Why do certain metric units only take certain prefixes?

The issue I find interesting, which was touched on once or twice above, is that the length & time are defined in terms of single units, while mass is defined in terms of a kilo-unit.

That gives rise to the MKS “flavor” of metric/SI (yes, I know technically those are very different terms), which uses a unit, a kilo-unit, and a unit, for the three fundamental properties.

Meanwhile, somebody came up with the CGS “flavor”, using a centi-unit, a unit, and a unit for the same three fundamental properties.

Seems to me next time we’re draining this swamp we ought to go with a system that uses a unit, unit, and a unit. That’s why they’re called “fundamental units.”

Shame somebody didn’t have the wisdom to simply rename the grave something else to calm the post-Revolutionary hotheads rather than saddle us with this silly Kilogram-as-primary business.

Medical people in America like the metric system, except when they don’t, and use it in their own idiosyncratic fashion sometimes. This is manifested when they measure volume: The only unit in common use is the cc, or cubic centimeter, which does not admit prefixes of any kind. The cc is exactly equivalent to the milliliter but this fact is not used. They will comfortably deal in thousands of ccs rather than switching to liters.

The advantage of sticking to one unit in all circumstances is simplicity, and the concomitant increase in calculation speed and reduction in the risk of being misunderstood. The advantage of the cc over the ml is unclear to me.

OK. Yes, I agree I was wrong, you were right to correct it, I stand corrected, than you very much for your correction.

They could always use cubic decimetres (i.e. litres)…

By the by, according to my copy of Medical Abbreviations, ccs are deprecated in writing prescriptions in favour of mLs, because the figure “cc” is too easily misinterpreted when handwritten.

I’ve lived around medical people my whole life and I still do. I think I can safely say the ones I know think in ccs. It is the volume measure of choice whether they are describing novocaine dosage or just how much dinner they had to pump out of this guy’s stomach. I don’t recall if I’ve ever seen anything but cc written on a script.

None of that contradicts what you’ve said in the slightest.

Hijack: back in ancient times when I was a 5th grader and we got to the “metric unit” in the mathematics textbook, we were introduced to the prefix “myria-” meaning 10,000 of something.

I have no difficulty seeing why this oddball increment would not crop up very often (the important ones are divisible by 1000, less important ones are centi and hecto, deci and deka, who the heck describes things a 3.2 ten-thousands of something?). Question is, has anyone ever encountered this prefix outside of dictionaries? Is it used for anything at all besides giving 5th graders another prefix to memorize?

[/hijack]

AHunter3: I’ve never encountered myria- as a prefix in actual use, but the noun, myriad, exists and is in use. Usually it just means “a lot of,” without giving a specified number (“There were myriads of flies in my room”), but I also read in in the sense of “a number of 10,000 of something.” I remember reading somewhere that this meaning of myriad was in use it ancient times - Greek writers would speak of “three myriads” of something when they meant “(approximately) 30,000.”

It’s interesting to see, btw, that ancient number nomenclature was quite similar to ours, although the system of characters to write numbers down was totally different from the Arabic numerals we use. The Romans spoke of decem, viginti, triginta, etc. up to centum just as we speak of ten, twenty, thirty, hundred and so on (and they also passed these words on to today’s Romance languages - dix, vingt, treinte, cent in French and diez, veinte, treinta, ciento in Spanish, for example. One wonders why they didn’t come up with the system we refer to Arabic numerals if they were that close.

According to Wikipedia myria- got the scientific thumbs down in 1960. Probably because it was only used to torture 5th graders.

I’m sorry, that’s just weird. Normal people use kilopascals. :slight_smile:

Please tell me you’re kidding.

It reminds me of the old-school electronics usage of µµF (micromicrofarad) instead of pF (picofarad), when measuring small capacitors. But that seems to have died out; it was deprecated in the 1980s when I was in electronics school, and I mostly see it in old electronics books for the sixties or before.

For some reason, though, nobody ever wanted to use nanofarad. They always said “1000 picofarads” or “0.001 microfarads”.

And don’t forget the people who can’t spell. Should that sign giving the distance to your restaurant say “12 KM” or “12 kM” or “12 Km” or “12 km”? And then there are the people who make signs in upper case only, and think that also applies to metric symbols.

No wonder some people don’t want to touch Gm and Mm.

The metric ton, the measurement of mass equal to 1000 kilograms (okay, okay, 1 Mg), is often spelt ‘tonne’. I think the explosive megaton is allways spelt ‘megaton’.

But that brings up another question… is the ‘ton of TNT’ involved a metric ton or a US ton (2000 pounds?) or what? And what is it really measuring? Liberated energy in the explosion? Outward pressure? Something else?

Hectopascals are equivalent to millibars, which are a common unit for pressure measurements when reporting atmospheric conditions. The rough equality between 1 bar and atmospheric pressure at sea level results in typical pressure readings on the scale of 1000 millibars and lower, so retaining three significant figures in hectopascals or millibars can be achieved without using the decimal point. In kilopascals, that same precision requires a decimal point; thus millibars and hectopascals persist for the sake of forecast brevity.

Normalcy is in the eye of the beholder: Bureau of Meteorology website. :slight_smile:

In all recent usage I’ve seen (since the late 70s/early 80s), the megaton has been arbitrarily set equal to 10^15 calories (4.184x19^15 joules). This was deemed 'close enough" to the actual energy of detonating a megaton of TNT for DoD purposes. The traditional definition is, I believe, now an obsolete holdover, as far as I’ve seen in the scientific literature. Then again, unless one actually works in nuclear weapons design, the megaton is basically a didactic device anyway.

