I have a KERN DS 100K0.5 industrial platform scale, range up to 100 kg, resolution 0.5 g, more or less like this:
The specifications say reproducibility 0.5 g, linearity 1.5 g.
When I stand on the scale, each time the reading is different, in a range of over 50 grams. So the reading changes within 0.1%. That might sound good, but when I put a large 5 kg jar on another scale with 0.1 g accuracy I get almost the same reading each time, only different by 0.1 g max, so can achieve 0.002% reproducibility, and my aim is to achieve the same 0.002% reproducibility while weighing myself (for the purpose of studying the rate of body water loss through perspiration and breathing over time).
I notice that the reading depends on the way I stand on the scale, if I hold my arm just a few cm forward for example, the reading is already different.
How can I equalize the readings so it would not depend on the way I stand on the scale? I tried sitting down curled on it, to make myself contained in a sphere closer to my center of gravity, but it didn't help much.
I tried not only using the scales averaging "animal weighing" mode, but also repeating that measurement 10 times, and taking the average after rejecting highest and lowest value, but the standard deviation of that heavily averaged measurement (n-1, divided by square root of the number of measurements) is still about 5 grams, and the process takes a long time.
How do the forces of my body gravity act on the scale differently if I have my arm a few cm forward? Maybe be not the average, but the highest or lowest of a series of 10 measurements would be most close the my actual mass?
How can I make it work more like with the glass jar, that each time I put it on the scale the reading is the same?
If not with this scale, what other kind of scale and/or weighing technique could achieve reproducibility close to 0.002% when weighing myself? The absolute value does not need to be accurate within 0.002%, only the difference between two consecutive measurements have to be accurate within 1 gram, so for example if I weight myself, drink a glass of water, and weigh myself again it will tell the mass of drank water within 1 gram.