[klodolph]

> That's how it's used, but the units don't measure the same thing.

I'm having some real trouble parsing this sentence right here. You're saying that the two DIFFERENT UNITS "fluid ounce" and "ounce (weight)" measure DIFFERENT things? Yes, different units measure different things. Or maybe I don't understand what you are trying to say here.

If I said "kilogram force", you'd know exactly what that is.

You are also not confused when I say that water boils at 100 degrees centigrade, and a right angle is 90 degrees.

And don't even get me started on the number of people who argue that 1024 bytes is called a "kilobyte".

[Karellen]

I think the GP's point was that "fluid ounce" is a measure of volume, not mass or weight. If you're measuring a liquid whose density is different from water, a fluid ounce won't weigh anywhere near an ounce.

"degrees" is an interesting one, as it's really just another word for "amounts" or "units" or "subdivisions". 100 degrees centigrade is 100 amounts of centigrade. Or actually Celsius, as "centigrade" means "100 subivisions". The name of the unit is just "Celsius".

Really, degrees of arc should probably have been given a "proper" name too, but it's too late for that. Or you could switch to radians ;-p

Language is fuzzy. People know what you mean when you say "degrees centigrade" or "fluid ounces", so it's fulfilled its purpose. But that doesn't mean that some "proper" terms for measurements aren't weirder than others. "Celsius" is straightforward and unambiguous. "fluid ounces" is kinda weird. And yes, "degrees (of arc)" is a poor choice of unit name, worse than "fluid ounces". That still doesn't make "fluid ounces" good though.

[klodolph]

> I think the GP's point was that "fluid ounce" is a measure of volume, not mass or weight. If you're measuring a liquid whose density is different from water, a fluid ounce won't weigh anywhere near an ounce.

Yes, and equally so, if you are measuring a substance other than water, the milliliter will have a different mass than a gram.

[Karellen]

But "millilitre" isn't named after a unit of weight.

A fluid ounce of mercury doesn't weigh an ounce. But a millilitre of mercury still takes up one one-thousandth of a litre, which is itself one one-thousandth of a cubic metre.

[klodolph]

Funny thing, "millimeter of mercury" is a unit of pressure, but "millimeter" is a unit of distance.

"Fluid ounce" is a unit of volume, but "ounce" is a unit of mass.

"Pound" (or "kilogram") is a unit of mass, but "pound force" (or "kilogram force") is a unit of force.

I understand the argument, it's just not a very compelling argument. Explaining the argument one more time is not really convincing. If I'm going to embark on a mission to remove ambiguity from measurement systems, my first choice would be to abolish the use of KB=1024 bytes. The "fluid ounces" and "ounces" thing just does not seem like it poses as much of a problem.

Addendum: I'm also not trying to be contrarian here, and I'm not trying to advocate for some kind of supremacy of US customary units over SI. Most countries have some form of non-SI customary units still in use and some amount of SI adoption. The SI units are preferred for lots of reasons, but it's not like the various systems of customary units are illogical or nonsensical. There's an explanation and a logic behind them, like how US customary volumetric units follow powers of 2, or how a "mile" is a thousand paces (nice round power of 10). The ounce / fluid ounce relationship is has the same physical explanation as the gram / milliliter relationship, because they are both approximations for the density of water, as a reference point. Neither relationship is exactly the density of water, because the respective units have been redefined, and the density of water is now something you look up which happens to have a value close to 1.0 with those particular choices of units. Just as the meter was originally defined as one millionth of the Earth's meridian, the mile was originally one thousand paces, and after standardization, neither fact is true.

Again, I can understand why people want to advocate for SI, but that doesn't mean that customary units are illogical or nonsensical.

[Karellen]

> Explaining the argument one more time is not really convincing.

Sorry, I wasn't explaining again because I thought you weren't convinced of my point. Based on your previous reply, I thought you hadn't understood my point. Because if you had, the "well, a millilitre of other liquids doesn't weigh the same either" seems like a complete non-sequitur. I couldn't figure out what point you would be trying to make with that comment if you had understood me. (Knowing that you did, I'm still not sure? Like... so what?)

[klodolph]

I guess my response is also "so what?"

If you measure a liquid other than water, then a fluid ounce does not weigh an ounce... so what?

Even water, the density is 1.0 oz/fl oz, but that's as many significant figures as you get. You would often want more significant figures, even in non-scientific contexts.

[taneq]

"mmHg" is also on my list of units that are stupid. :P

Likewise I understand your argument (yes, of course I know "fluid ounce" is a unit of volume and "ounce" is a unit of weight), I just think naming a unit of volume after a unit of weight is dumb.