The difference could be wear and tear on the equipment. The ones I have came into the lab about 7 years ago from another part of the university. No here remembers what they were used for before that. For all I know, they were used in the freshman chemistry labs where things seem to be constantly broken from misuse. ________________________________________ From: piclist-bounces@mit.edu [piclist-bounces@mit.edu] On Behalf Of Sean Breheny [shb7@cornell.edu] Sent: Wednesday, September 30, 2009 2:05 PM To: Microcontroller discussion list - Public. Subject: Re: [OT] Analytical balance question That is very interesting but still puzzling to me. As I said before, I get about 1 milligram repeatability with a 150g mass. I'm wondering why mine seems to be so much better than yours?! As for what I am using it for - nothing, really. I bought it because the guy was selling it for only $60 and I knew that if it were in working condition, it would be worth far more than that. I thought it was likely that I could fix it since it didn't show any external damage. That's why I am considering re-selling it, but instead of as-is, offering a 7-day right of return or similar, along with some proof that it works. I am trying to determine whether it is worth spending the time and money to establish that it works to the factory specs. I am also considering just keeping it and using it for weighing parts for model aircraft, although it is still way overkill for that. Sean On Wed, Sep 30, 2009 at 1:30 PM, William Wilson wrote: > 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 > -- 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