bathroom scale hysteresis

Sat May 28, 2005 12:35 am   

Sneaky scales fudge weights deliberately.

I suspect that many modern, microprocessor -controlled bathroom scales
have programmed in hysteresis to increase user confidence in the
scale's accuracy. The scale remembers fresh measurements, say 166.2#,
and if it next measures same weight +/- a little (within a short time),
say 166.8#, then the scale decides to report the *original* measurement
166.2#. Neat. Sneaky. User believes the scale is highly repeatable.

On such a scale, I weigh myself multiple times and get the same reading
(to the 0.2#) each time. 166.2#, 166.2#, 166.2#, 166.2#, ...
Then I weigh something different (myself holding a load), to reset the
memory.
Then I weigh myself again... Now, get something like 166.8#, 166.8#,
166.8#, 166.8#, ... very solid again, but 0.6# different from 1st
series of measurements.

I tried 6 models (2 or 3 brands) at a retail store display and find
this "feature" common.

Is it important? Maybe yes in the following scenario -- in some sports
like wrestling, boxing, judo, you have multiple competitors weighing in
at the same time, same scale, with possibly very similar weight. Some
competitors are concerned with as little as 0.25#. In this case it
seems one competitor could inherit the weight measurement of the person
in front of him.

In a perfect world, competition weigh-in equipment should be
certified/calibrated. But since the bathroom scale appears so
repeatable, some competitions now use modern, microprocessor
-controlled bathroom scales.

Somebody's going to say... "you shouldn't do that". Right, I agree.
But (a) it's happening, because (b) this hysteresis (memory)phenomenon
isn't widely known, I suspect.

Anyone care to confirm? Contradict? Repeat the experiment on their
own scale? Comment? In my experiment I didn't bother to determine
what weight difference resets the memory -- 1#? 1.5#?

Regards,
John Ruckstuhl

Sat May 28, 2005 2:38 pm   

"Robert Baer" <robertbaer_at_earthlink.net> wrote in message
news:ndYle.1270$s64.431_at_newsread1.news.pas.earthlink.net...

I've noticed with balance scales that the weight shifts by several pounds
depending upon where you stand on it ... I assume you're supposed
to stand more or less in the center, an inch one way or another can
make a difference of a pound or two (important if you're doing a
sport with weight divisions).

-JS2

Sat May 28, 2005 6:03 pm   

"I've noticed with balance scales that the weight shifts by several
pounds
depending upon where you stand on it ... I assume you're supposed
to stand more or less in the center, an inch one way or another can
make a difference of a pound or two (important if you're doing a
sport with weight divisions)."

This would be the incentive to repeat the last reading. This would make
the electronic scale seem more dependable, stop customers from worrying
about water weight gains and losses. It would take a very small amount
of digital memory and programing. I am sure there is even memory for
two or three "normal weights". There is probably a mass produced chip
that includes this program and you will need to find one with a
different chip to change the repeat. I bet you even paid extra for the
"added accuracy"!

Mon May 30, 2005 7:47 am   

Our bathroom scale has exactly the same problem - the same reading is
repeated when you weigh yourself a number of times.

If the first reading were, say 160.3, further readings would be either
160.3 exactly, or something outside the range of 160.3 plus/minus 1.

These results, I believe, is a result of clever firmware and not
mechanical or sensor characteristics.

Wed Jun 01, 2005 2:55 pm   

Hi,
Thanks to all who took time to reply.

Yesterday I went to a Bed, Bath, & Beyond store where there were
several models on display.

I brought with me several 0.5 liter water bottles (approx 1 lb.) and
half-consumed one (approx 0.5 lb.) so that I could adjust my weight in
0.5 lb. increments. I tried 3 models and found two that I think
exhibited this programmed hysteresis, and one that did not.

The $40 Thinner TH300 uses load cells and reports to 0.5 lb.
Holding an empty shopping basket and full pockets, I measured at 181.0
six times in a row. Then, holding ~0.5# in my basket, I still measured
181.0. Again, holding ~1#, I still measured 181.0. Then, when holding
~1.5#, apparently past the programmed hysteresis threshold, scale
reported 182.5.

The $50 Tanita BF679 reports to 0.2 lb. It did not have hysteresis.
Each measurement was independent, reporting like 178.2, 178.4, 178.6,
178.4, ... (I wasn't holding an empty shopping basket for this one).

The $60 WeightWatcher WW60 (Scales by Conair) uses load cells and
reports to 0.1 lb.
I (plus basket) measured 180.7 six times in a row. Then holding ~0.5
lb., I measured 180.7 again. Then, holding ~1#, I measured 181.9 .

So, I see this programmed hysteresis in some models, with a breakaway
delta of ~1#.

I had thought I originally noticed this two months ago in several
Taylor models and a Tanita, but I wasn't able to test any Taylors
yesterday and my one Tanita test didn't show it.

Best regards,
John Ruckstuhl

Thu Jun 02, 2005 8:49 am   

I suspect that many modern, microprocessor -controlled bathroom scales>

There's no such thing you jerkoff.