Re: text conventions/spec. round off &c

Subject: Re: text conventions/spec. round off &c
From: "Ridder, Fred" <F -dot- Ridder -at- DIALOGIC -dot- COM>
Date: Tue, 14 Apr 1998 10:07:10 -0400

Helen Cygnarowicz queries:

> can i please have your reactions to this statement:
> centimeters is not an accepted international unit of measure. it
> should be either meters or millimeters.
> can i also have your thoughts on this:
> it is not necessary to list a dimension to five significant digits,
> e.g., 210.98 mm wide. this number should be rounded up to 211 mm.
> it was my impression that in a Specification list, the exact dimensions
> should be used, but in a text reference, rounding up is ok. am i off the
> mark here?

Well, I was intending to leave this one alone, but we all know about

Regarding centimeters, I think the acceptance issue is one of audience/
industry/discipline. The meter is the fundamental, defined unit of length,
but I fail to see how a unit that is a simple decimal derivation of that
can be considered "not accepted", particularly when it is so useful for
measuring things on a human scale. For example, I'd give my height
in metric units as 185 cm, not as 1.85 meters (that pesky decimal is
too easy to miss) or as 1853 mm (inconvenient number and implies too
great a precision).

Regarding rounding, I strongly agree with L.G. Carlingshouse's reply:
the number of digits used in a stated dimension implies the precision
of the measurement and the tolerance of manufacture. Saying that
an object is 211 mm means that it can actually be anywhere from
210.5 to 211.5 mm, which is clearly very different from saying that it
is 210.98 mm. If that's the precision of the manufacture, that should
be the precision of the stated dimension. The only exception would
be the case where you use an approximate dimension to distinguish
among similar items of clearly differing sizes (e.g. use the 211 mm
spacer rather than the 187 mm spacer).

One case where false precision can creep into dimensions is when
converting between metric and inches. If an object's dimension is
defined to 1/10 inch tolerance, there is no justification for giving a
metric conversion to anything finer than 1 mm (roughly 1/25 inch).
Only if an object is dimensioned to 1/1000 inch tolerance should its
metric dimension be given to hundredth of a millimeter (and at that
point, it's probably also appropriate to explicitly indicate the tolerance
since 1/100 mm is less than half as big as 1/1000 inch). Also, be
sure that the conversion factor that is being used has an appropriate
number of decimal places for the number of significant digits in the
dimension itself.

Fred Ridder (mailto:f -dot- ridder -at- dialogic -dot- com)
Senior Technical Writer
Dialogic Corporation, Parsippany, NJ

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