The Strange Case of the Bathroom Scales - Page 1
Forum section: electronics / projects

author: German_EE
author_rank: Super Contributor
timestamp: April 25, 2017, 06:03:25 pm
content: About a year ago we replaced our analog bathroom scales with a digital unit, the girlfriend wanted to work out things like BMI whilst I was just interested in the adverse effects of all that chocolate. Right from the start we noticed that the first time either of us stepped on the scale it weighed high then on all subsequent weighings the results were about the same. For example, the first time I used the scale tonight I got 98.2 Kg then the next three times I got 96.6 KgFast forward to tonight and she turns up with a new set of bathroom scales, now with LED backlighting so we can weigh ourselves in the darkSo, I take the plastic strip off the coin cell and weigh myself. The first reading was 98.1 Kg and the next three readings were 96.6 Kg. What's going on here, why would two sets of scales from two different manufacturers (with presumably different mechanisms and software) both weigh high the first time they are used?Note that in all cases the scales were in exactly the same place, in the interests of scientific accuracy there are two small marks on the bathroom floor where we place the front feet. The floor is tiled so it won't be moving and the duplication of the final three meetings show that I'm standing on the same spot on the plate each time.

author: helius
author_rank: Super Contributor
timestamp: April 25, 2017, 06:08:30 pm
content: Mechanical: there is an elastic part with a long restoration time.Electronic: dielectric absorption in a capacitor affects the ADC

author: RoGeorge
author_rank: Super Contributor
timestamp: April 25, 2017, 06:24:12 pm
content: I guess different scale manufacturers might use the same chip/sensor/firmware/schematic, or the same guidelines (e.g. from an application note).Do both scales appear to have the same functions/settings/etc.?My digital scale (no fancy functions, just the weight) always shows the same weight, no matter if it's the first weighting or not.

author: QuantumLogic
author_rank: Contributor
timestamp: April 25, 2017, 06:34:32 pm
content: It's because you sucked in your gut the 2nd and subsequent times.Seriously, what helius said.Experiment: Try adding more and more time in between weighings (5 minutes, 10 minutes, 30 minutes, etc.) to see if the result follows some sort of curve. You would need to be extremely bored and determined to know why it is happening to do this, I think.

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 25, 2017, 06:36:30 pm
content: It's just there to make you feel better:1sttime: How much?2ndtime: Oh, that's better!

author: ale500
author_rank: Frequent Contributor
timestamp: April 25, 2017, 06:48:29 pm
content: Don't buy by Aldi or OBI.I wouldn't trust those scales, balances "Wiege" to have any accuracy to less than 2 kg, and as you saw, precision is something you cannot expect either...In a related note, why the heck does the original battery last forever but any replacement only last a couple of weeks ?

author: NivagSwerdna
author_rank: Super Contributor
timestamp: April 25, 2017, 09:18:23 pm
content: The scales are lying to you. It got two consistent readings and decided to use those until it saw a mass that was significantly different from the first (or taken many hours after).Try weighing youself... and then something of a significantly different mass... and then weghing yourself again.Some scales try and remember a small sample of people... so have the potential to cheat as well.I have an interest in weighing scale dissection (https://aardvarklabs.wordpress.com/2013/02/03/reverse-engineering-the-salter-ultimate-accuracy-scale/)... please take yours apart and post pics!Also... as helius said "Mechanical: there is an elastic part with a long restoration time."... sounds very plausible... if this is the first time these scales have been compressed.

author: DBecker
author_rank: Frequent Contributor
timestamp: April 25, 2017, 10:22:03 pm
content: I wonder what the base accuracy of the scales are?It's fairly easy for a firmware-based meter to promise "no missed counts", despite not having the base resolution. There is big gray area between averaging, interpolation and just plain cheating. And once you've moved past "interpolation", you can convince yourself that synthetic repeatability of a measurement somehow benefits a user.

