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bathroom scale hysteresis

Discussion in 'Misc Electronics' started by [email protected], May 28, 2005.

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  1. Guest

    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
     
  2. GaryG

    GaryG Guest

    What models/brands? I find it hard to believe that the manufacturers would
    do that, because it would require additional programming in the scale, and
    most modern scales are quite accurate as is.

    FWIW, I have a 4 year old Tanita, accurate to 0.2 lb, and I've tested it
    like you suggest on a number of occasions. In all such tests, (weigh in,
    weigh-in holding a light weight of some sort, then weigh in again) the scale
    returns to within 0.2 lb of the original reading, and usually to the exact
    original reading.

    If I step off, reset, and step back on (without changing anything), I get
    the same reading about 65% of the time. The other 35%, it varies by 0.2
    lb - I assume this is when my actual weight is somewhere between the two
    reported weights.

    Perhaps your test was flawed somehow? Or, maybe you were testing some
    crappy scales?
     
  3. Uncle Al

    Uncle Al Guest

    [snip]

    The reproducibilty of an electronic bathroom scale depends on the
    surface underneath. If it is stone or rigidly laid hardwood you'll
    get good precision. If it is soft like linoleum or carpet, you
    won't. Any need for true precision weighing cannot be satisfied by a
    cheap consumer scale.
     
  4. Luhan Monat

    Luhan Monat Guest

    Exactly the same experience I had with a digital scale. It worked out
    to a 5 pound window - I dont know the time window. Showed weight to
    within 0.5 pound as I got on and off with increasing hand-held weights
    until about 5 extra pounds.

    I still have it, plan to rework the electronics with a loud voice output
    and leave it in the bathroom to catch unsuspecting house guests....

    "Ow, oooh, get the heck off me!"
     
  5. "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"!
     
  6. Rich Grise

    Rich Grise Guest

    Not only have I never heard of such a thing, I've seen the workings
    of scales that go to great lengths to avoid just that. If your scale
    shows different weights when the object is on different places on
    the platform, then you have been robbed.

    Cheers!
    Rich
     
  7. Rich Grise

    Rich Grise Guest

    Now this is just plain silly. Who in their right mind would go to
    such great lengths, just to produce a reading that isn't even what
    the weight on the platform is?

    The phenomenon that the OP, John Ruckstuhl, mentioned:
    "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."

    Is from hysteresis. He steps on the transducer, it's been sitting all
    day stabilizing, so it reads low. He steps on the scale holding weights,
    It shows his 166.2 + the weight + some unknown error. He steps off
    the scale, and it springs back to something greater than zero; the
    scale waits until it's done bouncing, re-zeros itself, and now when
    he steps back on it's biased high. The effect would probably be even
    worse if he just stayed standing there and set the weight down.

    The point being that the sensor takes a "set." And, probably not
    so much these days with little strain gauges, but with mechanical
    scales, there's sticktion to deal with.

    Cheers!
    Rich
     
  8. Luhan Monat

    Luhan Monat Guest

    This would not explain my experience, just picking up a little extra
    weight each time and getting back on the scale. It just would not move
    until about 5 pounds total had been added. Only explanation I have is
    that is it 'faking' the accuracy; and doing it quite intentionally.

    That's why is on my list of projects for re-engineering. Maybe Mavin's
    voice "You standing on my like that is just so Depressing!"
     
  9. ck

    ck Guest

    A properly designed and calibrated scale should not exhibit this behavior.
    Calibrations should be performed according to NIST Handbook 44, which
    includes a shift test, a return to zero (testing mechanical hysteresis), an
    increasing load test and others depending on the type of scale.

    Also, ISO 17025 for calibration labs require all as found readings to be
    measured and reported before any adjustments to calibration are made. As
    left readings are also recorded. Acceptance tolerances and maintainance
    tolerances are specified in Handbook 44 and vary by Class specifications.
    Most legal-for-trade requirements defer to Handbook 44.
     
