Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

International System of Units overhauled in historic vote

npl.co.uk

Post reply on HN
Print view View original

541–550 of 589 posts

Re: International System of Units overhauled in historic vote

#541

Post by thaumasiotes »

kiriakasis wrote: post #479
thaumasiotes wrote: post #382

Earlier quoted context omitted.

That's not a base unit, or a unit of any kind. What's it doing in a list of SI base units?

It is a base unit, a unit for lots of stuff, and it is not dimensionless it is 6*10^23/mol

I'm pretty sure the mole is not defined as 6 * 10^23 mol^{-1}.

There is a concept of "the Avogadro constant", which is defined to have units of mol^{-1} (at least, according to a cited statement on wikipedia), but that is not a coherent concept -- since mol is dimensionless, mol^{-1} is also dimensionless.

Just look at https://en.wikipedia.org/wiki/Mole_(unit)#Criticism :

> Since its adoption into the International System of Units in 1971, numerous criticisms of the concept of the mole as a unit like the metre or the second have arisen:

> the number of molecules, etc. in a given amount of material is a fixed dimensionless quantity

> the mole is not a true metric (i.e. measuring) unit

Or look at https://en.wikipedia.org/wiki/Atomic_mass_unit :

> One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol.

amu and g are both units of mass, so 1 amu = 1 g/mol is an explicit statement that mol is dimensionless.

Calling mol a unit won't accomplish anything except corrupting your dimensional analysis. mol is not analogous to the SI units meter, second, ampere, gram, kelvin, etc. -- it is analogous to the SI prefixes kilo-, mega-, milli-, micro-, nano-, etc.

Re: International System of Units overhauled in historic vote

#543

Post by gpderetta »

jetzzz wrote: post #480
blhack wrote: post #232

Earlier quoted context omitted.

I'm one of the weird people that actually really likes imperial units. For instance: 1 foot -- base 12. This is a superior base to 10. It can be easily divided into 4ths, 3rd, and 2nds. Base 10 can only easily be divided into 2nds. 1 inch -- an easily identifiable unit of measure for smallish things. About the width of my thumb. A pretty good unit. For low precision, inches become 1/2, 1/4, 1/8, 1/6, 1/64, etc. Each…

> I also think that F is seriously superior to C. Both are not metric though. Metric unit for temperature is kelvin.

True although C is just an offset from K, i.e. only absolute values change, temperature deltas are the same in C and K.

Re: International System of Units overhauled in historic vote

#544

Post by gpderetta »

dcuthbertson wrote: post #213
beloch wrote: post #206

Earlier quoted context omitted.

I like to commend my American friends for their undying loyalty to the memory of the Empire. U.S. customary units are derived from units that helped the British conquer the world, but even the British themselves have shamelessly abandoned them for the units of a filthy, monarch-less republic. It's good to see that Americans still subconsciously yearn for the firm ruling hand of their rightful Queen. Note: Americans h…

That's some well metered ribbing. No worries. We can take it.

> metered

Ah!

Re: International System of Units overhauled in historic vote

#545

Post by Moru »

gilbetron wrote: post #504
blhack wrote: post #232

Earlier quoted context omitted.

I'm one of the weird people that actually really likes imperial units. For instance: 1 foot -- base 12. This is a superior base to 10. It can be easily divided into 4ths, 3rd, and 2nds. Base 10 can only easily be divided into 2nds. 1 inch -- an easily identifiable unit of measure for smallish things. About the width of my thumb. A pretty good unit. For low precision, inches become 1/2, 1/4, 1/8, 1/6, 1/64, etc. Each…

I'm glad you made this point, because I am the one that usually does and gets hammered for daring to even hint that imperial might not be totally insane! The temperature one I've always found compelling. Most humans only rarely experience temperatures outside of 0-100F. That makes sense to me. Celsius makes little sense, except for 0 being when water freezes. It compacts the scale too much for everyday use. However,…

The last couple of years we have had between -36 F to 97 F. Or -38 to +36 C. I like to be able to tell when the roads might get slippery (below +3 C or so), when the car will get problems starting if you forgot to plug in the heater (about -25 C), When it's time for the shorts (+18 C?).

Also handy to know that I need 3-6-3 ammounts to make pancakes. 3 eggs, 6 litres of milk and 3 dl flour (that will be meassured with grams because easier, can just pour everything into the bucket and press "tara"). Flour weighs 60 g per dl. Put bowl on scale, start it. Pour flour in the bowl until it reads 180 g. Reset and pour in milk until it says 300 g. Mix until perfect. Add eggs. Reset scale and pour another 300 g and mix again. Start frying!

Re: International System of Units overhauled in historic vote

#546

Post by Moru »

kbenson wrote: post #528
aengvs wrote: post #384

Earlier quoted context omitted.

> No, I represent a quarter as 25/100's to show how it would be accurately represented in metric. 1/4 meter is not pure metric, it's applying a non-metric modifuer to a metric amount. The metric representation of 1/4 meters is 25 centimeters, which is 25/100. What? Fractions aren't exclusive to the imperial system. 1/4 of a metre is metric, just as a 1/4 of an inch is imperial. Everything below is a perfectly legitim…

Fractions are mathematical modifiers that can applied to any quantifiable measurement. That said, I imagine they aren't generally used in professional context in metric, because they generally aren't in imperial either. You don't see architectural plans that say 1-1/2 feet, they say 1'-6". Is it common to see fractional amounts in metric when used in a professional context?

Usually we write everything in mm for professional context.

Re: International System of Units overhauled in historic vote

#547

Post by Moru »

autokad wrote: post #511
kiriakasis wrote: post #474

Earlier quoted context omitted.

Well it proves a lot in my opinion. Feet are really non uniform in length by a lot.

go to google and type "about definition"

You can't both say that meter isn't exact enough when meassuring 1/3 and also say that 25 mm wrong on a food is ok...

Re: International System of Units overhauled in historic vote

#548

Post by Moru »

bnjms wrote: post #524
kiriakasis wrote: post #475

Earlier quoted context omitted.

And is also more resolution than the error warrants

I think I meant less for forecasts and more for weather apps and thermostats. I really think it could help with adoption. But if it’s not warranted by the error then maybe I’ll reconsider.

The precision on most digital thermometers for home use is (surprice) +-1 degree C :-)

Re: International System of Units overhauled in historic vote

#549

Post by zzo38computer »

The old definition of kilogram (with IPK) is no good, so it is very good that they are correcting it.

Of course you can still use imperial and other units in whatever context they are useful, and so SI units can also be used in the context where they are useful. But still I am glad they change it, because the old definition of kilogram is no good.

Re: International System of Units overhauled in historic vote

#550

Post by kbenson »

Moru wrote: post #546
kbenson wrote: post #528

Earlier quoted context omitted.

Fractions are mathematical modifiers that can applied to any quantifiable measurement. That said, I imagine they aren't generally used in professional context in metric, because they generally aren't in imperial either. You don't see architectural plans that say 1-1/2 feet, they say 1'-6". Is it common to see fractional amounts in metric when used in a professional context?

Usually we write everything in mm for professional context.

Thanks, that's what I assumed (because it makes sense). For metric you often go two orders of magnitude smaller, but I'm thinking a good amount of the reason to usually go straight to millimeters instead of also using decimeters is that decimeters just can't easily represent many common fractions of a meter (1/4, 3/4, 1/3, 2/3), and a base 10 system can never represent some of those perfectly without resorting to a mixed format.
Post reply on HN