TL;DR: All values should be given primarily in metric and optionally, in non-metric units secondary.
When i find a question or answer, i want to be able to understand it.
In order that everyone understands the question, it should use metric units.
We already enforce a language, which much harder to learn and where it is very subjective to determine which one is better.
So we should absolutely enforce the metric system which is much easier to learn
and objectively the better system.
It would make engineering also easier and more of a joy than a pain when people would use the metric
system everywhere.
This site is about engineering, it is expected to use the metric system in engineering.
Everything else is unprofessional (I would call it Gebastel oder Basteln in German, basically means
crafting like a young child would do).
There are many places where the metric system is not used for some stupid reason, but that does not
make it less unprofessional.
Why we should use the metric system
This part is maybe a bit of a rant, but since i see how many refuse to use the metric system and
that there are people defending it, i think it is somewhat necessary.
- The metric system is just far superior compared to the historic units
- The metric system has well defined units
- The metric system has clearly defined unit symbols.
- Other proper system can make sense but they are not used in engineering.
- For complexer dimensions, like voltage, all people use metric units.
- It would simplify a lot if we would use the same system everywhere.
- Everyone knows the metric system.
The metric system is better
The metric system, especially the SI subset, has prefixes and they are of factors 10^N between them. Makes it really easy to convert and compare values of different magnitudes.
The second advantage of the prefixes is that they work on all SI units, except degree Celsius, the same. 1 km = 1000 m, 1 kV = 1000 V, 1 kΩ = 1000 Ω, ... So you only have to learn the prefixes ones and can use them for all dimensions.
Compare that to the British imperial units that have factors between them of 1760 (British land mile vs British yard), which is hard to convert in your head.
Even worse, some British units do not have whole numbers as factors between them but ratios. Take a look at this:
https://upload.wikimedia.org/wikipedia/commons/9/9e/English_Length_Units_Graph.svg
And that is only for the length, you have to learn a similar complex system for time, weight and speed.
You may argue that we don't use most of this, but still, it is way more complicated than the metric
system even when we limit the units to mil (which isn't even in the diagram), inch, foot, yard and mile.
AWG
Take a look at units for stranded wire size. When we use metric units we just use square mm, for the
area. Easy, the value is proportional to the amount of copper needed and inverse proportional to the resistance.
It also gives you immediately an idea about the current capacity (assuming copper under somewhat room
conditions, lets exclude exotic stuff like superconductors).
Compare that to AWG, probably the worst unit.
It does not give a size, instead it gives a value where you need the logarithm to calculate it from
the diameter: n = -39*log(d/0.005 in,92) + 36
Or to do it vice versa: 0.005 in * 92^((36-n)/39)
Nobody can do that in their head and most engineers can't do that on a calculator without looking up
this formula. And yes, the area of a stranded wire is important for things like resistance.
And it has the bad sideeffect of a larger value the smaller the wire. Most people not familiar with
AWG will assume a larger number means a larger wire, making it easy for someone to use a wire that
is not large enough.
The AWG is even worse by not using negative numbers if the value goes below 0. It uses 00 for -1 and
making 00 different from 0. Which makes it horrible for programmers.
What do they use if the formula results in -0.5 ? 00.5 ? I don't know.
Definitions
We have one body, the BIPM, that is responsible for coordinating the definitions of the SI units.
Which resulted in a system where 1 m is 1 m and everyone knows how long it is and there do not exist
competing, incompatible definitions.
There where some changes through history and very small changes, like the prototype metre which was 0.02% shorter than the previous definition, but that is miniscule
and not relevant in engineering.
Many non-metric units have different incompatible definitions.
Some of them only have small differences. For example the German Pferdestärke, which literally means
horseforce, which basically every dictionary will translate as Horsepower, is 735.49875 W.
On the other hand, the British Horsepower is about 745.7 W, about 1.4% more.
When parts between a German and a English speaking country get exchanged, they often confuse this units, which many don't notice since the difference is small.
A value in horsepower can often mean either one of this and without more information you can not know which one is used.
