Metric, good or bad?

Despite living in a strongly metric country, I've often found mils to be a handy unit for traces - as you say, they are typically whole units. I can imagine how 6/6 mil looks, but 0.16/0.16mm doesn't have the same feel.

Other than that, metric all the way.

We usually do mechanical design (sheet metal, machining) in decimal inches. And PCB layouts, except we create BGAs and such in metric, whatever's on the data sheet.

One surface-mount part that we use was dimensioned 900 on a side. Microns.

Some of my aerospace customers design jet engines (stress, thermal calculations, everything) in Old English units. Like BTUs and slugs and feet and lbf and lbm and ounce-inches. That's scary.

David Brown wrote in news:q45u1u$aoc$ snipped-for-privacy@dont-email.me:

The metrics for the metric system...

The seven units of the metric system and their fundamental constants:

Meter length. Distance traveled by light in a vacuum in

Second time. Exactly 9,192,631,770 cycles of radiation of an atom of caesium-133.

1023 elementary entities.

Candela luminous intensity. A light source with monochromatic

of 1/683 watt per steradian.

Kelvin temperature. Boltzmann constant, or a change in thermal

elementary charges per second.

NIST has since refined Plank's further...

Do they measure capacitance in Jars?

The building trade has "hard metric" and "soft metric", e.g.

ou input the data in inch units you can convert to metric perfectly. 1 inc h = 25.4 mm. But if you input data in metric and try to store it in inch es it doesn't convert so well and round off has to happen. 1 mm = 0.0393

cause they were typically whole units, 6/6 mil was a common trace and space . Now trace and space are commonly fractional thousandths, so metric is ju st as good. Some BGAs require 3.5/2.5 mil or 0.09 mm/0.0635 mm or 90um/63.

more than 12 mil for 6/6 mil trace and space which turned into 0.15 mm if I recall. This was close enough to 12.5 mil that it worked ok for routing t o TSSOPs and such. Routing a 0.5 mm QFP I had to switch to a 0.5 mm grid.

ing hole locations on PWBs?

be

eel.

Only in Leiden (which happens to be in the Netherlands) and not recently.

That's because we Brits understand those sort of things...

The way I see it, metric does not help or hurt when you are under a cm or i nch. If you use fractional inches fine, but any actual advantage of the Eng lish system is the ability to divide by three. Feet, inches, 12 gozinta mor e numbers than 10.

People fave designed just fine without ever even knowing what an inch is. T hey got used to it. Actually they DO know but they do not think in English. Me, I am, or was pretty sharp at the fractional to decimal parts of an inc h. Some people don't get it.

Unfortunately I think in English so when I look at most datasheets etc. tha t are in metric it is a little bit less easy to grasp the size of something in my head right away. Got to keep 254. and .3937 in mind.

In act, I read somewhere that they recently (within maybe five years) actua lly changed the definition of an inch to make the math come out better. I a m pretty sure they changed the meter standard but not sure if it really mat ters, usually...Anyone catch wind of that ?

Yes, in 2015 the metre was redefined to be exactly 40 inches. And Pi is exactly 3. And god created the world in seven days and on the eighth day he created jurb and sayeth unto him, "Go forth and blather insanely, and looketh thou not upon that which the heathen calleth facts, for verily they are fake news."

Cheers

That doesn't sound right :-)

The meter was redefined by some sort of molecular motion or some shit. The inch, only to make the math jibe.

snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:

quote:

The Academy chose the meridian definition. This defined one meter as one ten-millionth of the distance from the Equator to the North Pole.

... The definition of a meter has changed since 1791.

Today, a meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.

Because it's not. An inch is 2.54cm, exactly, so a meter is 39.37 inches (exactly), not 40 inches. Of course, the Pi==3 comment identifies it as bing a troll.

I guess some people are a bit rusty at math.

Rick C.

Coming in late (and without the original post to reply to so apologies):

I'm bilingual - grew up with imperial, learnt metric in school.

One are where metric (specifically S.I.) absolutely pounds any other system into teh ground is pure science, especially engineering and physics (chemists are still a bit weird in their use of units)

Everything is a ratio of 1 (bearing in mind the base mass unit is a kilogramme, not a gramme - that's just a naming anomaly left over from the CGS system).

So 1 Joule per Second is 1 Watt,

I did an electromagnet calculation for a big horseshoe magnet I made in school metalwork in the 80s but using my old man's 1940s engineering books and it nearly broke my head... I would much rather work in SI for that type of work.

But for everyday, I still prefer imperial for cooking (butter is still basically an 8oz brick within 10% (now 250g = 8.8oz, good enough for cooking) so quickly can subdivide down to a 1oz slice by eye. Egg is 2oz give or take. And so on...

Woodwork, I tend to metric, but if working with the neighbour we sometimes play "inches or mm today" for fun.

I switch back and forth depending on where the particular client is located or what they prefer. If they want the documentation in another language then it's metric almost by default.

Yes, it's usually all imperial there. To my surprise often even at European aerospace companies, despite people in Europe sayig otherwise.

Quote "... featuring Airbus' modern comfort standard of 18-inch wide seats ..."

Sometimes they have the metric numbers in brackets behind the imperial ones though not for stuff such as engine thrust:

I thought THAT was what they changed about the inch, to make it exactly 39.37.

Maybe I am remembering it wrong. DLUNU I think nailed it about the meter, but I am having trouble finding info about the inch.

Maybe I should put my browser in stealth mode and go check out the official NIST site...

No, the exact value is 25.4 mm to an inch. We all rely on that. If someone made your change, then it'd be 25.4000508, gaahhh!

Its actually more complicated than that. The US has two different definitions in use - the one you gave above and the land survey one.

The U.S. Survey Foot is defined as exactly 1200/3937 metres.

See:

John

In the early days of IC layout, when people cut patterns out of rubylith with xacto knives and had lots of inch rulers around, the consensus was that there was 25 microns per mil.