Why is my digital scale so unreliable?
LoopyByteloose
Posts: 12,537
I like to think I am all about good tools and good solutions.About a year and a half ago, I decide to monitor and manage my weight. So I purchased a digital scale locally to do so.
Even with fresh batteries and a temperature controlled environment, I am getting reading that fluctuate wildly. The simple reality is that a mechanical scale may not have been entirely accurate, but it was at least consistent. But the problem is that they are harder to buy.
I don't think an electronic digital solution is really appropriate for everything... especially weight scales and thermometers. But these have pretty much moved over to digital.. regardless. About the best that I can do is visit the hospital a few blocks away and go up a few floors to a clinic that has a public scale in the waiting room.
I am becoming a retro reactionary about my gadgets... it is about reliability.
http://www.ebay.com/itm/Large-Vintage-Toledo-Scale-Honest-Weight-No-Springs-Antique-D5-MH-/171036602237?pt=LH_DefaultDomain_0&hash=item27d2936f7d
Even with fresh batteries and a temperature controlled environment, I am getting reading that fluctuate wildly. The simple reality is that a mechanical scale may not have been entirely accurate, but it was at least consistent. But the problem is that they are harder to buy.
I don't think an electronic digital solution is really appropriate for everything... especially weight scales and thermometers. But these have pretty much moved over to digital.. regardless. About the best that I can do is visit the hospital a few blocks away and go up a few floors to a clinic that has a public scale in the waiting room.
I am becoming a retro reactionary about my gadgets... it is about reliability.
http://www.ebay.com/itm/Large-Vintage-Toledo-Scale-Honest-Weight-No-Springs-Antique-D5-MH-/171036602237?pt=LH_DefaultDomain_0&hash=item27d2936f7d
Comments
Hospitals here in the UK changed over to electronic ear thermometers a few years ago.
Yes, Taiwan changed over to electronic eye thermometers as well -- eliminated mercury and problems of serilization. And they actually use video camera IR to screen the front door for people with a high fever.
But if I am baking bread or a turkey, I would prefer something traditional.
The scale I have is unbranded purchased at the local Carrefour (a French hyper-market chain akin to Walmart). Searching the internet, I have found a lot of posting complaining of flaky digital scales. But the only factual complaint seems to be that the reading change with variation in room temperature -- not a problem in my A/C controlled room.
For the past month or so, when I have started it up -- it would not read Zero, but something like 7 or 8 kilos. And then offer up numbers that varied wildly. I changed batteries several times and the darn thing is telling me that I have gained 5 kilos (which is alarming and upsetting)
++++++
The end of the story.
Feeling it has broken, I pulled it apart for a postmortem peek. It is all very simple on the inside. I cleaned a bit of dust from the interior and reassembled.
Now it is behaving itself... and I am back down to my normal weight.
So I guess I will just keep using it until it becomes frustrating again and then order an expensive, real mechanical scale for roughly $90USD plus shipping.
Users should be warned that this is a device that is likely to cause manic and violent mood swings.
I used to have a mechanical one where I always had to go through a physical calibration to get it to roughly behave.. a bit like tuning a guitar as in when you overshoot (tuning too high) you have to go down and ease your way back up again.
Eventually I got fed up at that and bought an electronic scale. This one always shows the same with the same "input" so to speak, but it also seems to average over some seconds - you wait a moment, and then it'll stop and freeze on a weight. From what I can tell by various means it's both accurate and stable. I do make sure though (as you must with every scale) that it is always on a flat, no-carpeted surface, in fact it's always placed at the exact same spot whenever I make a measurement.
-Tor
For it to be accurate, it needs two things:
1. Time to calibrate. (Many scales will give an error if you step onto it before it's ready, but not all. Scales that doesn't give an error can be considered junk)
2. Evenly placed load. Moving the heel of one foot an inch to the side will completely invalidate the results.
In addition, I've found that you always want to keep both 'outside' and 'inside' factors as similar as possible every time you step on the scales.
(I weigh in after doing the number 1/number 2 in the morning, but before washing or showering. Not going to mention what I'm wearing or not... )
Oh, and 'fashionable' scales with a glass plate?
Should be outlawed!
I've lost 35 pounds this year. Hallelujah ;-)
Well, this one has only ONE pressure sensor and a set of mechanical arms and linkages that are supposed to balance a load from all four corners. It may have been that the linkage became misaligned and jamed. So disassembly and reassembly did wonders. There also seems to be a capacitance switch for turning on the processor that times out after a period without load. I guess I should just be a bit more gentle with it.
For a good quality human weight scale, get one from a medical supply, but it will not be cheap!
The "Legal for Trade" scales (lower down on following link) are guaranteed to be very accurate. Here are some...
http://www.scalesgalore.com/Physician_Scales.cfm
I agree. All the digital bathroom scales I have take apart have been strain gage based.
Having dealt with strain gaged projects the first thought I have regarding readout fluctions is that one or more of the gages have unreliable bonding. If you can get at the gages try the erasure test.
