Question about those weight balance scales where you slide the weights across the bar and get the arrow to hoverbalance to indicate your weight. I have one at home, and its properly balanced at “0” when empty, but i weighed myself and then went to Drs office who had an electronic scale (weight to tenths lb) and 15 min later under identical conditions i was 4.5 lbs heavier on his scale. My home scale is 10 years old but is in like new condition and basically gets lightly used 2x a week and balances at zero. Is it still possible to be 4.5 lbs off even if it balances at zero or is it more likely the doctors electronic scale (which is presumably certified as a medical device) is the culprit or could it be equally either? I guess im wondering if it balances at zero on the manual weight scale doesnt that pretty much mean it will be accurate within ounces or can it still be significantly off and still balance at zero when empty?
Bathroom scales are notorious for underweighing by a few pounds. It’s sort of like vanity sizing. Your doctor’s scale, whether counterweight or electronic, is undoubtedly more accurate.
I can’t answer your question, but I’ve found I weigh 2-5 lbs more on my doctor’s scale than my scale at home. And it’s not because I’m wearing clothes at the doctor’s office since I weigh myself at home with clothes on. (My doctor has a counterweight scale and I have a good electronic scale.) I asked a nurse once about it and she said they don’t really care what you weigh exactly since they are only looking for differences in weight between visits. As long as their scale is consistent that’s all they really care about.
Are you also wearing the same clothes on your scale as on theirs? It’s not likely to account for 4.5 pounds but this may be a contributing factor.
I find the opposite. My bathroom scale can vary by up to 5 pounds depending on whether the weight is more on my heels, toes, or in between. The doctor scale (digital) is consistently a few pounds less.
As I understand, those balance beam scales are supposed to be pretty accurate; a balance with weights is more reliable than a spring which can get less “springly” with age and is not manufactured for fine accuracy.
“Accuracy” can be measured in various ways. This thread, so far, seems to be focusing on, “If the scale says 100 pounds, is it really exactly 100 pounds?”
I have a totally different problem with my electronic bathroom scale (which gives weights supposedly accurate to 0.1 pounds), and I would love to know if it also plagues the counterweight versions.
Here’s my normal procedure for weighing myself: I get on the scale and see what it says. Then I get off, wait for it to turn itself off, and then weigh myself again. And then a third time. Sometimes two of the three readings will match, but usually the three measurements are spread over a range of a pound or two.
So this one scale gives very different readings over a period of time when the weight of my body could not have varied by more than a few micrograms.
On a practical level, I have to resign myself to this point of view. I concede that it is a very practical way of looking at it. But I wish I had more confidence in the system.
Regarding accuracy of the counterweight scales, it’s been years since I’ve worked in a place that uses them, but I recall that they were required to be officially calibrated regularly.
You would think that the fact that the indicator is hovering between the up and the down limits (with no weight on it) that that would mean it is accurate, but maybe not necessarily. I remember the calibration dude coming’ round with a set of special calibration weights.
mmm
In principle a beam balance should be more consistent, and has much less reason to be inaccurate than an electronic scale. There is nothing springy that needs calibration or may vary change in characteristic. You need accurate balance masses, which are difficult to make change, and accurate location of the pivot points of the beam. One would expect thermal expansion issues to cancel out due to the inherent nature of the design. So overall they should perform well.
Electronic scales seem to be generally a beam spring with a strain gauge. There are many reasons these may vary in reading. By bathroom scales are a beam spring with an insane mechanical gearing system that turns the dial. There is so much slop in the mechanism and stiction, plus sensitivity to how level the ground is that it is +/- one kilogram at best. It has four feet, and thus having it sit perfectly level requires a perfect bathroom floor, which is hard to achieve with tiles. Rotating it in place by 90 degrees is enough to change the reading by a kilogram.
Any spring scale is arguably open to more variation in operation, but like anything, throwing effort and money at the problem can compensate for many ills.
In commercial settings, regular calibration is often more about ensuring that nobody is attempting to swindle customers by fiddling the scales as worrying about the scales going out of calibration by natural means.
I have found a huge variation in the scales used by doctors. A few just use domestic quality scales. As noted, they are really only concerned with trends, not absolutes. Unexplained weight loss always gets them worried.
Note that the two measure different things. Balance scales measure mass; spring scales measure weight.
Obviously one is guessing. Any scale needs to be calibrated to be accurate and if so should be within a couple percent, a few pounds. Your weight might fluctuate by as much as several pounds a day depending on fluid intake and flow, and losses due to heat, etc. If clothes are estimated, the result would be even less accurate. Weight loss is concerning if it is significant (10%+), sustained and unintentional.
I get highly repeatable results from a cheap electronic scale on bathroom tile. If I step off, let it reset and step on again, I will almost always get the same reading to 0.1 kg precision. The scale sits with all 4 feet on a single tile, and I always align the scale with the edge of the tile and stand with my feet in the same position on the scale.
This of course says nothing about accuracy; I checked the documentation for the scale when I bought it and it made no claim at all regarding accuracy.
I got the same with my co-tenant’s scale. It’s on the garage floor so that’s very level and I didn’t move the scale between weighings. The only thing is, I should have gotten a different weight since I removed shoes and things from pockets. Certainly more than a tenth of a pound. I concluded that it has some significant hysteresis and that I’d need to wait longer between weighings or do something like weigh an object that has a significantly different weight.
Not really - garage floors are slightly sloped towards the doors (mine is sloped 1/4" per foot) so that liquids (rain, melting snow) drain out of the garage.
Might be a 2% error (one part in 48)…
Sounds like a crappy design. I have a pretty cheap electronic scale, but it has a separate strain gauge for each foot. I tried weighing with all my weight on the right, and then with all my weight on the right ball of my foot, and it gave the same value each time. I added a 3 lb mass to make sure it wasn’t just forcing the same reading, and it still gave the correct result. It may still be a few percent off in absolute terms, but it’s very consistent regardless of the mass distribution.
That was the standard bathroom scale design most of my life.
Very true; digital home scales and cheap top-loading balances certainly use a strain gauge. But for completeness sake, I’d like to mention that the electronic balances you find in laboratories do not use a strain gauge. They use an electromagnetic force restoration (EMFR) cell. The EMFR cell consists of a coil, magnet, optical position sensor, and closed-loop control mechanism. It’s a rather ingenious mechanism.
It’s true that top-loading balances measure weight, not mass. But they can (and often do) accurately report mass if they are properly calibrated and not moved.
Only approximately. 1 kg of styrofoam will always weigh less than 1 kg of lead (assuming the nearby humans can breathe).
Yes, a buoyancy correction must be used when the density of the thing you’re weighing is different than the density of stainless steel. 
A slope of 2% is typical for garage floors, but that’s not the magnitude of the resulting measurement error.
The normal force measured by the scale varies with the cosine of the slope angle. A 1:48 slope is 1.19 degrees, the cosine of which is 0.99978. So a scale placed on a sloped garage floor would be expected to read low (due to said slope) by 0.0217%.
I did an experiment with my bathroom scale. I’d keep reweighing myself and the last digit would stay the same (it was 0.2 lbs resolution). But if I’d hold a book or something and weigh myself, then put the book down to reweigh my real weight, it wouldn’t repeat the previous real weight. I think they’re redisplaying the last weight if they think it’s close, probably to avoid people returning the scales because the last digit doesn’t repeat on reweighing.