Low cost air quality measurement is difficult. PM is tricky but almost manageable with laser scattering, toxic gases at typical inquinant concentrations are almost impossible to get right with electrochemical sensors, no matter how much money you throw at them.
Weight measurement is easy, they could give you precise and accurate results but most customer base doesn't understand anything of physical quantities measurement and would panic they gained 50g in a minute if they showed up the real fluctuating values by stepping on the scale twice.
It's because of how load cells (and scales) are manufactured. They're made for weight classes like 100g, 1kg, 2kg, 3kg, 5kg, 10kg, etc. So to make a scale for a particular weight you pick the load cells you'll need for the range you want.
Higher quality scales don't select higher quality load cells; they're all the same (reasonable quality) for the most part! Instead they just use more load cells and just take an average of all their values, multiplied by the number of cells, to measure weight.
So if you want a damned accurate industrial scale for measuring weights around 1kg you'd use like twelve 100g load cells rather than a single 2kg load cell or two 1kg load cells. Of course, then the hard part becomes, "how do I distribute this weight perfectly evenly across all of them?" so you have to take tolerances into account to make sure that at no point can weighing part of your scale rub against the side of its container or accidentally get something wedged between the scale part and the case.
There's loads of other things to consider as well but for the most part the accuracy of the scale is a factor of how many load cells its using, not the quality/manufacturing tolerances of the load cells themselves. Because even with the most precise manufacturing methods load cells will still drift and be impacted by temperature. All you can do is calibrate regularly, test them, and hope for the best (or use a completely different technology).
They want the highest resolution so they can see their 10 gram progress when dieting. But they want the measurement to be stable enough to not cause them too much anxiety.
Do they? 100g is already quite granular, and you can lose that in a day.
I have two scales which are giving me different measures - I've been weighting myself and assume I'm in the middle of both. It's a bit annoying, but works for my goals.
The other one, weight, is easy enough and if it's not accurate it is for commercial reasons. Either because what I said or what you said. Both could be true.
Even the cheapest load cell is pretty damned accurate and the average between the four of them is going to be accurate within 0.1kg (~0.22lbs). Having said that, load cells have this problem where their values drift over time but only if there's a constant weight on them. If you step off the scale and allow it to re-calibrate itself (which is what it does every time it turns on) there will definitely not be enough drift to matter from a, "how much do I weigh?" standpoint. All it takes is like 2 seconds without weight for a load cell to "settle down" back to its not-actually-very-noisy resting position.
I've tested loads of the cheapest possible load cells money could buy along with some load cells I took from an expensive transparent scale that had cracked. They were all far more accurate than I was expecting and any differences between them was negligible.
The longer you wait before weighing something the less accurate it'll be but only by tiny, negligible amounts. Like, if you're weighing something that's 100kg and you waited ten minutes after calibration (something most scales won't allow because they all have something like a 30-second timeout) you'd still get a value that's within 0.01 of 100kg. They don't drift that much at rest unless there's some serious temperature variations or you had something resting on the scale the entire time.
Is it a big problem? No, but there's a fake decimal and an attempt to hide that the decimal is fake. Not cool.
I've never seen a human weight scale that went beyond a single decimal place and in all honestly they're going to be very accurate. If you get on the scale and it says 201.2lbs that measurement is going to be accurate within ±0.1lbs (actually, more than that but you only get one decimal point of precision). If you put the scale next to something that's emitting a ton of electrical noise then maybe it'll be off by 0.2lbs but I doubt it.
My cat sat on it, read 9.4lb. I stood on it, it gave my weight, then I picked up my cat and it read (my weight + 9.4)lb.
I was impressed!
Annoying, but it works.
Because digital scales have features that analog ones do not. And contrary to an earlier assertion that 'almost all' scales lie to get consistent results, this is very much not true. There are reputable digital scales that do not behave that way.
What I wrote sounded more conspiratorial than I am planned, but I stand by the opinion that coperate interests are overrepresented in regulations.
They system has found an equilibrium: customers are willing to pay ~$X for a scale. For $X you can afford Y accuracy.
If you increase the accuracy, great, but now your scale costs more then $x.
The bodyfat measurement is good enough to be usable and the same scale is working in our household for almost 10 years now I believe.
It has wifi and bluetooth, so it always syncs up with your phone and can track multiple people. I don't really trust the body measurement stuff but use it for measurement per measurement information. The heart rate is kind of nice to track too.
0. https://www.theverge.com/23573362/anker-eufy-security-camera...
Measuring human weight is harder than it seems (we don't stay still).
Most scales use a strain gauge sensor that detects bending of a metal arm through changes in resistance. It's pretty noisy and needs averaging. There's some integrated circuits to relate the resistance to weight. Most do some factory calibration only around the target weights (median adult weights).
Look instead for a scale that explicitly states that it is accurate over the whole range (e.g. baby to 220lbs). Usually there's tradeoffs - e.g. the displayed weight won't 'settle down'. I have one like this and it works better than any previous one I had (tested by adding calibrated weights), but I hesitate to recommend since it's a low cost consumer model.
Accurate enough for weight control (doesn't drift from day to day), no "stuck weight" problem mentioned either. Just need to wait 2 seconds for self calibration to finish
Humans are constantly changing weight, with every single breath you take.
I also tested on small weight and what my kitchen weight calls 3.016KG my bathroom weight calls 3.0KG so I'm pretty sure it's still reasonably accurate in absolute scale.
You need to weight youself and then weight yourself again with something of known weight in your hands.
Floor scales usually aren't good at a small weights, though I wouldn't be surprised if they are not bad at it nowadays.
The only project I found was FitScales and it has been dead for a long time now because Android Bluetooth changes made it incompatible with newer devices.
Personally, I've only ever had one scale in my life that I thought might be using memory to give false consistency. But even then, it was short-term, not long term. I.e. if you stepped off and back on, it might bias towards giving you the same weight. If you walked away for a bit, then came back with a small extra weight in your hand, it was accurate.
It is fascinating how much one's weight fluctuates during the day (or night which is even weirder if you don't eat/drink anything!)
That works with my NoName NoSmart TwentyBucks Scale scales too.
Fat is 77% carbon by weight, and glucose is 40% carbon by weight (C18H34O2, C₆H₁₂O₆) . All off that carbon is broken down to CO2 and removed by the blood and lungs.
2000 calories of fat is 0.57 lbs, and 0.44lbs of carbon. 2000 calories of glucose is 1.14 lbs, and 0.46lbs of carbon.
The carbon masses are similar because the energy we use comes from breaking the carbon-carbon bonds these molecules.
This means you exhale about a 0.45 lbs of carbon per day for baseline metabolism. Each pound of fat you lose is another 0.77 lbs of carbon on top of this (over however long you lost it).
Compairing to your numbers, the approach above means you exhale 200g of carbon a day, and 8.3g/hr. If you sleep for 7 hours, that is 58 grams of carbon exhaled (pretty close).
I say that because it seems like certain tenths of a pound are outright impossible to achieve.