For thisreason, I actually like teh distinction between kilo-/mega- ton and the common metric tonne, especially when figuring quantities that often use mega-/kilo- tons as ilustrative units (like meteor impacts) A “kiloton” (of TNT) is not a unit of mass; it’s a unit of energy, and I find that using a different spelling (tonne) or unit (kg) for mass helps keep things clear for the student

If you want a gripe, why not look at “large Calories” (properly: kcal = 4184 J) and “small calories” (cal =4.184, also incorrectly called a 'gram calorie)). That leads to endless confusion. Believe it or not: I’ve gotten in major arguments with physicists and the like who simply refused to believe that a “dietary calorie” is a kcal, not a cal! (Even though, as a physician and molecular biologist, biochemistry is kind of my bread and butter) I end up sending them home to work out how many cal they’d need to keep a 70 kg mass with 1.73 m^2 of surface area at 37C in a 20 C environment. [1] It’s the only way thay’ll accept the correct order of magnitude --and even then they often come back quibbling with e.g. the surface area figure (a standard textbook figure in medicine)

It’s actually surprising how often patients\quacksters\sci-fi authors use the Cal/cal dichotomy to defend a flawed intuition of biological efficiency.

1. The more chemically inclined among you can confirm that the combustion of glucose or dextrose (exactly the same thing, but let’s not go there) yields 3.7 kcal/g (666 kcal/mol) and not . If you still have trouble attributing that much energy to a sugar cube, imagine a life-sized human statue made of dessert gelatin. Could you keep that jello figurine at body temperature using only a 3W christmas tree light? Obviously not. Yet a 3W christmas tree light consumes 3 J/sec * 3600 sec/hr * 24 hours/day / = 61.95 kcal/day

Er… that should be:
*"In all recent usage I’ve seen (since the late 70s/early 80s), the megaton has been arbitrarily set equal to 10^15 calories (4.184x10^15 joules).

and the footnote should read:
1. The more chemically inclined among you can confirm that the combustion of glucose or dextrose (exactly the same thing, but let’s not go there) yields 3.7 kcal/g (666 kcal/mol) and not . If you still have trouble attributing that much energy to a sugar cube, imagine a life-sized human statue made of dessert gelatin. Could you keep that jello figurine at body temperature using only a 3W christmas tree light? Obviously not. Yet a 3W christmas tree light consumes 3 J/sec * 3600 sec/hr * 24 hours/day / 4.184 J/cal= 61.95 kcal/day
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The countless other typos are left as an exercise for the reader.

Well, they’re easy to convert between. it could be worse. You could be using pounds per square inch and I could be using inches of mercury…

Even with the typos, that’s one of the best explanations of the whole calorie issue I’ve ever seen. I’ve recently become aware of it, as I get more into exercising, and the exercise machines here give estimated calorie usage as you exercise. I was telling my trainer that the calorie mentioned on the machine was the amount of energy required to raise a kilogram of water by one degree Celsius, but I didn’t mention the rest of it.

Food packages here give energy content in large calories as well.

I think we should be using joules for all this as well.

And I’m not going to get into house energy supply, where the electricity bill comes in kilowatt-hours and the natural gas bill in cubic metres of gas (but at what pressure? Shouldn’t it be in kilograms or something?), and I don’t know at all about the oil bill… 1 kW·h = 3600 J, and one cubic metre of gas when burnt yields 37.5 MJ.

The thread has drifted a bit before I could give my 2 cents, as for why people use some prefixes in some instances, but not in others. (The question as to why a kilogram and not a gram is the basic unit was best answered with the difficulty of making a precise gram as brass cylinder in Paris when the base units were decided upon. I think that sounds plausible.)

My WAG, based on my memories of high school chemistry, biology, physics, astronomy, and popular science books: in some areas, like biochemistry, you use nano- and micrometers because things are always the same size, and it’s easy to picture them, if you know the size of a normal cell is this many nano - or mycrometers, then a virus is a lot smaller than that, etc.

But in Astronomy, giving distances in kilometers is better than in Giga and Mega, because distances vary wildly.

As for kiloliters and similar: where would you use this? In everyday life, you buy a liter of milk; and I doubt scientists often use megaliters.

Lastly, while I learned the correct prefixes, I found simplest to use was the base 10 expression, esp. when doing calculations. I guess that’s why my teachers preferred it, too. So when a newspaper says sth. is a hundred thousand kilometers, everybody can write down the base 10 for that, but not everybody knows right away how many zeroes a Giga or Peta has. (Even with computers: you know that 1.44 MB is very small today, 14 GB is okay, 144 GB is big enough for video files. But how many zeroes does it have in Bytes?)

And expressing things in base 10 avoids the confusion that always happens when untrained translaters mistranslate English Billions to European (German) Billions (it’s Milliarden, and a german Billion is more than a Milliarde.)

I read hectopascals far more frequently than kilopascals - actually, to me the usage of the latter seems weird. YMMV.
hPA has the advantage of being identical to the millibar, another (now non-SI) unit of pressure some people might be familiar with.

Hectolitre is the standard unit to express the beer output of breweries in Germany, btw.