Push down on the gage surface with a pencil erasure and note the change in reading. If the reading returns to about the same reading as before the erasure test then the gage is most probably well bonded. If, however, the reading fluctuates during the test and doesnt reliably return to the original readout, then the gage is not well bonded. The gage can be rebonded but that takes some skill. I can offer suggestions if desired.
Cheers, David
All bathroom scales are garbage. To get the cost down they take shortcuts like using half-bridge strain gage sensors, folded sheet metal levers if they have a mechanical component, and axles through the sheet metal instead of proper pivots and bearings. Properly made electronic scales are really too expensive for consumers to afford, but the minimum you should consider if accuracy is important is a mechanical scale with a beam atop a column as sold for health care. Health-O-Meter is the one we sell when someone calls us about a personal weight scale.
My own personal scale is a Detecto half-height column beam which was made in the 1950's; I rescued it from the Dumpster and rebuilt it myself in the 1980's. It has cast iron beams and hardened pivots and bearings, and nothing remotely like it has been made for over 30 years.
If you really want an electronic scale you can get an bottom of the line industrial model with a short column for USD$300-$400. These are sold more for packing and shipping than personal weighing so they have larger platforms and better load cells so they are more tolerant of non-centered loads. We still consider these nonrepairable garbage but they are at least meant for frequent and heavy use and will serve a lot better than anything deliberately made for weighing humans.
What would it take to make a good scale with an expandable MCU hardware sensor, etc... platform?
Optional Bluetooth LE interface, TLS light sensor, "BMI sensor", and per-user storage would be great.
At any given time we have at least one scale in our showroom that can weigh you reliably to within 0.1 lb. You can eat lunch or take a bathroom break and read the difference. Those scales start in the USD$500 range and for something we don't consider crappy, more like USD$800.
Exactly. We have two electronic bathroom scales of very different design. Both work great...as long as they are on a hard flat surface.
Today attempted to get weight, somewhere between 84.5 and 90 kilos is the range of samples. It seems obvious to me that the sensor is likely inferior, and you get what you pay for in any bathroom scale - digital or mechanical.
It doesn't help that Taiwan is often the final destination of second best consumer products that may have been rejected by Europe or America, especially at the lower end of the market price range. These days, HTC phones and pads are awash in the market.
I just have to buy something that works. If I have to import and spend $100USD, I'd rather just have a good mechanical scale without the batteries. I haven't seen any solar models.
Can you hold your nose and recommend one of these inferior designs? Accuracy to within 2% seems fine to me as long as the measurements are reasonably consistent. My own personal weight management is a relative thing ... I just need to know if I'm making good progress instead of bad progress.
http://www.amazon.com/EatSmart-Precision-Bathroom-Capacity-Technology
I have to visit one of the 3C consumer goods outlets witha 7 day no questions asked return policy. Most places won't accept return.
Manufacturer -- none
Brand -- none
Warranty -- none
The label says.. "8867V1 16/04/2007 Made in P.R.C"
And the label has an icon that indicates that it should NOT be thrown in the trash (a bit ironic considering the quality).
Inside there is one tiny cheap potentiometer for adjustment, but I don't dare touch it
Screening people via IR for fever is a holdover from the SARS scare...
Digital scales when misbehaving will drift...the erratic behavior you are seeing is unusual behavior.
Might that tiny cheap pot be the source of your erratic problem?
As it is....
I start the scale and wait for it to find zero and it goes to something like 60 or 70 kilos and stops. If I stand on it after that, it still reads the 60 or 70 kilos that it stopped at. For one day, it found zero and seemed to give me a good reading.. but no longer.
It is junk. It is going to the recyclers post haste.
I am actually more concerned that I am loosing track of my weight and going off my diet. Time to just buy something else.
+++++++++++++
Saving the load cell for a hobby project might be interesting, but I am not going to bother. I am mostly annoyed with myself for buying junk.
I picked up an Oserio (I guess a Taiwan brand) with warranty and a 7 day return policy from the retailer for about $20 USD. Measurements seem to vary about +/- 0.1 kilo.
And the best thing is that my weight is fine, hasn't gone up (as the junker scale was telling me).
Interesting fact;
Back when I was studying I had a class 'Microcontreoller technology'(or something like that) which was a project course(build something sensible with a i8051 in it) the teacher told us about a previous project for a group in that class.
Someone had built a 'back unit' for a stove, with not just a clock and timer, but with a thermometer.
(This was when most such backs had mechanical clocks)
The students had been told by the stove manufacturer that the thermometer absolutely HAD TO be calibrated correctly for 'room temperature'.
Large deviations in the higher temperature ranges didn't matter as long as they were consistent.
After all, not many customers would have the equipment to test if 200degree C actually was 200 degrees, instead of 180 or 215 degrees...
But if it was wrong at room temperature, anyone with a $1 thermometer would notice.
(That project actually ended up in a stove... )
The same with bathroom scales. If it shows the correct weight for up to 20Kg, and a consistent result for anything heavier, who's going to notice if it's off by 5Kg at the 100Kg mark?