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Hi everyone:
There is nothing worse than using a measuring device that does not
state the error. Since the usual rule of thumb in science in that case
is to assume it os only accurate to half of the smallest scale
increments, that would make the scale accurate to 0.5 kg.
I used a spreadsheet and took the numbers from 70 to 90 in 0.5 kg
increments, converted them to pounds using 2.2046 pounds/kg, and then
rounded them all off to the nearest integer. That gives values in
pounds ranging from 158 to 198. When I looked at the converted values I
found that 167, 179, and 189 did not show up. In this case the pattern
appears to be that every eleventh number is skipped. When I did it with
just integer values for kg, I was missing every second value, so the
scale is likely more accurate than 1 kg.
Since David's numbers are not "skipping" every 11th number, it may be a
round off effect combined with some other internal error in the scale.
It would be interesting to take the scale and increbentally add 500 g
lead masses to it to see what the readings would be....
Pat
On Dec 3, 2006, at 5:19 PM, David & Alison Webster wrote:
> Dear All, Dec 3, 2006
> There is, as usual, a lull in NatureNS traffic as we near winter
> and this lull provides an opportunity to comment on something that I
> found interesting; a digital readout with 'missing numbers'.
> I have weighed myself on digital bathroom scales, that have
> readout to the nearest pound, for about 255 sundays and readout spans
> 192 to 168 lb. It soon became clear that some weights were favoured
> relative to others. For example for the 91 weights on one page (May 4,
> 2003 to Jan 23, 2004), the readings cover a range of 172 to 185 but
> some intermediate weights were not represented.
>
> The number of readings at each weight within this range were:
> 172; 7
> 173; 0
> 174; 6
> 175; 3
> 176; 9
> 177; 0
> 178; 15
> 179; 7
> 180; 20
> 181; 0
> 182; 15
> 183; 0
> 184; 8
> 185; 1
> indicating that some weights, such as 177, 181, and 183 are likely
> to be filtered out by some distortion in the instrument and appear as
> some different reading.
>
> Much of the world uses metric so I looked at the implications of a
> kilograms to pounds retrofit that was tacked onto scales that had been
> designed to read to the nearest kilogram. For one artificial case,
> with input being a series of weights difffering by 0.05 kg (76.50,
> 76.55...); assuming output rounded to nearest kilogram, then converted
> to lb by dividing by 0.45 and then rounded to the nearest pound, the
> frequency of weights is either zero or 20 within the range 171 to
> 187--
>
> Weights with a frequency of 20 were 171, 173, 176, 178, 180, 182, 184
> & 187 while
> weights with a frequency of zero were 172, 174, 175, 177, 179, 181,
> 183, 185 & 186.
>
> The gaps in the artificial readouts don't exactly match gaps in the
> actual readout (177, 181 and 183 do but not 179) but both examples
> show that a rock-solid digital readout to zero or more decimal places
> may not be as reliable as it appears.
>
> Yours truly, Dave Webster, Kentville
>
>
>
>
>
>
========================================================================
==
Patrick Kelly
Director of Computer Facilities
========================================================================
==
Faculty of Architecture and Planning
Dalhousie University
========================================================================
==
PO Box 1000 Stn Central 5410 Spring Garden Road
Halifax, Nova Scotia B3J 2X4 Halifax, Nova Scotia B3J 2X4
Canada Canada
========================================================================
==
Phone:(902) 494-3294 FAX:(902) 423-6672 E-mail:patrick.kelly@dal.ca
========================================================================
==
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Hi everyone:
There is nothing worse than using a measuring device that does not
state the error. Since the usual rule of thumb in science in that case
is to assume it os only accurate to half of the smallest scale
increments, that would make the scale accurate to 0.5 kg.
I used a spreadsheet and took the numbers from 70 to 90 in 0.5 kg
increments, converted them to pounds using 2.2046 pounds/kg, and then
rounded them all off to the nearest integer. That gives values in
pounds ranging from 158 to 198. When I looked at the converted values
I found that 167, 179, and 189 did not show up. In this case the
pattern appears to be that every eleventh number is skipped. When I
did it with just integer values for kg, I was missing every second
value, so the scale is likely more accurate than 1 kg.
Since David's numbers are not "skipping" every 11th number, it may be
a round off effect combined with some other internal error in the
scale. It would be interesting to take the scale and increbentally add
500 g lead masses to it to see what the readings would be....
Pat
On Dec 3, 2006, at 5:19 PM, David & Alison Webster wrote:
<excerpt>Dear All, Dec 3, 2006
There is, as usual, a lull in NatureNS traffic as we near winter
and this lull provides an opportunity to comment on something that I
found interesting; a digital readout with 'missing numbers'.
I have weighed myself on digital bathroom scales, that have
readout to the nearest pound, for about 255 sundays and readout spans
192 to 168 lb. It soon became clear that some weights were favoured
relative to others. For example for the 91 weights on one page (May 4,
2003 to Jan 23, 2004), the readings cover a range of 172 to 185 but
some intermediate weights were not represented.
The number of readings at each weight within this range were:
172; 7
173; 0
174; 6
175; 3
176; 9
177; 0
178; 15
179; 7
180; 20
181; 0
182; 15
183; 0
184; 8
185; 1
indicating that some weights, such as 177, 181, and 183 are likely
to be filtered out by some distortion in the instrument and appear as
some different reading.
Much of the world uses metric so I looked at the implications of a
kilograms to pounds retrofit that was tacked onto scales that had been
designed to read to the nearest kilogram. For one artificial case,
with input being a series of weights difffering by 0.05 kg (76.50,
76.55...); assuming output rounded to nearest kilogram, then converted
to lb by dividing by 0.45 and then rounded to the nearest pound, the
frequency of weights is either zero or 20 within the range 171 to
187--
Weights with a frequency of 20 were 171, 173, 176, 178, 180, 182, 184
& 187 while
weights with a frequency of zero were 172, 174, 175, 177, 179, 181,
183, 1