Hi all,

I have a question that I hope someone here can answer - someone who is
familiar with high-accuracy analytical balances.

I recently bought an as-is Scientech SA410 analytical balance on eBay
for about $60. This is a 410g capacity unit with a repeatability to
0.1milligram. It has a wind screen with sliding doors. This model is
still sold and goes for about $3500 USD new.

It did not work when I received it but I discovered that the only
apparent problem was that a TVS device inside was shorted. I replaced
it and it seems to work.

I don't have the proper weights to calibrate it. However, I do have a
150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
balance, I get 150g to within about 2mg. However, removing and
replacing the weight causes a variation of about 0.5milligram up to as
much as 1mg. In other words, the overall calibration seems close if
not very close, but the repeatability is off by a factor of 5 or 10
from the spec. The unit always returns to exactly zero when the weight
is removed, and lower value weights have less variation in repeated
readings. I handle the weight using a piece of sheet plastic to avoid
getting oils from my hand on it, so I don't think that is the reason
for the discrepancy. I also have been operating the unit on a large
granite-slab table we have at work so vibration does not seem to be
affecting it much.

If I place the weight on different parts of the weighing pan, I can
make the reading vary by 10s of milligrams. I am wondering if perhaps
the repeatability problem I am seeing is due to inexact placement of
the weight.

Can anyone tell me if it is normal for weight positioning to make such
a difference? It almost seems like the repeatability spec is rather
meaningless if so. I do notice that the weighing pan seems to have
been modified on this unit: instead of being a concave circular pan as
shown in the manual, it looks like someone attached (welded? glued?) a
flat plate to the top of it.

For my own purposes, its present capabilities are more than enough. I
am toying with the idea, though, of re-selling it, stating that it
does work, hoping to get more money for it. If I were to do this, I
would probably spend the $50 or so to get some more accurate weights
to perform the proper calibration on it.

Thanks,

Sean
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