Serious SI Question

[zdas04]

What is the convention for exponating units with multiple letters? I saw "100 km³" the other day and can't for the life of me figure out if the intent was "100 thousand cubic meters (10⁸L)" or "100 cubic kilometers (10¹⁴L)".

MathCAD says that 100 km³=100 (km)³" but MathCAD treats "km" as a single unit. Is that the official convention or just a MathCAD convention?

In my terms, 100 cubic kilometers is 3.5 TCF and 100,000 cubic meters is 3.5 MMCF.

David

 

[KENAT]

Myanmar?

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[IRstuff]

Well, you guys are welcome to your own opinions, BUT, BIPM is the official "keeper" of all things SI, and Tableau 5, Prefixes SI, lists "c" for centi as a recognized prefix for SI units, making cm a legitimate unit designation.

TTFN

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[hokie66]

Put it another way. Does anyone here use centimetres? If so, why?

 

[KENAT]

It was the main unit we dealt with in school up until our teens.

At least we never spent much time on dm.

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[VEBill]

Why? The faint echoes from the CGS system (as opposed to the MKS system).

 

[IRstuff]

I use cm, because that's what one of my simulation program wants for focal length and aperture. In any given military procurement specification, it would not be unusual to find:
meters, yards, feet, nautical miles, and knots all in the same document.

TTFN

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[ScottyUK]

theTick - USA, Canada, Libya?


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If we learn from our mistakes I'm getting a great education!

 

[zdas04]

As stated above it is USA, Liberia, and Myanmar, but it is really farther than that. I teach classes around the world, and find that more people can comprehend FPS units than can agree on metric units. People who use kPa seem to be in the minority everywhere. A lot of folks use kg/cm^2, a lot use bar, and all seem to understand psi. Oil field volumes are even worse. In Alberta it is E3m3 (I guess k(m3) is too easy to confuse with km^3), but in BC they never heard of E2m3. In Austrailia the common volume unit is Joules (and they assume 1000 BTU/SCF for the energy content). Everyone seems to understand barrel and SCF (usually pronounced "scuff").

David

 

[hokie66]

Looks like it is be discipline specific. Australian structural/civil engineers use only mm, m, km for length; kPa, MPa for pressure and stress. Tonne is common.

 

[zdas04]

You know hokie, "tonne" is the only unit that I have confidence in when I see it. If I see "ton" it might mean metric ton (tonne), short ton (2000 lbm) or long ton (2200 lbm which is somehow different from the 2200 lbm tonne), but tonne seems to always mean 1000 kg. I'm starting to see Mg more often because purests don't seem to like tonne so I expect it to evolve into something less precise (I figure I'll see "tonne (2000 lbm)" in something soon).

David

 

[btrueblood]

But hokie, mm is just as derived a unit as cm is.

Gun calibers for quite some time were described in cm, at least calibers over 20mm or 2cm.

I still calibrate my test tanks and flow meters using cylinders and flasks that are calibrated to the nearest cm^3.


...

ok, I made that one up, really they calibrate in mL. Convince me again how they are different?

 

[patprimmer]

cc and litres are the units of volume I use.

I convert tank sizes ti cm before calculation to keep it uniform and not so many zeros to count which pop up if you use mm then covert afterward. I don't really care if it's correct SI or not.

Also, I still mix units to what I can envisage or that my mind relates to by example.

I still like psi also. I know what a pound is and I know what a square inch is. An mpa or kpa is just some abstract number that I need to process to evaluate its influence.

I use kph for speed and km for distance on land, but mph when water-skiing or speed boat racing and time for distance when water skiing.

I still find myself thinking how many 8 X 4 sheets do I need for a wall 3100 X 2700. I then convert everything to mm.

Also, 3000mm is 3 metres, but 3100mm is 3100mm not 3.1 metres. Don't ask me why, it's just what occurs in my mind.

Regards
Pat
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[hokie66]

Pat,
Why use cc and litres when ml and l are the same thing?

bt,
I know anything other than m is a derived length unit, but as a structural engineer, see no reason to use anything which is not a multiple of 10^3. When converted to volume, the 10^3 relationship still holds. Millilitres, litres, m^3, etc. Same for units of mass. If you go through the list of derived relationships, say Pascals, it is N/m^2, while kPa is kN/m^2, and MPa is N/mm^2. Centimetres just don't play a part.

 

[patprimmer]

ccs or ml plays a part when your measuring equipment and existing data are all in that unit. Also it reduces the number of zeros to be handled and possibly be miscounted. I guess it's kinda the same reason why the USA is still the only major country to not have gone metric. I also guess because I'm kinda lazy in this regard and do the least amount of conversion possible to get things into line

cc and ml are effectively the same thing and I substitute either way without conscious thought.

Litre vs l is a bit different. l can be confused with a 1 so the full word removes possible misinterpretation. Same as I refuse to use gas for gasoline and always say petrol as that cannot be confused with LPG.

Regards
Pat
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[hokie66]

Not sure if I confused you or myself.

 

[Skogsgurra]

I noticed that happens to me also quite often. But, as confusion is the first step on the path to knowledge, I think that is a good thing.

Gunnar Englund

http://www.gke.org

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[patprimmer]

I reread my last post. I was obviously confused and said some fairly nonsensical things.

Regards
Pat
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[zdas04]

Anything you want edited?

 

[patprimmer]

No, it's not doing any harm.

Regards
Pat
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[btrueblood]

I said, convince me that a cm^3 is not a mL. Do you know how a liter is defined, hokie? Pretty sure it's still a cube 1 dm (10 cm) on a side. Do you really want your doctor to give you a shot with a syringe calibrated in m^3? That's a lotta little zeros for him to read, and he's getting kinda old...

About your 10^3 units thing, I don't disagree for primary units. Meter, kilometer; gram, kilogram;... and cc, liter is the same.

Oh, baby growth charts use cm units, suspect a lot of other medical metrics uses same. Most SI scales (rulers) use cm divisions. It's just a convenient, human-scale unit, like inches.

My biggest peeve is the kgf (force/weight) unit, or kgf/cm^2 as a pressure. It takes away one of the better reasons to use SI, since you now have to convert the force unit into its proper form (Newtons) before using it. Like Pat said, I think in terms of psi - ask me the modulus or yield strength of a given material and I'll give you the value in psi, not Pa or kgf/mm^2. I think kgf/cm^2 was invented by somebody who wanted a unit that made sense in human-scale terms, and didn't want to use psi because his teacher kept swatting him when he did that...