Yeah, for folks with experience in machining (in the US), this is a "tenth" (a pretty fundamental unit in machining is a thousandth of an inch, or a "thou", for very high precision work, that can be divided into "tenths", or ten thousandths of an inch).

A mil, or a thou, really isn't good enough for high precision work. 25.4 microns is a lot. A surface flat to that isn't really that flat. An instrument that measures to that isn't really a very high precision instrument. Microns are a pretty good unit.

EDIT: There's this rule of thumb (I might be getting this slightly wrong) that one in a million accuracy/precision is about what you can expect to achieve without resulting to exotic means (at least order of magnitude). So microns of flatness over a meter e.g. Trying that with imperial units just breaks my mind, what's a million mils. Sure I can calculate that. But it's like a pretty useless measure. So accuracy of one thou over a million mils is sort of weird. (1000 inches which I'd have to translate to some normal unit... 25.4 meters ;) ).

Yeah, that's why tenths (~2.5 microns) are used when thousandths aren't precise enough.

I'm definitely not claiming it's a good, I'm just describing the system I have seen used in the handful of machine shops I've spent time in.

I think YZF just mis-parsed your sentence, and thought you meant a thou is for very high precision work, while you meant that a tenth is for very high precision work.

(That said, I'd add -- measuring to one part in 10^6, for anything except time, is challenging but still very doable; machining to one part in 10^6, for anything beyond very simple shapes, does require pretty exotic methods or very great care, and to verify it you have to measure to one part in 10^7).

Oh, yep, there's an extra comma that makes my comment a bit ambiguous. Alas, it's outside the edit window...

I think "a tenth of the thickness of a human hair" would be an appropriate layman description of the approximate size.

30 or so coronaviruses side by side.

How many would fit in a football field?

Like, a grip of library of congresses worth?

A few micrometers is around 1E-4 inches or 3E-6 yards for those of us who think in freedom units (aka real units).

You joke, but this would (perhaps even more absurdly, really) be called a "tenth" by most people who care about imperial units at this scale (as explained in a sibling comment)

This is pretty anti-intellectual isn't it? People, in my opinion rightly, make fun of measures expressed in libraries of congress or Olympic pools or African elephants, and then an article actually uses a normal SI measure and that gets disparaged too? What are authors supposed to do?

There was nothing disparaging about the comment. There is also nothing anti-intellectual about comparing things you don't know to things you do, in order to contextualize information. And he did that by first converting to mm, so did not abandon SI measurements at all. I agree that when we are talking about more common measurements it can be gratuitous or grandstanding, much like talking about things over 1x size in percent, as e.g. 450% bigger, etc. But that is not the case here.