author: BrianHG
author_rank: Super Contributor
timestamp: April 25, 2017, 10:56:00 pm
content: It's the zero-ing effect at first power-up & sometimes the faceting springs which hold down the top of the scale to the main base. These springs supposed to be loose, not to add or subtract from the weight, yet, hold the top down so the scale doesn't fall apart. Also the mechanics of the levers from the 4 corners to the center single load cell also slip and re-center at times after use, then a large relax time. Only really expensive 4 load-cell, 1 per corner of platform scales are immune from this problem, but, they have a different issue.As for the chocolate, I used regular Toblerone to loose weight from 151kg (333lb) to 92kg (210lb), then had to switch to dark Toblerone to go from 92kg (210lb) to 86kg (190lb), then removing the Toblrone to go down further to 79kg (174lb). Though this took years, I'm have programmed my body to loose weight when eating chocolate. It's fun at major dinner events having nothing but extra rich chocolate moose cake, of thick chocolate fudge, like 1/4 a cake, and all it does is speed up my heart, excitement, some sweat, with the end result of loosing weight, burning all the calories of the big dinner instead of gaining weight just by having a chocolate desert. IMPORTANT NOTE: I had a few chunks of Toblerone 15 minutes before each workout and trained FURIOUSLY 3 days a week for a decade, 10 months a year, and reserved the chocolate deserts only for special dinner events when I'm on vacation or out with the family. I programmed a physiological response to eating chocolate, every time I eat some, my body thinks it's about to do a full workout and my metabolism goes through the roof to the extent that I feel it.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: April 26, 2017, 09:26:05 am
content: Quote from: BrianHG on April 25, 2017, 10:56:00 pmMany cheap scales have four load cells, one in each corner.Quote from: BrianHG on April 25, 2017, 10:56:00 pmExcellent advice. I imagine the latest changes would make it even more effective

author: Jeroen3
author_rank: Super Contributor
timestamp: April 26, 2017, 09:40:16 am
content: Quote from: German_EE on April 25, 2017, 06:03:25 pmIf your tool shed casts a shadow on the display you are going to need that LED backlight.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 26, 2017, 09:53:04 am
content: My experience is that bathroom scales are inherently unreliable and results are not repeatable.This was after testing several scales from German retail sellers. The wife was very unhappy!And even a reputable brand as KERN was not better.Well, I opened the KERN scale (Picture) before I returned it to the retail store and could not believemy eyes. It was so cheaply made that after I saw the inside I was no longer surprised about theunreliable repeatability.(And I always thought that Kern was a good quality German made brand)So, I ended up getting a reliable packing scale for the bathroom and all problems are solved.This one even came with a calibration certificate.

author: bitwelder
author_rank: Super Contributor
timestamp: April 28, 2017, 12:48:52 pm
content: Did you try instead of weighting something in the range of 100Kg (which bring a difference of ~2Kg between measurements) to put on the scale something around e.g. 10Kg and see what's the error there? i.e. if for instance it's proportional.

author: BrianHG
author_rank: Super Contributor
timestamp: April 28, 2017, 01:13:19 pm
content: Quote from: NivagSwerdna on April 26, 2017, 09:26:05 amOMG, what is that tiny thing with missing pieces. Talk about cheap. I wish I took a photo 4 years ago when I bought over 100 x 400g (the original full large size) Toblerones at the supermarket after Valentines Day. They were on special for 1/3rd price, and considering the regular price here was 10$ each... It took a year to go through them.

author: German_EE
author_rank: Super Contributor
timestamp: April 28, 2017, 03:16:42 pm
content: OK, an update on the scales situation. I manged to take the cover off the old scales but I was forbidden by SWMBO to dismantle the new set. There are four load cells in the old scales but the PCB only has five or six components including the LCD display and the work is done using a COB device with no markings. It then took me half an hour to line up the zebra strip until the display worked properly, I'm not impressed.I tried a 20 Kg load (4 x 5L water bottles) on both scales and got similar results, the first measurement gave 20.1 Kg (old) and 19.9 Kg (new) and then measurements after that were, on average, 20.0 Kg (old) and 19.8 Kg (new).I also tried standing on both scales at the same time and that's when stuff got REALLY strange. No matter how I stood and which scale was used on which leg I got 44.4 Kg on the old scale and 60 Kg on the new, I don't weigh 104 Kg!

