There is a lot of cheap digital scales out there, typically small and battery powered. Basic ones have 3~4 digits, 0.1g or 0.01g resolution at best and cost peanuts, but for some $40 it's possible to get scales with advertised 1mg resolution and 50~200g capacity, such as this "One Hung Low 8068-series Professional Digital Jewelry Scale" piece of sh!t I had a pleasure of testing today.
Amazingly, this scale collects a lot of positive reviews on auction sites, including people saying they tested it with calibration weights and/or compared against professional lab scales.
Well, I can only hope that they got paid for those reviews because if they did it for free they have to be irredeemable idiots
It's not hard to notice that something is off. The scale ships with a calibration weight included and sure enough, calibrating the scale and weighting the calibration weight gives 100.000g as expected, as many times as you want. But then you take it off, put some random junk and suddenly re-weighting the junk gives results that vary by 2~4mg
That's if you are lucky and it's a light junk, because heavy junk (like 160g) may vary by 13mg.
Actually, 0.08% is not a terrible result, but how is the 100g cal weight always coming bang on? I guessed the answer quite fast, and that's what really pissed me off. Yes, they cheat and the results are rounded to nearest full grams. More detailed testing near 1g revealed that results which the scale believes to be ±10mg away are shown as 1.000g, some results further still are displayed as 0998~1.002g, near 1.030g it flips randomly between honest reading and 1.003g and further it works normally. Similar behavior was observed near 2g and 3g and that's where I stopped caring about this turd anymore.
Below is a linearity test I conducted by weighting increasing number of identical metal washers, 50 pieces ~33mg each. Blue is the reading, red is deviation from linear fit multiplied by 100. The washers are not calibration weights, they may not be identical and this may contribute to some of the vertical "noise". We see that measurement-to-measurement noise is 5mg peak-peak (including contribution by washers). There is some waviness at the beginning, which could be nonlinearity or - perhaps more likely - drift. (Drift could be cancelled by re-taring the scale between each measurement, which I didn't do.) And then there is a 20mg "glitch" near 1000mg, because the scale assumed that I am testing it with a 1.000g calibration weight and didn't believe its own sensor, which in itself actually seems far more accurate than 20mg
Can't wait for DMM manufacturers to catch the idea, if they haven't already
(Yes, I know that Zotek hides offset voltage of their meters by replacing low readings with zero, I want more
)