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Common mistakes when using the metric system

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Re: Common mistakes when using the metric system

#51

Post by isoprophlex »

creata wrote: post #41
tkgally wrote: post #32

I thought this looked like good advice until I got to: > Mass/weight distinction > Incorrect: He weighs 70 kg; Correct: His mass is 70 kg > Incorrect: She weighs 50 kg; Correct: She weighs 490 N on Earth Really?! We’re not supposed to use kilograms to describe a person’s weight? And to describe someone’s weight, we have to use newtons and specify what planet they’re on? Good luck enforcing those rules.

Yeah, this is the only one that seems flat out wrong. When I'm on a bridge, I absolutely want to see "The bridge can hold up 3000 kg", NOT "The bridge can hold up 29 kN". Partly because "holding up" (as opposed to, say, withstanding) force sounds nonsensical, but primarily because knowing the maximum load in kg is much more useful.

You never know! Tomorrow that bridge might be on the moon, and how on earth (or how on the moon, indeed) will you know if it will hold up when you walk your herd of elephants over it!

Re: Common mistakes when using the metric system

#52

Post by jeroenhd »

Tor3 wrote: post #31

The most common error I see is "kph" which doesn't even have any possible meaning. 'kilos per hour' would in most places be taken as "kilograms per hour". As in "my weight increases by 3 kph when I visit McDonalds"

kph seems to come from people that normally work in miles but switched to metric for one reason or another. The common notation I almost always see for kilometres per hour would be km/h, in rare occasions with `h` exchange for whatever letter the local translation of "hour" starts with.

I don't think anyone would confuse kph for kilograms per hour (after all, that would be kgph or kg/h, and I don't think I've ever had to calculate kilograms over time outside high school anyway). Usually, context clues make it pretty clear what's being described.

Re: Common mistakes when using the metric system

#53

Post by jeroenhd »

lobochrome wrote: post #11
Hadarai5 wrote: post #3

I've never seen Mtr, sqm or gr. Even as a child, I knew the short of those unit names.

sqm instead of m^2 is also just a way of typing it on a mobile keyboard.

More people seem to find the ² on mobile keyboards than they do on desktop keyboards. It's still rather uncommon, though.

If anything, I would've expected phone keyboards to turn m2 into m².

Re: Common mistakes when using the metric system

#54

Post by immibis »

tkgally wrote: post #32

I thought this looked like good advice until I got to: > Mass/weight distinction > Incorrect: He weighs 70 kg; Correct: His mass is 70 kg > Incorrect: She weighs 50 kg; Correct: She weighs 490 N on Earth Really?! We’re not supposed to use kilograms to describe a person’s weight? And to describe someone’s weight, we have to use newtons and specify what planet they’re on? Good luck enforcing those rules.

It's all pedantry. Physics makes a distinction between mass, a (mostly) invariant property of an object, and weight, the force proportional to your mass and the planet you're standing on divided by the square of the radius. Only physicists care about this distinction.

Re: Common mistakes when using the metric system

#55

Post by poulpy123 »

ikari_pl wrote: post #14

I was surprised to see "L" as the "only correct" (though as minor mistake) abbreviation for a liter, being certain "l" or "ℓ" is the way to go, as that's how I was taught so far. Turns out that SI actually allows both "l" and "L", but I would only ever use the uppercase if it's a sans-serif font in a non-prefixed unit ("L" as opposed to "ml"). I've never seen any can or bottle using "mL" or "cL". Some of the examples…

gr. is not uncommon in france. Not in formal publications, but in day to day life you may find it. I know that I use it sometimes

Re: Common mistakes when using the metric system

#56

Post by minetest2048 »

> Use 1008 MJ monthly electrical energy consumption instead of 280 kW⋅h monthly electrical energy consumption

> Avoid common non-SI units (suggested)... For serious science and engineering, SI units like metres per second should be used instead.

Yeah good luck with that... electrical power engineering speaks in kWh not MJ

> 150 pJ gamma ray instead of 938 MeV gamma ray

Particle physicists will hate you if you take electron volts from them

While I'm at it, do you know that attaching suffix to SI unit is not allowed by SI / IEC /ISO? So abbreviating megawatt of thermal energy as MWth or decibel referred to 1 milliwatt of power as dBm is technically illegal:

> "When one gives the value of a quantity, it is incorrect to attach letters or other symbols to the unit in order to provide information about the quantity or its conditions of measurement. Instead, the letters or other symbols should be attached to the quantity."

https://physics.nist.gov/cuu/pdf/sp811.pdf

Yeah... all of my spectrum analyzers, signal generators, software-defined radios, antennas and RF interface control documentation is not ISO/IEC compliant...

Re: Common mistakes when using the metric system

#57

Post by beAbU »

It would have been nice to know the risks of making each of these mistakes, since honestly I only found a few places where a mistake can have real negative consequences.

As with all things, context is king. I'm not going to be confused when my colleague asks me on slack if it's OK to email me a 15mb (millibar) document - I know what they meant. Nor am I going to worry if it's actually MiB vs MB, in this context it does not matter.

Also, although it probably be nice that we all standardised on the correct prefixes, literally nobody speaks in terms of gigameters or teragrams or whatever. We carry with us an internalisation of real world measurements, and use that to compute relativity when we read/hear these numbers. It's roughly 1000km from my home town to the coast, so if if the moon is roughly 400,000 km away, then that's 400x the distance, pretty far. My car weighs roughly 1 ton, so Hafthor's 501kg deadlift record is half my car - pretty impressive!

Re: Common mistakes when using the metric system

#59

Post by jeroenhd »

greatpatton wrote: post #5
ndsipa_pomu wrote: post #2

So, when using the kelvin temperature scale, it's lower case when using "kelvin", but uppercase when using the abbreviation "K"? That annoys me as kelvin is a name and even my phone wants to capitalise it.

Same for "2.4 GHz wireless network" but it's supposed to be 2.4 gigahertz if fully spelled. I suppose there is a logic behind but it's not obvious.

I've always assumed "giga" is capitalised because it one might confuse a lower case g for "grams". Technically, 1gm is one gram-meter (grams * metres), whereas 1Gm is a thousand kilometres. You don't often encounter weird combined units like gram-metres outside of physics, except maybe for kWh (kW * 1 hour), but I don't see why you wouldn't be nice to physicists and give them the extra clarity.

Other order of magnitude indicators are capitalised to distinguish them from their smaller counterparts (millimetre/Megametre).

Wikipedia states that the distinction is because units named after a person always start with a capital letter. I've always assumed Hz has to be capitalised to prevent confusion with the hecto prefix (hHz being 100Hz). I don't think there's a unit or order of magnitude that's abbreviated to `z`, so in theory "Hz" could just be "H",

I think Hz is more readable, but that's probably because I'm used to it.

Re: Common mistakes when using the metric system

#60

Post by physicsguy »

Lots of these are country dependent in terms of what is familiar to people. To some degree that matters if your primary communication is not international but domestic.

For e.g. in the UK sqm is very common, particularly when looking at property. On RightMove which is the most popular property website here, for e.g. you can see here that the default is `sq ft` with `sq m` given underneath: https://www.rightmove.co.uk/properties/140100683#/?channel=C...

In terms of things like mixture of units, I appreciate you want to give in one unit but I think the things chosen are a bit strange. I've never seen anyone write '15070 grams', it'd almost always be given as 1.507 kilograms.

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