# Metric and Imperial units

From Wikipedia:

The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth’s circumference is approximately 40000 km. In 1799, the metre was redefined in terms of a prototype metre bar. The actual bar used was changed in 1889.

I wonder whether the temperature change in the measuring stick used from the Pole to the Equator was taken into account with that original definition.
The Metre, originally, seems about as accurate as the barley corns.

I can imagine the conversation in 1793: Now, it is 50,000 sticks long, so if we divide that by 5 and multiply the result by 4 we have 40,000 sticks and a bit, which is just a little over a yard so that will be OK because it cannot be the same as those Imperialists!

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OT, but did you know that France once defined GMT as “Paris time, diminished by 8 minutes and 26 seconds” (or something like that). Also UTC isn’t an acronym for a sensible phrase; it just annoys both the Anglophones and Francophones equally.

Anyway, back to your mil and micron argy bargy.

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Still OT. Does not surprise me. England and France have occasionally had differing opinions on various matters over the years.

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The Metre, originally, seems about as accurate as the barley corns.

There exist only a single earth, so the accuracy is a lot higher than when you base it on something that exist billions of times, in different sizes.

If you have the track alone, maybe not. Try to solder a GND pin when you have a (large) GND plane. It still works but you need a lot more patience. With 200µm width track, the isolation between pad and GND plane is better than with 1mm width tracks. At least i had difficulties with some PCBs because of this problem.

Well, that is problematic. It is just such a unnecessary problem we create by having a system and a different pile of units that are incompatible to the metric system and even incompatible among themselves.
And don’t forget, there is not just the inch, there are many different units that are all called inch and have a different incompatible definition. Which was the main reason we adopted the metric system.

We learned your language, where it is very subjective to determine which is a better language. Please use at least the metric system, which is objectively simpler and more consistent.

So you don’t actually understand what an inch is.

OT but I was educated in both imperial and metric units. I seem to remember my engineering course switching from FPS to Metric with the change in year, just like that. Which was strange because I had already used Metric in high school. But then I am nerdy and used to remember conversion factors like 1760 and 5280, as well as the whole system of linear units in Imperial. Oh and that reminds me of the Asian system of weights I used in markets. So pfft, languages (natural and computer), units, etc. I can cope.

My favourite FPS engineering unit name was the slug.

If you have object A which has a grid of 2mm (lets say a mounting panel where you want to mount a PCB to) and object B with a grid of 2.54mm (for example a PCB with evenly spaced holes (i German we say Lochrasterplatine, i don’t know the exact English word, there is breadboard but this term is ambiguous)), you have a harder time, no matter how good you can deal with mentally / how easy it is for you to convert one to the other.

I have written this before on this forum.
If I need to make a TO220 or DIP16 I will use a .1 inch grid.
If I need a BGA I will use a .8mm grid

If I have both on a PCB I will alternate grids. I am completely conversant with both Metric and Imperial systems and very happy to swap back and forth on the one board. I am more interested in making a PCB functional rather than worrying about what the measuring system shows on the grid setting.

Both systems are used in the manufacture of components, so I believe it is of great benefit to be fully conversant with both systems.

Eg.
6 x .1 inches = .6 inches is a lot easier than 6 x 2.54 mm
12 x .8mm = 9.6mm is a lot easier than 12 x 31.49606mil.

While it sounds reasonable to all agree on one standard, Americans cannot agree on much of anything and trying to change that vaguely reminds me of this one:

Back in the 1970s there was some effort to get USA on the metric system and that effort fizzled miserably.

Yes in my opinion that is dealing with engineering life as it is.

BTW: In using our kitchen scale, I have come to appreciate the advantages of working in grams. But my wife is originally from a country that more used the metric system, and she is more likely to work with pounds and ounces.

You lost me on that one. I know that an inch = 25.4 mm. I think that is all I need to know. I use it often enough so that it is easy to remember.

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I use the calculator I bought about 20 years ago. Into its one memory field I have written 0.0254 and never changed it. I’m not sure but I think I had to rewrite it when I have changed batteries.

[rant]

Yes and no. There are multiple incompatible definitions of inch. On of them, the international inch, the currently most used one, is defined as 25.4 mm. But this inch doesn’t exist since a very long time, in fact only about 70 years. There are other units also called inch that aren’t 25.4mm.
A historical definition of inch was:

three grains of barley, dry and round, placed end to end, lengthwise

While we don’t use so many different units with the same name any more as we had 200 years ago, we still use different units with the same name. Some examples:

• A inch can be 25.4 mm, as you know. But it can also be 33+1/3 mm = 1/30 m, Taiwan uses this inch for some measurements.
• A pound can be 500 g (if you are in German and want to buy bread or vegetables and ask for a pound, you will get 500g). It can be 453.592 37 g or it can be about 373 g. Or it can be a force of about 4.9 kN, 4.45 kN or 3.66 kN.
• A ounce can be 28.349523125 g, 31.1034768 g, 100 g or it can mean something completely different and mean volume, thickness or force.

