The 0.15 mg is the repeatability. You are correct. I didn't move the decimal correctly. I still don't trust the sa410s below a gram though. The 5 of them that I routinely work with swing all over the place with milligrams. ________________________________________ From: piclist-bounces@mit.edu [piclist-bounces@mit.edu] On Behalf Of Sean Breheny [shb7@cornell.edu] Sent: Monday, September 28, 2009 4:27 PM To: Microcontroller discussion list - Public. Subject: Re: [OT] Analytical balance question Are you sure you are reading that spec correctly? I just checked my manual again and it definitely says 0.00015g for the repeatability. That's 0.15mg (I had said 0.1mg before so I was a little off but not by much). Sean On Mon, Sep 28, 2009 at 2:38 PM, William Wilson wrote: > 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 > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist