The manual I have with my SA410 says that the repeatability is 15mg,
so your measurements seem about right to me.

As for the flat plate, mine does have a concave plate on it, and a stainless
steel flat plate that fits on top of that to help with whatever you want the
mass of  (rezeroing required obviously). The plate should lift up easily, 
but it is possible that someone glued it in place to keep it from getting
stolen - or they glued a flat plate over an acid hole in the concave one.

The SA410s are not very accurate when get down to milligrams. I don't
trust mine below a gram. If I need something below a gram I use a different
scale. Or as one of my assistants just said, "It's meant for massing marijuana,
not cocaine" 


_______________________________________________________________________________
Cris Wilson
Information Resource Consultant
College of Architecture, Arts, & Humanities
Clemson University
Report computer problems to aah_computers@clemson.edu


-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Sean Breheny
Sent: Sunday, September 27, 2009 11:43 PM
To: Microcontroller discussion list - Public.
Subject: [OT] Analytical balance question

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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