Ok, you guys have officially gone over my head! Love this place! 
It’s just a plain old cheap scale with the numbers and the “pointer” thingy. It’s not in the bathroom, so moisture wouldn’t affect it. I’ve irritated my roomie so much that he looks at me with disgust and refuses to talk to me if I bring it up! Mission accomplished; except that he still thinks he’s right! 
Buy a new scale.
But…but…how will I know if it’s accurate?
I just loves me some small-business gravel-pit mechanical-conversion truck scales; they always like to have their scale serviced on Thanksgiving or Christmas (because that’s typically off-season for gravel, plus they’re closed for the day and won’t lose any business, no matter how slow it is).
We crawl underneath and find approximately a truckload of gravel underneath jamming up the works; when we ask them when was the last time they cleaned out underneath the scale they go all 
and say, “Clean? Underneath?? Ain’t that what we’re a-payin’ you for???”
And we and say, “Sure, if you don’t mind a $225 trip charge per tech, two-tech minimum, and paying $254 each an hour to scoop gravel out of your truck scale by hand. And from the looks of it, we’ll be here a few days.”
They then typically get guys like you out on Christmas day to scoop gravel by hand out of their truck scale.
And just so this isn’t a hijack: blahblahblah bathroom scale blahblahblah
Not that I know of. If it’s your run of the mill checkup, the doctor doesn’t really care all that much if you’re within a few pounds of your previous weight.
If you’re in the hospital with a health condition where excess water is of great concern, like kidney disease or congestive heart failure, then daily weights being accurate is much more important. We’re supposed to weigh you in the same clothes (generally a hospital gown), the same time of day and using the same scale every time and report a gain or loss of more than 2 pounds to your healthcare provider. Many hospital beds have scales built in, and a *good *nurse will load it up with all the things you’re likely to use in the bed (gown, sheets, blankets, pillows, even things like SCD booties) and then they zero it out before the patient gets onto the floor.
You should see the room at Weight Watchers weigh-in, though! People there are pretty careful about either wearing the same outfit as last time or stripping down as far as public nudity laws allow. (No one but the counselor sees your weight, but the weighing is done in an antechamber where everyone files through in turn, and generally two or three people are stripping down behind you while you’re on the scale.)
Registered just to revive this old thread and say it is a great resource for anyone looking for a basic understanding of precision, accuracy, linearity and calibration. Lots of good topics discussed in a very everyday way.
I also think it is absolutely awesome that nobody was concerned in any way with the OP’s actual weight. I love that she could have been 300 pounds or 70 pounds and the discussion would have been pretty much the same.
If reviving such an old thread for a remark like this is a no-no here, please advise and I will not make the same mistake again.
Eureka!
Last Monday I got weighed on a digital scale at the doctor’s office. Now I have a very good idea how much I weigh and the scale was 15 to 20 pounds off. After I mentioned it, a couple of the doctor’s employees tried the scale and said the same thing. I’ve lost 60 pounds in 9 months and would like to weigh 15 to 20 pounds (or a whole lot more) less. This was a weight and nutrition doctor and this happened at 2 in the afternoon and I was the first person all day to mention the scale was off. I think if you’re going to try and lose weight you should be honest with yourself about how much you weigh. My digital scale at home runs a few ponds light, but is extremly consistent. I like that in a scale. If your scale gives the same reading every time I wouldn’t sweat 10 pounds off as long as you know its off.
Let’s throw some data into the fold, to breathe some life into this zombie.
Home scales are pretty accurate at 50kg. That’s about 110 pounds. They are less accurate at lower weights and higher weights. Digital scales are more accurate than dial scales.
This absolutely agrees with my observations in the 4 years of home nursing I’ve done since this thread was started. Scales are not perfectly accurate, and the heavier you get in realistic adult human weights, the less accurate they are. But they are not 10 pounds off, they’re more like 2-7 pounds off for dial scales and 1-3 pounds off for digital scales.
They’re even more off for very light weights. If you’re trying to weigh a small child, or a bag of sugar, you’ll get a more accurate result if you hold the child and weigh together, then subtract your weight from the total.
Oh, and doctor’s offices scales? Terrible. Just terrible. I don’t know if they’re not recalibrated as frequently as they should be, or it’s rushed technique or what, but the numbers on the doctor’s visit notes are often 10 pounds off from what I know my patient weighs.
Get yourself some dumbells and stack them up on the scale as standard weights.