You, as an English speaker, may assume a inch is 25.4 mm.
In Taiwan, a 寸 is 1/30 m ( about 33.3 mm ). When google translates 30 寸 (Which is 1m ) to English
you get 30 inches (which is only 762 mm).
https://translate.google.com/?hl=de&sl=zh-CN&tl=en&text=30%20%E5%AF%B8&op=translate
You can be more specific and use the term international Inch or British inch and 英寸, but how often do you specify which unit version you use?
If you talk about ounces, you not only have a bunch of different units of mass, all named ounce, but
there are also units called ounce that measure volume and some that measure force.
In PCB design, 1 ounce of copper does not mean a mass, weight nor volume but a thickness of 35 µm.
Now we have a unit name that is used for 4 different dimensions, 3 of them have have more than one definition.
Compare that to the metric system, where you know 1 metre is 1 metre and is not suddenly 0.95 m or
12 kg when you go to a different country.
Symbols
There are 29 SI-Unit names and 24 Prefixes**. With them, all SI-Units can be written. All having a
clearly defined symbol that are distinct.
This makes it easy to write them and it is always clear which unit is used.
Assuming people write it properly, which is sadly often not the case.
Do you know if 1 mil is: 0.001 inch, 1 international mile, a Swedish mile, someone who meant mm but
didn't know how to write it, or a Biblical mile?
The context may gives you some clue, but it would make things easier when you can look at the unit
and know exactly what it means without having to understand everything else first.
Is 1 lbs = 1 pound or 1lb = 1 pound-second ?
Other systems
There are other systems, besides the metric system and besides historic units.
One example would be the Planck system, that is only based on natural constants, without using
made up factors.
The Planck system makes more sense than any other system on paper.
But since neither the Planck system nor other systems are used for engineering, we should not start
to use them and making things even more complicated as they already are.
Complex Dimensions
As soon as the Dimension gets more complex, like voltage or electric resistance, everyone uses
metric units.
For voltage, everyone uses Volt, wich is 1 V = 1 kg*m^2 / (s^3 * A)
Nodoy uses a unit like lb * mile * inch / (s^3 * A)
If we already agree that we use the metric system for this dimensions, why not use it for all?
One system for everything
Converting between unit system is bad, it is a source for failures and errors. It can also make
things incompatible.
I don't know how much resources we waste because we have to convert between units, have more
failure, and less compatibility, but i would not be surprised if it costs humans 100s of Millions
of euros per year.
So using a single system for everything should be a goal. We should make it easier for the future
Generations, not harder.
Since the Metric system is better than non-metric units, everyone should use metric units wherever
possible.
Most of the world switched to the metric system about 150 years ago, because it is better what they had before. What stops the rest from also switching?
The unit system everyone knows
Everyone learns the metric system in School (hopefully).
Ok, there are some that don't, but we should expect some level of knowledge when using this site.
So we should expect everyone that uses this site to be familiar with the metric system.
On the other hand, most schools probably don't teach how non-metric units work.
We should not expect that everyone who uses this site to be familiar with non-metric units.
It makes it understandable for everyone when we use the metric system.
When we use non-metric units many people will not understand it.
Comparing it to the natural Language
We enforce that all users use English.
Understandable that we want a single lingua franca on this site.
Not arguing against that.
But then we can also enforce a common unit system.
Opinion part:
Learning English takes years, learning the metric system does not take long.
What bothers me is that often the ones where English is there native language, so they didn't had
to learn it as second language, are the ones who often refuse to use the metric system.
It is disrespectful when we use years to learn someone else's language, because it is the only
language they speak, in order to communicate with them (again, not against this, since someone of
us had to learn a foreign language), and they don't even bother to use the metric system, which is
very easy to learn and objectively better.
That is a rude way of saying F**** You and your effort trying to communicate with me.
**Most of this Prefixes and many of this Unit names do most people not need to know. Average Joy doesn't need to know what Henry or Tesla means, or how many Watt a Yottawatt is.