  10. Rich Grise

    Rich Grise Guest

    I've answered this in a different branch - the sensor has a certain
    amount of hysteresis, but also excellent "repeatability" as long as
    you haven't moved the sensor's threshold in between, which is what
    John says is happening. The sensor takes a "set", or something very
    much like that. But I guess I'd have trouble blaming sticktion when
    they're using strain gauges - but anything, to measure how hard it
    is being stressed or strained, has to have some kind of change take
    place, or there wouldn't be any signal. Generally, that's movement
    of some kind. So, if it's something that flexes, it simply doesn't unflex
    back to _precisely_ the same position, for any number of reasons. At least
    not right away. Probably after awhile, yes. So when he first gets on, the
    sensor has been relaxing all night, so has no "set." He steps on the scale
    first time, and puts whatever "set" that 166.2 puts on it. Then, when
    he steps off and on, The force on it is the same, so it doesn't add any
    "set", and since the thing has good repeatabilty, it reads the same. Then,
    he steps on it with weights on, and it reads, say, 191.2. (It would be
    very interesting to see this number, if the weights are calibrated). This
    increases the "set" to S(191.2). Now, he puts 166.2 on it, but it doesn't
    have enough force to even _reach_ the higher "set," so effectively zero
    has backed away by 0.6, and it reads 166.8.

    Nothin' to it! ;-)

    Cheers!
    Rich
     
  11. Luhan Monat

    Luhan Monat Guest

    Sorry, something is getting missed here. I kept picking up an extra
    pound of stuff each time and then getting back on the scale. So it had
    some chance to re-zero. But all it did was report exactly the same
    weight each time. When the accumulated extra weight was about 5 pounds,
    then, and only then, did it jump to a new weight. Then again, it
    claimed that to be the real weight as a dropped off a pound at a time
    getting on and off the scale.

    The software in the scale was obviously fudging the numbers to produce a
    false sense of accuracy.
     
  12. James Meyer

    James Meyer Guest

    I'd be willing to bet that there is a sticker on the scale or a note in
    the owners manual with a phrase like "... not legal for trade".

    You bought a freakin' bathroom scale fer cripe's sake, not a NIST
    certified research instrument. Get over it.

    Jim
     
  13. Carl Ijames

    Carl Ijames Guest

    For about $180 you can get a freon charging scale with 400 lb range and
    accurate and reproducible to a fraction of an ounce. I guarantee there
    is no funny stuff like you are seeing in the firmware :). I forget the
    brand we got last year at work but it has those specs.
     
  14. I think it is quite common. Mine does the same thing. What I do is
    move about to get the maximum, then the minimum, then split the
    difference (typically about 5 kilos!).
     
  15. Luhan Monat

    Luhan Monat Guest

    I am 'over it', hun. This discussion is about whether the error was
    just 'cheap design' or 'intentionally fraudulent'. It was only of
    interest to me at the time because such technology (micro controllers)
    had not been used before to fake extra accuracy.

    I simply recommended to my friends that they buy analog scales:
    inaccurate maybe, but at least they could show moderate weight gain or
    loss reliably.
     
  16. GaryG

    GaryG Guest

    I think you just got a bad scale. My 4-year old Tanita does not have that
    problem at all. I can step on and off, and always get the same reading (or,
    within the 0.2 lb resolution). If I step off, drink an 8 oz glass of water,
    and step back on, it shows that I've gained 1/2 lb. If I weigh myself
    before and after a crap, it shows (appropriately) 1.5-2 lbs lost.

    No weigh (get it?) would I go back to those crappy old analog dial scales.

    GG
     
  17. Rich Grise

    Rich Grise Guest

    Then your scale is lying to you. It actually has a resolution of 5 lbs,
    but since the designers thought that wouldn't sell well, they just
    make up some random interval, or maybe interpolate based on the time
    of day or barometric pressure or something.
    Yes. Obviously. :)

    Did you get it for $5.00 at an auction? ;-)

    Cheers!
    Rich
     
  18. Pig Bladder

    Pig Bladder Guest

    What a load of crap! ;-D
     
  19. Tam/WB2TT

    Tam/WB2TT Guest

    John,
    My guess is that the hysteresis is caused by friction in the mechanical
    workings.

    Tam
     
  20. John Miller

    John Miller Guest

    Occam would agree...
     
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