My mistrust of the SA410s below a gram is based on using them. When running repeatability checks with a 250g calibration block, ours usually have an error of +/- 0.75g. Thus my distrust of them below a gram. Out of curiosity, I decided to run a repeatability check this morning using a 5 mg calibration block. Much to my surprise, the SA410 did come in at 5.00 mg every time. So then I tried increasing mass calibration blocks and the inaccuracy got larger with more mass. I haven't curve fit the data, but it looks roughly like the inaccuracy changes parabolically with the mass. So your trust in the values that the SA410s depends on what you are massing with the inaccuracy increasing with mass. On the ones I checked this morning the repeatability was +/- 1.27g at 395g, +/- 0.5mg at 100g, +/-0.2mg at 50g, and in the expected error range below 50g. For me, I typically mass things in the 300g range so my data says don't trust it to one gram. But the data also shows that I should trust it to one gram below 100 grams. Results may vary with the phase of the moon and the number of dead fish you wave. ________________________________________ From: piclist-bounces@mit.edu [piclist-bounces@mit.edu] On Behalf Of Sean Breheny [shb7@cornell.edu] Sent: Tuesday, September 29, 2009 11:56 PM To: Microcontroller discussion list - Public. Subject: Re: [OT] Analytical balance question 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 strip; > although, the power strip is more for convenience to turn on/off several > 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. For 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 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 >> > > -- > 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