I am sure there are many more variants still in use i didn’t mention.

The image you showed doesn’t fit the use of metric system. There is a single system that is actually used and can be called system, the metric system [1]. There are no real competing standards. The British imperial units are not a system. It is a pile of units. With the adoption of metric units we did go the other way than the image implied. Before, there where a lot of incompatible units, all with the same name, in use. Something like pound was the most extreme. If you did go from one city to the next, the definition was different. This incompatible definitions was the main reason we adopted a better system. Only a small part (UK + USA) where too stupid to do the same where it matters. And because of this stupidity we now have to suffer this problems.

And they are not even consistent. For example, as soon as you use more complex dimensions, like Voltage, everyone uses metric units. 1 V is 1 kg*m^2/(s^3*A) . If you use inch to measure you PCB, please be at least consistent and use lb*in*mile/(s^2*A*h). Not that any sane person would ever use such a unit, but that would be the logic consequence of using non-metric units.

On top of all that, you have to actually learn the metric system before you can understand the more modern definitions of this non-metric units. So why use them when you have to know the metric system anyway?

[1] There would also be the Planck system, which is only based on natural constants, which would make more sense in theory, but no one uses it and we don’t know the exact values since we can’t measure G very accurately. But since we talk about SI vs ancient units and not about SI vs Planck, we can ignore that.

[/rant]

I already learned your language, which was a lot harder than learning the metric system, can you at least use the metric system? Thank you.
It would also be a lot more professional.

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My understanding early on was that despite using metric for many things, a lot of PCB fab houses used mils as the defining measurement to determine via sizing and track width/spacing and feature sizing. This may have been due to the fact that certain vendors of manufacturing equipment used inches instead of meters…I think it’s best if everyone is comfortable with both and just use what your fab house asks for.

At the risk of continuing this OT diversion, I honestly don’t know what your complaint is about. I am familiar with both systems and use the appropriate one. When dealing with legacy ICs, I use the 0.1 inch (ok 2.54 mm) grid. When I have those mm grid chips, I use mm. In daily life here, it’s metric.

Look face it, both systems for dimensions in electronics will be in use for as long as the machinery and products are around, and metric will slowly gain prominence. Will that do? At least we aren’t dealing with gallons and Farenheit in KiCad.

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Edit: That comment has me laughing out loud.

If you have 10 carpenters who build a house with no plans and little communication, it will be a bad house. But that does not mean that it is not a house. (At least it is a house before a storm blows it apart.)

I think that any American pcb designer can work with metric units. But if one’s employer or customer wants inches, we do what is requested. And as (I think @retiredfeline) said, if you have an IC with pins on 50 mil or 100 mil grid, it is easier to use mils or inches than to convert all of that to mm.

Sorry I use gallons and Fahrenheit because that is what I grew up with. Gasoline and milk is sold in gallons. Our car odometers and signs on the freeway show miles. When I was a child in 1st grade I learned to show a date as month, day, year and that is the way that most web forms want it today.

For engineering I use Celsius.

At least you named it correctly. Even here some people refer to it as Centigrade.

TiC: I’m starting a campaign to get people to use Kelvin instead of Celsius. Advantage: no measurements will ever be negative. Of course referring to today’s temperature as 308K could be a bother, but I’m sure people will see the light. Wanna join?

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Well OK. But what to do when the temperature outside is zero degrees? Go swimming in liquid helium?

The other boss in the house showed me one of her cook books.
Measurements include: teaspoons, tablespoons and cup-fulls. They are not metric, but the results taste good.

React. Verrrry slowly.

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It is a pile of units that not quite conveniently fit together.

An inch has never been defined as three barley corns. It is the equivalent of three barley corns.
The yard was the original definition, but a yard, much like the metre, is pretty useless. Both are too big for small things and too small for big things.
How often has anyone seen a PCB size quoted in metres.
So the foot was developed. The foot is about the same length as the appendage on the end of a leg. Next was the inch. I suspect 12 was used because more numbers will divide evenly into 12 than 10.
IT MUST BE REMEMBERED that the Imperial system was built long before measuring rulers were invented, produced and distributed. People needed something practical to relate to.

My favorite is the stone, a measurement of weight. The original definition I remember reading somewhere was something like: a rock large enough to be comfortably lifted with two hands.

Anyway, I will move to metric exclusively when all the breadboards, DIP16s, TO220s, Chinese plug-in modules, and Kicad Imperial grids have all been buried in landfill and new tooling results in the distance between the pins of the above mentioned components becoming 2 or 2.5 or 3mm, not the stupid (my opinion) 2.54mm.

If everyone (even people whose work does not involve numbers) learned scientific notation then we could avoid that problem and measure everything in " *10^-XX light years."

BTW I think this discussion should win the “Let’s get as far as possible off topic” award.

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