bathroom scale hysteresis

john.ruckstuhl · 2005-05-28T00:35:03+00:00

Sneaky scales fudge weights deliberately.I suspect that many modern, microprocessor -controlled bathroom scaleshave programmed in hysteresis to increase user confidence in thescale'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* measurement166.2#. 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 thememory.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 1stseries of measurements.I tried 6 models (2 or 3 brands) at a retail store display and findthis "feature" common.Is it important? Maybe yes in the following scenario -- in some sportslike wrestling, boxing, judo, you have multiple competitors weighing inat the same time, same scale, with possibly very similar weight. Somecompetitors are concerned with as little as 0.25#. In this case itseems one competitor could inherit the weight measurement of the personin front of him.In a perfect world, competition weigh-in equipment should becertified/calibrated. But since the bathroom scale appears sorepeatable, 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)phenomenonisn't widely known, I suspect.Anyone care to confirm? Contradict? Repeat the experiment on theirown scale? Comment? In my experiment I didn't bother to determinewhat weight difference resets the memory -- 1#? 1.5#?Regards,John Ruckstuhl

May 28, 2005 43 Replies

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Chris Jones 21 years ago

snipped-for-privacy@gmail.com wrote: [snip]

[snip]

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.

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Noah Little 21 years ago

Occam like that, too.

Noah

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Seth Breidbart 21 years ago

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

Don\'t ever masturbate after getting capsaicin on your hands. -- Patrick Arnold

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Seth Breidbart 21 years ago

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

Oh well, never mind. I have the electrodes and the conductive gel and a serviceable familiarity with sendmail.cf -- Tanuki the Raccoon-dog

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