[time-nuts] OT Prototype Boards

[Hal Murray]

bob at evoria.net said:
> If you google "25.4001 conversion" you can find lots of tables using that as
> the conversion factor online.  I don't know where the error came from or why
> it's quoted so regularly. 

Thanks.  I never would have thought to search for 25.4001.  That's an amazing 
calibration on the quality of information out there on the web.

I still remember 39.37 inches per meter from early school years.  I wonder 
what grade that was.  I don't remember that there were any digits past 39.37 
but neither do I remember that there weren't any more.  I don't remember any 
discussion of accuracy back then.

My guess is that the conversion charts on the web come from somebody starting 
with 39.37 rather than 25.4 and being smart enough to do the arithmetic but 
not sharp enough to understand the accuracy or round-off issues.  I wonder 
how many of them are carried over from before the inch was redefined in 1959 
as compared to starting with 39.37.  You do have to be more than a little 
geeky to pay attention to things like this.  It's under 2 ppm.

My copy of Machinery's Handbook (copyright 1984) says 100 inches is 
2,540.00000 mm.  It also says 0.03937 inch/mm and 25.4 mm/inch with no 
discussion of the accuracy.

At the few ppm level you have to pay attention to temperature.  (The wiki 
page on micrometers discusses temperature and they are only good for 0.0001 
inches, 100 ppm.)



-- 
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