Conversion translators need to be smarter then just converting two numbers
by a conversion factor. They need to do some sensible rounding using the
principle of significant figures. In the example below, there is 2 feet
with one digit and an answer in centimetres with 4 digits. Thus the result
implies an accuracy much more then the original intent.
If a number like 2.000 feet were entered I could see a need for a result
like 60.96 cm. But when a number like 2 feet is entered, then the result
should be rounded to the same number of significant figures and intended
accuracy. Since the result, if rounded to two digits would be 61 cm, we can
see that this still is more accurate then the original. Thus, the result
can be further rounded to 60 cm and maintain the same level of accuracy.
In mechanical design using inches, rarely does one see dimensioning below
1/16 inch or 1.6 mm. Not everything made is done in decimal inches in
thousands. That language is foreign to many inch based users. In most
cases decimal inches are more precise then needed for the application.
The use of the millimetre is actually a better replacement and gives better
accuracy then the fractional inches it competes with. Products converted
from fractional inches can easily within standard fractional tolerance
ranges be converted to whole millimetres without any loss of precision. A
dimension of 1/8 inch can easily become a dimension of 3 mm. The reason for
converting to whole millimetres is that they can easily be read on a
standard millimetre scale without guessing where the real converted value
lies.
Much of the older drawings that the company I work for still uses shows
dimensions in fractions with a notation in the block that all fractional
dimensions have a tolerance of ±1/32 inches. This is very close to ±1.0 mm,
meaning a dimension of 1/8 inch (0.125 inch) can very anywhere from 0.09375
inch to 0.15625 inch (or 2.4 to 4.0 mm), thus as you can see 3 mm is an
acceptable alternative to 1/8 inch.
The most important thing to learn when metricating is that the result of a
conversion factor is not the real answer and if one thinks it is so, then
one is going to have a hard time dealing with non-rounded numbers. Metric
results need to be workable on metric measuring devices. If you want to
make metrication and the conversion process as simple and accepting as
possible, these simple facts need to be stressed.
Dan
----- Original Message -----
From: "Jim Elwell" <[EMAIL PROTECTED]>
To: "U.S. Metric Association" <[email protected]>
Sent: Saturday, 2005-10-22 11:31
Subject: [USMA:34969] Re: Metric converter web site?
At 21 October 2005, 10:30 PM, Pat Naughtin wrote:
To provide a link to a metric conversion page is not, in my view, a good
idea.
The process of metrication is not encouraged by metric conversions; or to
be more blunt, metric conversions are a direct impediment to metrication
progress.
I agree with your position that doing conversions impedes the process of
LEARNING metrication.
However, once someone knows metric, they may still have to do a variety of
conversions in their work. This is particularly true of engineers, who are
my company's main audience. I certainly know metric, but regularly refer
to my company's rulers or to ASTM/IEEE 10 2002 for conversion factors.
Terry's comments on Google's calculator were interesting, both from the
point of a feature of Google that I was not aware of, and from the bigger
viewpoint of the efforts of Google to surmise what you want from pretty
minimal information.
For example, merely typing "two feet" yields:
two feet = 60.96 centimeters
Now, what if I were interested in bipedal animals?
As interesting and useful as the Google calculator is (and I am sure I'll
use it now that I know of it), I would still be interested in a site that
can convert just about any known unit into metric, but not the other way
around.
Jim
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