When done, you can become buff with a nice new set of weights to work out with every day.
Doctors’ scales are not any better than any other scales. I know my office scale is about 5 pounds heavier than everybody else’s but I’m reluctant to buy a new one because we are more worried about change in weight than in absolute weight. We basically just subtract 5 pounds off every weight.
There’s no reason to expect any particular accuracy with exercise weights. When I used to lift weights regularly, there were two brands of 45-pound plates at the gym I frequented. I could bench-press 225 pounds several times when using one brand of plate, but when I used the other brand, I could barely manage one or two reps. And it wasn’t just a bad day; this happened repeatedly to me, and other folks noticed it too.
If you want to be sure your dumbbells (or any other test weights) weigh what they say, take them to the post office. The lobby is generally open 24/7 so people can access their PO boxes, and they usually also have a DIY postage machine that includes a scale. That scale gets calibrated regularly by your state’s bureau of weights and measures, and should be accurate to within a fraction of an ounce. The USPS package weight limit is 70 pounds, so I assume that scale will be able to weigh dumbells that weigh that much. Show up some evening after they’ve closed, drag a dumbbell in and see what it weighs. Repeat for one or two other dumbbells, and now you can pile them on your bathroom scale and know your actual test weight to within a couple of ounces.
That s if it is 4.0 pounds higher no + -
Go to a thrift store and find the bathroom scales.
Lay them all out and weigh yourself on each.
Now do you believe those crappy scales lie?
My old MD had a real, live balance scale. Stone simple, slide the weights until the beam was level.
That scale I trusted. The new! improved! All Digital! scale? Not so much. But it IS nice, in its own way… :rolleyes:
This is why I love it here. A fun find from over five years ago. Gave me a smile this evening.
I’m not sure if I should open a new thread or zombify this one but…
Would low batteries in a digital strain gauge bathroom scale cause error in a particular direction? How large in absolute terms would the error likely be?
Needless background: Mrs. Charming and Rested is on a diet and exercise program that by all outward appearances seems to be working well for her. She feels better, her blood pressure has gone down a little and her old jeans fit a little better. She doesn’t weigh herself regularly so after two months of this, she jumped on the scale and got a low battery message. She changed the batteries and her indicated weight was exactly the same measured to three digits. She doesn’t particularly care about the absolute weight but she would like to know if she’s made a little progress.
I’m guessing the strain gauge will tend to register a higher weight with new batteries. Reviewing this thread, I’m also intrigued by Napier’s experience.
So I see several possibilities to explain my wife’s indicated weight not changing:
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Her weight really hasn’t changed even though her measurements seem to be a little smaller – Not impossible if she’s replaced some fat with some muscle, but I find it a little unlikely that she would build new replacement muscle essentially exactly as quickly as she lost fat. Every indication is that it’s really hard to build a meaningful amount of new muscle in such a short period of time.
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The weak battery the last time she weighed herself caused the scale to read low. The new batteries today caused the scale to read true and the error in the last reading almost exactly offset her actual weight loss – This was my guess this morning before reading this thread.
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The scale has a “last reading” smoothing function, as suggested by Napier’s experience. It just so happens that since my wife doesn’t use the scale very often and she changed batteries, the error induced by the low batteries and the change in weight told the scale that essentially nothing had changed, so it just showed the last reading. Seems plausible.
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Again relying on Napier’s experience, if we’re considering that the digital readout was programmed to increase user satisfaction, I thought of another possibility. If changing batteries can induce a meaningful change in the indicated weight, the scale could have a casual recalibration program that applies every time the batteries are changed. I would imagine that the program could work something like this: if the scale detects that the batteries were changed and the weight on the scale after the change is pretty close to the last recorded weight, the scale just recalibrates itself assuming the person weighs the same as before the battery change. I have no idea if any scale actually works this way but it doesn’t seem impossible. It would prevent the problem of people’s weight changing dramatically every time they changed the batteries, although it could induce entirely unnecessary calibration error in a case like my wife’s. I doubt this is it only because I can’t imagine a real engineer being satisfied with such a clunky calibration protocol.
The nurse at my GP’s surgery has an old fashioned beam scale with a height scale attached in her room. It is mostly made of brass and she said that it came from the original GP practice that was there in the 1950’s. Children enjoy moving the weights around to make it balance and show how heavy they are. The problem is that it reads in stones and pounds, so there is a conversion chart to Kgs on the wall adjacent.
And everyone’s phone has a calculator in it nowadays…