This image has been posted here before, but it serves a useful purpose...
• ACCURACY means that what your scale reads is actually the true weight. It's nice when several scales all give the same reading, but that's NOT accuracy. The only way to achieve accuracy is to use "check weights", which are themselves traceable to the
Bureau of Weights and Standards.
Secondly (and this is my pet peeve about electronic scales), is to zero the scale with a 20gram (308.6gr) check weight, and then believe your pistol loads around 4.5gr are correct. No sir, the scales need to be "zeroed" in the range where you will be using them. IOW, you need to "zero" at 5gr in this instance.
• REPEATABILITY means that when you come back a week later, a month later, a year later... that you get the same reading. No YouTube video, no authoritatively written testimony, no 5 star rating can prove repeatability. "The proof is in the pudding."
There is basically 1 failure mode for a balance beam, fouling of the pivots. That is to say inaccuracy on a balance beam is most likely a percentage of the reading, in which case the accuracy is plotted as a straight line. (The Green line in the cartoon.)
Whereas there are proabably 20+ failure modes for an electronic scale. So many, that most are hidden from the "average Joe". And these failure modes can come in
multiples, where a cool draft may
raise the readings at the low end, and then have internal frictions that
lower the readings above a certain range. (The Red line in the cartoon.)
And of course, while you can see that "fluff ball" fouling your balance beam pivots, but you will never see software errors, cool drafts, low power, dead batteries, friction in the mechanism, stretching of the load cell, etc, etc in your electronic scale.
