. In case the degree is used to measure angle, there is no space between degree and number as in 20°. In case measured quantity is expressed by a an expression containing several units, mathematical operations are allowed. It should be kept in mind that English “per” implies division, so is a vehicle is moving at one hundred kilometers per hour, that should be written properly 100 km/h, not 100kph or something similar. In case of ambiguities, it is acceptable to use parentheses, such as 17 ps/(nm*km).
SI units are much easier to work with than English/American units — but the insistence on capitalizing/not capitalizing certain letters sometimes borders on the nit-picky.
On a lighter note: Can you imagine how comical American culture would be, metricized?
A bathing beauty would measure 91-61-91 and weigh 54.5.
Grandpa Jones would sing “Thirteen kilometers and Louisville will pull into my view …”
How about “I’m 160, I’m 320, I’m 480, I’m 640, I’m 800 kilometers away from home.”?
Merle Travis would sing “This four-kilo hammer/Is a little too heavy …”
Even funnier, since velocity is a vector quantity:
“It’s a mighty rough road from Lynchburg to Danville,
A-lyin’ on a five-kilometer grade —
It was on that hill that he lost his air-brakes;
You can see what a jump he made.
He was makin’ forty meters down the hill ev’ry second
When the whistle broke into a scream —
He was found in the wreck with his hand on the throttle,
Scalded to death by the steam.”
Mack KD5OZA:That’s funny! Good one. Love it! Then there is Tennessee Ernie Ford’s song 14.515 metric tons of number 9 coal, and I owe my soul to the company store!