Digital scales

I check mine with a bag or two of sugar 1KG and always use it on a the hard bathroom floor, never noticed a problem.

I;ve also been weighin a couple of eggs on a small set of scales also always measured on a hard level surface the same surface is a good idea too just in case there's any micro singularities hiding.

Well whereever a scale is put it always reads higher than it should, don't argue with them just nod in agreement.

I made a small platform with 4 spikes screwed into each corner. They pierce the carpet and alow the scales contact the hard surface underneath

I disagree. My bathroom scales under read by an amount determined by the carpet thickness but they are extremely consistent about how much.

To make them work on any surface would require much more rigid engineering of the base plate which adds weight and materials cost.

My kitchen scales which get used on a level rigid surface are accurate to the claimed precision of 5g on up to 3kg. There is a bit of hysteresis and stiction on the very low end which means weighting several identical Xmas cards to work out airmail charges reliably.

Indeed. Our cheap glass-based scales are at least 12 years old and have a sensor in each of the four feet.

I have always assumed that each foot has a strain gauge and the output of each is added to give the final figure. I just tested this by placing the scales on a triangular-shaped piece of wood which allowed one of the feet to float in mid-air. No matter which foot floated, the weight shown was identical to that if all four feet were on the ground. So you are right; no matter how strange it might seem, it must be the carpet pile (and underlay) taking the strain away from one or more of the feet to allow inaccurate and/or variable readings to occur.

Screws instead of nails, too.

One teardown of an IKEA unit I just watched suggested the 4 load cells were wired in a Wheatstone bridge to sum up in an analogue fashion:

I wonder if that means any differences in each cell cause a non perfect summation and thus variation depending on how the load is distributed??? I don't know.

In message , fred writes

Well, my test this morning seemed to show that all you needed was the hard, flat platform. My tile just sat on the carpet and the scales placed on it read correctly.

Martin Brown has brought this to us :

I have a pair of digital bathroom scales from (I think) Aldi around ten years ago. They have a glass top and what seems to be a fairly solid cast alloy base/under frame like an H. You give them a kick, to wake them up. I regularly check them against calibrated scales and they always show to be on the nose (+/- 0.1Kg). They seem not to care at all what sort of surface they are used on, so long as you stand reasonably on the centre.

That isnt the way things like bathroom scales are DESIGNED.

That?s a fudge done by the scale, it keeps using the original reading for the later readings, doesn?t actually keep measuring the weight.

You can prove that by tricking it by getting it to measure two different weights and running a test with each weight used alternately. That stops it fudging and you will find that most domestic bathroom scales aren't very reproducible at all in that test.

They're designed to a price, to look good and to get a sale.

There seems to be little consideration made about the environment in which they are used, or their longevity, like so many consumer products.

I am certain they could design a reasonably cost effective set of scales that would function to +/- 2% accuracy and +/-0.2% repeatability across a range of floor types and off level floors *if they wanted too*.

But no one seems to. Just making the feet longer would help - then ensuring the load cells would sum fairly accurately with imbalanced loads.

Digital Scales those are what Sher sings on her new Abba songs cd. Brian

They are in fact designed to show your weight.

Turns out that it isnt easy to show you your weight reproducibly, let alone accurately. It is possible to do that and commercial scales obviously do that, but they are much more expensive and few are prepared to pay that much higher price.

That?s wrong with the type of floor they are used on.

They clearly do with commercial scales, but those are much more expensive than domestic bathroom scales.

It doesn?t and some have tried doing that with domestic bathroom scales.

- then

That is what they do.

Andew Jones laid this down on his screen :

I can buy and have bought, multi-meters costing hundreds of pounds and I have bought cheap ones at a fiver a time. The later I generally find to be very close to the accuracy of the expensive ones. The difference is mostly down to build quality and robustness. Likewise with scales...

Mine were fairly cheap ones, but I check and compare them against expensive calibrated ones, when ever I get the opportunity and find them to be spot on. Were I to find any discrepancy, my first thought would be that the reference I had checked them against was in error, rather than my bathroom scales.

I have a cheap Sekonda watch, which I found (yes found) abandoned on a bank roof. It is an electronic analogue type. That watch is accurate to one second per month, which is incredible accuracy for any watch which is not radio controlled.

I have a £5 set of cheap 0.1 to 100 gram digital scales which measure to 0.1 of a gram and a set of gram weight calibration standards. when I check their calibration, these always show up to be spot on.

So I am not convinced that cheap always means poor accuracy and I am someone who is obsessed about accuracy. I have voltage, currant and resistance standards, frequency standards - I like to know my equipment is accurate.

Yes, but bathroom scales are different.

The difference

With multimeters, yes, and how well the survive you doing something stupid like attempting to measure the mains voltage when on the current setting.

Not with bathroom scales with repeatability and accuracy.

How do you actually do that check ?

Were I to find any discrepancy, my first thought

More fool you. It is trivial to check your own bathroom scales with a know fixed weight.

Irrelevant to bathroom scales.

Irrelevant to bathroom scales which can be checked the same way.

No one ever said that with bathroom scales.

and I am

And it is even easier with bathroom scales.

I like to know my equipment

ISTR a pair of bathroom/bedroom scales that came with optional carpet feet to concentrate the force into for smaller 'points' to reduce that effect

I like it when cheap means "for occasional use". Same performance, but to be carefully kept in a box, and used a few times before being put back in the box, as opposed to something which has to withstand harsh daily use.

You can get some good "cheap" tools, which really make a difference.

Which are possibly optimised for accuracy at or around the average weight of a human and not, say, at a few kg.

Although it helps to have accuracy, people weighing themselves on bathroom scales shouldn't be relying on one measurement. It wouldn't be unusual for someones weight to change by a couple of kg during the day or overnight depending on how much fluid/food consumed and ejected.

It is nothing like a couple of kg when you weigh yourself in the same state at the same time every day, say just after you have got up and had a piss.