We all agree that lots of factors can affect it, but it does puzzle me why you "don't trust it below a gram." To me, that sounds like saying "I don't trust my car for trips longer than 1 mile" With my SA410, even though it is in questionable condition at this point, I was able to weigh one nylon 4-40 screw (about 200mg if I remember correctly), then weigh a bag's worth of them (100), and when I divided the total by the individual weight, I obtained the screw count to within +/- 1 out of 100. To me that says that it is clearly usable well below 1 gram. My experience so far would be trust all but the last digit (100s of micrograms). Sean On Tue, Sep 29, 2009 at 2:00 PM, William Wilson wrote: > We keep our balances and other sensitive equipment on lab tables. > They are not granite, but they are some kind of thick burn proof, acid > proof, scratch proof, shock proof, minimal vibration carrying, hard black > plastic material. (I can't remember what it is called). > > We do keep it plugged in all of the time to a surge protecting power stri= p; > although, the power strip is more for convenience to turn on/off =A0sever= al > things. > > It is sensitive to wind, temperature, and humidity. We had to move it > away from an air duct because of the air flow and local temperature change > when the AC would come on. =A0For the humidity problem (I'm in the south = east US) > we have just decided not to trust the last digit of resolution - not that= I trust it > below a gram anyway. > > _________________________________________________________________________= ______ > 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: Tuesday, September 29, 2009 1:32 AM > To: Microcontroller discussion list - Public. > Subject: Re: [OT] Analytical balance question > > OK, thanks for the input. Do you have your SA410 on a very solid > table? I found that moving it from an ordinary table to a granite-top > measuring table made a big difference in repeatability and stability. > I think the vibration really affects it. This thing is so sensitive > that if you leave one of the doors open and blow into it from across > the room, you see several digits of the display move when the "wave" > of air reaches it. Also, do you keep it plugged in all the time? > > Sean > > > On Mon, Sep 28, 2009 at 9:04 PM, William Wilson wrote: >> The 0.15 mg is the repeatability. You are correct. I didn't =A0move >> 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 Sea= n 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 stai= nless >>> steel flat plate that fits on top of that to help with whatever you wan= t the >>> mass of =A0(rezeroing required obviously). The plate should lift up eas= ily, >>> but it is possible that someone glued it in place to keep it from getti= ng >>> stolen - or they glued a flat plate over an acid hole in the concave on= e. >>> >>> 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 diffe= rent >>> scale. Or as one of my assistants just said, "It's meant for massing ma= rijuana, >>> 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 Behal= f 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 >> > > -- > 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