balances are universally available, anyone can use them. I think so, this doesn't seem good to calibrate 1mg, when I want accuracy of 0.01mg right?
Ok, I should have asked about the universal use of the balance. Then you probably need to keep to full range calibration.
We really haven't talked about the accuracy of the balances. I guess we could write a book on trying to figure out accuracies for balances.

But in short, I typically set tolerances much looser than the resolution. So for a balance with .01mg resolution, the tolerance might end up being much looser; at least .05mg. Of course, setting your tolerance will have a lot to do with your use/application. As far as tolerance on the balance, give yourself as much as you can. But, you will still need to determine how repeatable the balance is in your use range. That repeatability will be a component to your uncertainty.
Also, the reported uncertainty on your standard weights will probably be a factor. Some of those little whisker weights (1mg) can have a fair amount of reported uncertainty, that you would want to consider.
what if I calibrate with a weight, but I weight solid sample that is powder to a little bottle? that's not the same, is it?
No it isn't. Good point.

I suggested using a weight to solely determine the repeatability of the scale. However, I believe when you are performing R&R of the measurement system, you will want to use the product/sample that is normally used. Hopefully the gauge R&R experts (not me

) might offer a suggestion on that.
Also, since you're talking powder, I don't think evaporation/ temperature density will be a factor. But you probably know that better than I.
"If you are wanting to keep your uncertainty analysis simple, you can assume that many of the variances present would be accounted for during your daily checks." - I want to keep it simple, but as I said I suspect that calibration is pretty good, it's the weighting process that I am concered about
You're probably right.

But, the scales do contribute to the error, and you would want to know by how much.
don't understand, what do you call a swing? movement? they're stable. what is draft - controlled area?
Good questions; I should have elaborated. As far as swing, operators may notice the balances act a bit erratic when, the trash run is made, or the air handler kicks on and off, or at 2pm when it is the hottest, or the balances are most stable in the morning, etc. Operators are always a powerful source of information, and can help in trying to figure out what's going on with these processes.
Sometimes balances (especially your low range balances) can be affected by air flow in the lab; the A/C kicking on and off. That might have already been considered by you. If not, you may want to look at it. Simple hoods made of Plastic/Plexiglass or something can significantly help with air flow/draft considerations.