author: janekm
author_rank: Unknown
timestamp: April 28, 2017, 03:25:40 pm
content: Knowing that cheap scales require zero-ing before use (as they are temperature dependent along with other effects affecting accuracy), I have wondered how they can take an accurate reading without requiring being turned on (zero'd) before the measurement is taken.Seems you have found the answer

author: SeanB
author_rank: Super Contributor
timestamp: April 30, 2017, 12:41:50 pm
content: I have no problem with checking my mass, the scales I use just came back from the lab, and i have a certificate on there, traceable to the standard in Paris through only 4 intermediaries. Sadly they do not lie at all.Also was nice to see that my calibration masses are also showing no loss of accuracy, they lost only 0.01g from wear in the interval from handling, not bad on a 20kg masspiece. Also have a 5mg masspiece that is known down to sub nanogram level, though I bet it took 20 minutes on the Mettler Toledo precision balance there for it to be stable. Same for the 1.0000g masspiece.Was also chatting to the chief tester, and was warned about some of the Chinese masspieces you can buy, that are over 5kg, that they are both very porous, and thus will pick up water from the air, and also are also badly cast, and thus prone to breaking just from normal handling, along with some being out of the tolerances allowed for the class they claim to be, and no way to correct either, as they have no way to add lead trimming pieces and then seal them in.

author: BrianHG
author_rank: Super Contributor
timestamp: April 30, 2017, 01:28:30 pm
content: Quote from: German_EE on April 28, 2017, 03:16:42 pmIt would appear my old 1 load cell scale is doing just about the same. 175.0 lb (79.4kg) to 175.5 lb (79.6kg) (note that the scale only goes in 0.5lb steps, this is basically 1 lsb resolution noise) spread over multiple resets and re-weighs. Basically it has 4 arms to the 4 corners, basically 4 levers, dividing the weight down to 1 center low weight load cell.I think you need higher resolution ADC to fix your problem if your scale has the same sample bit size as mine. There might be nothing you can do unless you find a scale with 0.1lb sample resolution. It's probably not the load cell's or mechanical design fault.Don't forget to reset the scale (re-zero) in-between every use.

author: helius
author_rank: Super Contributor
timestamp: April 30, 2017, 02:47:45 pm
content: Quote from: BrianHG on April 30, 2017, 01:28:30 pmI use a very old scale with a zero button that also is required to power it on—makes it hard to forget. I also have an analytical balance in mothballs; I should get that working.

author: james_s
author_rank: Super Contributor
timestamp: April 30, 2017, 03:55:50 pm
content: Most of these digital scales you need to tap it to turn it on, then you wait for it to zero and then you step onto it to weigh yourself. If you are just stepping onto it then it has not had a chance to zero.The magical shrinking candy bars are one of those things that really ticks me off. I fully understand the need to increase prices but when the products are made smaller to hide the price increases I notice and I feel like someone is trying to cheat me and that makes me mad.

author: SeanB
author_rank: Super Contributor
timestamp: May 01, 2017, 12:10:59 pm
content: Kind of common, you see this with sweets a lot, each year the bar gets slightly smaller, around 2mm each cycle, and after 5 years it is a lot smaller in size, but a lot more expensive as well. Then you get the "new bigger" one, with the original size pack, and a new label, and a new higher price.Of course, while they claim to be chocolate, what they contain is as far from real chocolate as margarine is from real farm fresh butter, or the milk you buy in the store is from real fresh full cream milk fresh from the jersey cow.

author: ebastler
author_rank: Super Contributor
timestamp: May 01, 2017, 03:24:44 pm
content: Quote from: BrianHG on April 25, 2017, 10:56:00 pmI didn't realize that moose cake was a thing in Canada, but it makes perfect sense now!

author: G7PSK
author_rank: Super Contributor
timestamp: May 01, 2017, 03:41:07 pm
content: I just replaced our scales. The old ones showed the weight as being nearly 7 kilos lighter when standing on the left leg compared to standing on both legs and showed an increase in weight when standing on the right leg only of 2 kilos. The new one show slightly different wights when doing the same sort of gymnastics but not as marked. I checked the accuracy with cans of water and by weighing myself with the scales at my doctors and the new ones are within a few grams, which for body weight does not make much difference. The reason the scales were doing such crazy things, the platform distorts. the first set of scales was made from plastic for the platform and the second set is made of stamped steel sheet, not thick enough to not distort completely though. Both scales made by salters which is a large manufacturer of weighing machines in the UK. The doctors scales were the same make but obviously rather more expensive.

author: SeanB
author_rank: Super Contributor
timestamp: May 01, 2017, 03:52:01 pm
content: Made For Salters, not made by them. Big difference there, you probably will find a good number of very similar sized units, with only minor cosmetic differences, being sold for various price points, all using the same stamped set of levers and stamped knife edges on them.