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Fascinating that the second reply (or second best reply?) is an unreasonably angry response:

Who the hell needs all this .. Get lost [angry emoji]

There are a fair number of people who seem to take personal offense that the US (and to some degree the UK) don't use SI for everyday types of things and add it to their laundry list of grievances about the US generally.

Never mind the fact that SI is generally used for engineering and other areas where it has legitimate advantages (which miles vs. kilometers doesn't really in day-to-day life).

And there are a fair number of people who seem to take personal offense that SI units exist in any form or fashion, perturbing the natural way of the world.

We can't really know to what category this one belongs to.

I agree it's not worth the hate, but in my opinion SI units are much more convenient.

Using your own example, miles vs kilometers may not feel like a big difference, but the fact that you have proportional units (mm, cm, m, km) for any possible distance has the potential to save you a lot of trouble. Having completely different units for the same thing (land miles / nautical miles for distance), usually not proportional between them, is less practical than the SI at every level. Not only for an engineer but also for home owners and almost everyone of us.

I understand why there are some places in the world where they still use systems other than the SI, mainly for legacy reasons. But I think the practical benefits of a more coherent (and standard) unit system exist not only for the engineer.

That's fair enough. Whatever the advantages of Imperial in some narrow contexts, I'm not sure very many would argue for them absent legacy.

On the other hand, it's pretty much an academic debate because a switch isn't happening for mainstream use. The desire and political will just aren't there. The little push that once existed is essentially gone whatever some tech types might want.

I like arguing for arcane standards, and there are aspects of the imperial system I like.

Specifically, I tend to advocate for a base 12 system based arround the inch. Base 12 because the very structured multiplication table, which also makes for easy dividing. The inch because I find my lengtg estimation accuracy to be better captured by inches.

Your length estimation accuracy is dependent on what you used more.

I have never used inches outside screen diagonals. Yet I still feel inches better match my accuracy. My actual accuracy in inches is horrible though, because I never get into contact with them.

I assume that 12 inches to a foot is the reason that, at least when I was growing up, we were taught multiplication tables up to 12x12 rather than 10x10.

My favourite use of units is still the „2mm Scale Association“, a British model railroad club that builds their trains at a scale or two millimeteres to the foot (which is slightly larger than the standard N-track). It always blows my mind when I try to think how people come up with a scale like that.

Kinda a crazy ratio, but not -really-.

If you have modelmaking tools measured in mm, and a bunch of legacy dimensions of trains and buildings that are often round numbers of feet, it becomes tempting to use a scale like this. After all, you tend to get round numbers of mm to make things...

It might be convenient to make a starmap where e.g. 10 light years is 1 cm. Yes, the actual dilation factor is strange, but it's pretty easy to plot a set of cartesian coordinates measured in light years on a 1 cm grid.

> that SI is generally used for engineering

Except when it isn't? Are you telling me SI is used in engineering everywhere in the US? I've heard too many stories (especially accidents) to believe that in any way.

Nope. Hence generally. I haven't made a study of it but I assume Imperial units are more common in civil engineering and other areas that converge with local construction and building trades. Metric is pretty standard in my experience more broadly. (Note also that the better known examples of unit conversion accidents are pretty old as far as I know.)

don't they still use pounds per square inch ?

Now, in Europe (excluding the UK) they also still use some weird units from the past. Calories for example, and horse (!!) power.

A calorie was originally defined as as the amount of heat required at a pressure of 1 standard atmosphere to raise the temperature of 1 gram of water 1° Celsius.

But you're right that is sort of a weird unit from the past insofar as it's related to the SI system but isn't formally part of it. (It's basically now defined by its ratio to joules.)

Adding: There are a number of measurements that aren't necessarily obviously related but are actually the same thing in terms of units. Heat, work, and energy are all newton meters or kg m^2/sec^2 in SI units.

PSI is very commonplace though I'm not in a field where I use it in an engineering way. (I actually had to think for a second to come up with the SI equivalent.)

Pounds are one of the real bugbears of imperial in mechanical engineering with lb-f and slugs (i.e. "pounds" conflates mass and weight). Always hated that. I'd convert things to metric and then convert back when I had to work in Imperial.

