RE: Tolerance in text

Subject: RE: Tolerance in text
From: "Jonathan West" <jwest -at- mvps -dot- org>
To: <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Tue, 21 Feb 2006 14:29:06 -0000

Hi Diederik

> I have a few questions about measure tolerances wrote in text.
> --------
> Point 1:
> --------
> What is the best way to write a measure with tolerance - where
> both '+' and '-' are equal - in a text (or non-technical illustration)?
> I normally write:
> "Drill a hole with a diameter of 25 +/-0.2 mm."
> My colleque writes:
> "Drill a hole with a diameter of 25 ±0.2 mm."
> I _obviously_ prefer the first one.
> Because the '±'-sign means 'approximately' (the hole can be 24 mm
> or 26 mm) and not 'plus or minus' (the hole must be between 24.8
> mm and 25.2 mm).

No, the ± sign means plus or minus. ? means approximately equal. +/- came
into use on typewriters or printers that didn't have a ± symbol. I would
prefer using ± as it is more compact and as it *is* the symbol designed for
this purpose.

> --------
> Point 2:
> --------
> What notation is correct or more, preferrable:
> A. "Drill a hole with a diameter of 25 +/-0.2 mm."
> "Drill a hole with a diameter of 25 mm +/-0.2 mm."

The second of these options is better. You are avoiding any doubt by
specifying the units both for the base measurement and the tolerance. (I
would also use the ± symbol here of course.)

If you don't want to repeat the unit, then parentheses should be used as
follows to ensure that it is quite clear that the unit designation applies
to both the base and the tolerance

"Drill a hole with a diameter of (25 +/- 0.2) mm."

> B. "Drill a hole with a diameter of 25 +0.2/0.5 mm."
> "Drill a hole with a diameter of 25 +0.2 mm/0.5 mm."

Both of these are incorrect. It should be

"Drill a hole with a diameter of 25 mm +0.2 mm / -0.5 mm."

You must specify the minus sign for the second tolerance value, otherwise it
will not be clear what the second figure refers to.

If you are able to, even better would be to have the positive value as a
superscript and the negative vale as a subscript. I'll show this below, as
far as it is possible with plain text. (Make sure you have fixed width
characters displayed for this)

"Drill a hole with a diameter of 25 mm."

Where the "+0.2" is formatted using a superscript and the "-0.5" is
formatted using a subscript. This is how the writers of standards do things.
For an example, take a look here for how the British Standards Institute
states how tolerances should be written in British Standards

Scroll down to section 21.10. National and international standards bodies
are usually very strict about the way these things are expressed, because
their documents have to be as unambiguous as possible.

Jonathan West


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Tolerance in text: From: Links, Diederik

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