Not really a DIY question (unless it turns out that there is in fact a DIY fix), so apologies.
I have a bathroom scale with an LED display (branded Weight Watchers but I don't know the actual maker) which is a good few years old now. It's always been completely reliable but over the last couple of days the display has refused to stabilise: it flicks up and down over a range of about half a stone, never stopping on any one reading.
I'm assuming that it's an electronics problem rather than a mechanical one, but am I correct?
Are they on a firm/hard surface? We've some bathroom scales in our en-suite which do this on the bathroom carpet- which has a soft pile. On the bedroom carpet (more of a berber pile), they are fine. Put them in the other bathroom (a tiled floor) and they are fine.
It could be they've been moved and a fresh bit of carpet pile is causing your problem.
Have you tried replacing the cell/battery? I had this with the 10kg kitchen scales and a new CR2032 cell fixed it (always check the fuel before stripping the engine!).
When I started as a maintenance engineer (over 50 years ago), the golden rule was "always check the power supply first", It's stood me in very good stead over the years.
I had a problem with a hard disk TV recorder (a PVR) which would boot up and present its initial menus and would act as a tuner (converting aerial input from digital TV into SCART output) but would not show the recordings on the hard disc or allow new recordings to be made.
I suspected a failed hard disc and, because I had nothing to lose, removed the disc to see if it would spin up and could be read by connecting it to a PC. Answer: yes it spun up without any problem and Windows could see two partitions but could not detect filesystems on them. I later learned that they were probably UNIX-specific filesystems - I bet if I were to connect the disc to my Raspberry Pi nowadays the disc could be read; I might try it one day as a class exercise!
Something made me check the supply voltage and I found that the PSU produced a no-load voltage of 12V, but this dropped to about 8 V as soon as I plugged the lead into the PVR. Fortunately I had a spare PSU from a hard disc caddy, with the same connector (which was wired in the same polarity - I checked!) and was rated at the same voltage and at least as much current as the old one.
The device immediately sprang into life and had worked fine ever since. So I now always suspect the PSU whenever any device misbehaves and check that the on-load voltage is close to what the PSU is supposed to produce.
Spot on. When I opened it up I found that one of the two rear feet which act as sensors (each connected to the main PCB by a single wire) has come adrift from its fixings (very small plastic studs on the case, seemingly just push-fitted into holes on the internal flange of the foot.
I'll try refixing the flange (Araldite, maybe?) and see if that solves the problem. In the meantime, many thanks for all the thoughts and suggestions.
While many of them are utter shit, not all of them are. My Soehnle Spirito has been very reliable weight wise for more than a decade now and I have only recently stopped using it because its replacements load the weight reading into my smartphone automatically so I don?t have to fart around manually entering the data.
I have just recently replaced it with a Renpho ES-CS20M and it gives the same weight and is just a reproducible and shows small changes in weight when I hold something small when weighing myself.
Just after I got it I got a Easy Home Body Analysis Smart Scale 63684, an Aldi special buy which also is just as reliable weight wise and also gives the same weights. I got it because I could try it and return it for a full refund and just havent gotten around to returning it yet.
My new Salter Ultimate Accuracy Electronic Digital Bathroom Scales are increditably accurate. They do have to be on a hard surface. I use them on an piece of wood which itself justs rests on a carpet. When I weighed my suitcases I got the same result as the Gatwick Easyjet self check in.
Good price too. Problem is that I find it much more convenient to have the weight automatically loaded into the smartphone than to have to do that manually every day.
Not necessarily IMHO, even electronic scales are measuring the compression of some sort of "spring". In practice, there are various links and pivots and a little bit of friction anywhere will affect the result. Mine varies if you rock backwards and forwards. I have found you can get more consistent results if you effectively place each of your big toes on the same "mark" every time (with your heels in the same places, of course). As others have said, the flatness, hardness, and levelness of where you place the scales can also make a difference.
But it could be electronics too: battery connections?
The problem with beam balance scales is that they take a long time to get a reading because you have to add measuring weights in a binary chop until the beam is just balanced. And they don't give a reading on an analogue or digital scale: you have to manually add up all the balancing weights that you have added.
Spring or strain gauge scales gives an instant reading - though they *may* be less consistent. Is that the usual way in which they are "utter shit" - that reading the same object on multiple occasions gives different results? Or is it that there is a systematic error - eg they under-read at the low end of the scale and over-read at the high end, so a 1 kg weight added to a
1 kg weight and a 1 kg weight added to 100 kg will register as different extra weights?
I've seen scientific "balances" for laboratory use which can read to the nearest milligramme for overall weights up to maybe 10 grammes. Are you saying that this degree of accuracy is spurious? If so, a lot of lab readings would be found to be incorrect and people would soon start to complain. So strain gauges must be reliable.
I suppose one could argue that *any* device (not just weighing scales) that uses a spring to balance out the position of a needle on a gauge against an analogue of the quantity that you are measuring (eg voltage proportional to speed, weight, oil pressure, engine revs, temperature) is equally "shit" because you may not get a linear reading across the scale or may get different readings for several measurements of the same quantity, because of non-linearities in the rate of the spring (constant of deflection versus force) or because the needle sticks slightly on its bearings.
Aaaah, thereyougo, you got it. Just leave away the "uses a spring" bit, and it'll be more correct!
There is whole host of terms to describe the different ways a measurement is shit: repeatability, linearity, temperature dependence, systematic errors, random errors, drift, ...
A balance scale, a simple two-bowls-onna-beam thing, will fail systematically in many ways: a nicely sneaky one its the buoyancy in air of the objects to be weighed in air. Different *volume* of the weights and the object enters into it, at the rate of about a kilo per cubic meter, and dependent on the barometric pressure. It's pretty good in that it compares masses, and the local gravity pretty much cancels out. (Though there are special scales made to detect differences in local gravity -- and these scales are tabletop models, not huge things, see Cavendish and Eötvös). The there's friction, heat and temperature gradients, the beam bending under loads, dirt on the weights, down to gasses sticking to the weights and diffusing into them.
That said, I have a Soehnle kitchen scale to 15 kilograms, reading to one gram, probably strain gauge, ~25?. A test with a set of weights (once certified, but out-of-date) showed yeah, off by one or two grams above a kilo -- more than OK for kitchen work.
Beware some bathroom scales have cheat software to show the same weight if nearly-the-same-weight is applied within a short time!
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