And I still hear stones from time to time in the UK if you really want archaic.

Yeah, a lot of people in the UK use stones for weight. Even young people in my experience.

We still use PSI for tyre pressures in UK, probably some other things too.

They're given in N/m^2, but all the petrol station machines are in PSI, probably because the numbers just fall better.

I thought they were given in bars instead of N/m^2.

Aye, could be. Just checked my manual and they're in KPa and Bar.

On the UK railways, I think I'm right in saying that the only place imperial units are used is in measuring the curvature of track, which involves chains.

> which miles vs. kilometers doesn't really in day-to-day life

Standard units are much more useful in carpentry. It's a lot easier when you can divide by 2,3,4, and 6.

Oh yes, then you need to figure out if 7/16 is bigger or smaller than 3/4. No, thanks.

You can easily divide by 2 and 4 in metric, because guess what, you just add a decimal point to your measure and 10cm/4 is 2.5cm and not some crazy fraction or weird unit and the math is not harder if you're dealing with 10m as opposed to 10cm.

10cm/3 is 3.3cm or whatever the precision of your tools is. It's the same "problem" (it isn't) with dividing 1ft in 5 for example. You go by the tolerance of your tools

It's a lot easier to do things when you don't have to go "okay, what's the conversion between these two units again?". The problem isn't base 10 vs base 12, it's the fact that in SI, every unit scales in the same way (the prefixes). With imperial units, every unit scales differently. How many fl.oz to a tablespoon? No idea. How many teaspoons to a tablespoon? No idea.

Unless you are working with wold and the measurements are all nonlinear lies. https://en.m.wikipedia.org/wiki/Lumber#Dimensional_lumber

I've heard that and I don't disagree with it. But things fail really badly. E.g. 2 inches and 5 sixteenth

I find this hilarious when watching DIY videos on YouTube.

All the makers are talking about "5/16 of an inch" and I'm trying to convert this to a metric value and it doesn't make any sense at all, because "complex" fractions like this are not used in every day life.

Yep. Factoring is useful in a lot of cases.

Also, although not strictly a debate around metric, the Fahrenheit scale gives you more granularity without going to decimals and requires less use of negative numbers on a day to day basis. And, if one is really concerned about a scientifically relevant temperature scale, we'd be using Kelvin, not Celsius.

Kelvin IS Celsius, only starts at absolute zero. 0 C is 273 Kelvin, and 0 Kelvin is -273 Celsius. You add 273 to any Celsius measurement, and voila, it's Kelvin.

No. That's actually not quite correct. You add 273.15... The size of the degree is the same of course.

My point is that if you want to use a "correct" scientific measurement on a day to day basis you'd use Kelvin. As soon as you're converting, you're converting whether from Celsius or Fahrenheit.

ADDED: There's an argument for Celsius vs. Fahrenheit of course in so far as the size of the degree is baked into some other SI measurements. But there's no particular other reason that Celsius is superior on a day to day basis other than familiarity for some. There's some logic to basing easy to remember numerical points around water properties but it's not clear that actually has a lot of advantages for day-to-day questions about how hot or how cold it is.

The fact that degree is the same means that energetically 1 degree C is equal to 1K. So when doing scientific calculations, for example to calculate energy needed to change the temp by X, they are the same. The fact that the scale is based on water properties seem to be just so values are easier to relate to. We are water based, after all.

Can anyone provide any reason to use Fahrenheit scale, apart from historical ones? Legit question.

Sure. Where I live:

1. Fahrenheit is far more likely to cover the range of temperatures I encounter without having to use negative numbers. 0 is really cold. Anything below zero is really freaking cold. The temperature of the boiling point of water is mostly an academic point in my day to day life.

2. Fahrenheit provides about 2x the granularity of Celsius without having to use decimals

I'm not going to argue that if Fahrenheit didn't exist, we'd invent it. But it does have some advantages as an existing system.

ADDED: For engineering using SI units, of course Celsius and Kelvin make a lot more sense.

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