bathroom scale hysteresis

[James Meyer]

Occam would agree...

Occam might agree, but only if he never read the whole thread.

Jim

 

[Ken Taylor]

Occam might agree, but only if he never read the whole thread.

Jim

Occam should have grown a beard - much simpler.

Ken

 

[Andre]

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.

 

[GaryG]

About ten lines of code in C or C++; not at all hard to add.

-JS2

I have a hard time believing that they would do that. Why would they go to
the trouble of "lying" to users of their product?

If I noticed my scale doing that, I would not be happy. Fortunately, my
4-year old Tanita does not seem affected by this problem (I've tested it a
number of different ways, and never once noticed the problem described by
the OP).

Perhaps it's limited only to low-end scales.

Oddly enough, nobody complaining about this "problem" has mentioned brand
names either...

 

[The Real Andy]

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

Having spent some time in the weiging industry, i can assure you that
the cheap portable bathroom scales that you buy are neither:
a. repeatable
b. accurate
c. linear

In fact, most will become more innacurate in the most common weight
ranges.

In AU, a cheap set is $20, a good repeatable set is $400. The
expensive scale can be recalibrate and remains linear. Should it not
be linear a multi-point cal can be done. The cheap set can only be
thrown.

Most cheap sets are innacurate of the shelf as they are made in china
and fail to take into account local gravitational constants

 

[GaryG]

Having spent some time in the weiging industry, i can assure you that
the cheap portable bathroom scales that you buy are neither:
a. repeatable
b. accurate
c. linear

In fact, most will become more innacurate in the most common weight
ranges.

In AU, a cheap set is $20, a good repeatable set is $400. The
expensive scale can be recalibrate and remains linear. Should it not
be linear a multi-point cal can be done. The cheap set can only be
thrown.

Most cheap sets are innacurate of the shelf as they are made in china
and fail to take into account local gravitational constants

Do you have any experience with the newer digital models? Not the low-end
ones, but ones like the Tanita with ostensible 0.2 lb resolution, priced
around $50-90 (US).

For instance, http://tinyurl.com/49swe, or the more expensive German-made
Soehnle brand http://tinyurl.com/3t6kt.

GG

 

[[email protected]]

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

 

[GaryG]

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

Outstanding report...thanks!

My experience with Tanita has been the same - no hysteresis noted.

Consumer Reports magazine tested several models of body fat scales in the
last year or so. As I recall, they liked the Tanita the best - they warned
that body fat was quite variable, and probably not very accurate, but that
the body weight reported by Tanita was better than most other units (not
sure if they tested for hysteresis).

GG

 

[The Real Andy]

Do you have any experience with the newer digital models? Not the low-end
ones, but ones like the Tanita with ostensible 0.2 lb resolution, priced
around $50-90 (US).

For instance, http://tinyurl.com/49swe, or the more expensive German-made
Soehnle brand http://tinyurl.com/3t6kt.

GG

The high end Soehnle scales can be calibrated, but the ones show look
like you standard cheap crap. I could well be wrong here. Some Soehnle
stuff is quiet good.

Tanita, never seen anything but cheap crap.

The good scales I am familiar with are manufactured by a company
called AND. Try do a google for AND though!!

 

[[email protected]]

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

There's no such thing you jerkoff.

 

[The Real Andy]

http://www.andweighing.com/and.nsf/html/UC+Series

 

[Martin Underwood]

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

There's no such thing you jerkoff.

I presume that he was referring to scales with an LED display. I imagine
that these *do* have a microprocessor of sorts inside, to perform
analogue-to-digital conversion on the output of a strain-gauge attached to
the part you stand on, and then to convert this reading to signals which
control the segments of the display.

Now who's the jerkoff? ;-)

 

[Noah Little]

The "digital" bathroom scales I've disemboweled only digitized the rotation
of a disk. The disk would have had numbers on it, if it were a
"mechanical" scale. The disks had black and transparent radial bands, that
were read by a quadrature decoder/counter, and displayed on 3 LED digits.
The mechanics were exactly the same as in the old bathroom scales we've had
for a nearly a century.

That's true. But the point in question was the "no such thing" put
forth by a previous poster. That's clearly not true.

 

[Noah Little]

Do you think that this behaviour would result if the ADC resolution is
coarser than the display resolution, for example the LSB represents 1.5#?
Or maybe the ADC has missing codes. This wouldn't require tricky
programming, just a bad ADC.

Occam like that, too.

 

[Seth Breidbart]

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).

A balance scale at a gym I used to frequent had about a 5 lb
difference at the 200 lbs mark, depending on whether you moved the big
weight to the 200 lb notch, or moved it to the 150 lb notch and the
small weight to the 50 lb mark.

Seth

 

[Seth Breidbart]

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.

I have a Taylor bathroom scale, with a 1# hysteresis. I weigh myself
every morning. Each new weight is either the exact same as the
previous weight or at least 1.2 lbs different. (The scale shows
weight to 0.2 pounds.)

Seth