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 Post subject: Buying a balance
PostPosted: Thu Apr 09, 2009 12:42 pm 
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When I recently retired after a long career in science at the University of California, I became interested in gemology with my main interest being gem identification. So, I’ve started acquiring books and instruments. The books were the easy and fairly cheap part. The instruments are another story with lots of options and variations in quality. One necessary instrument is a balance (or scale, if you like). I have lots of experience with laboratory balances, mostly using Mettler and Ohaus balances. I wanted a balance that could weigh a 0.01 g (0.05 ct) gem with a reliability of +/- 0.001 g. This requirement meant a balance that could read 0.001 g. I wanted a capacity of 20-60 g and the ability to “weigh below”. “Weigh below” means the balance can be placed on a platform (a cheap plastic box works well) with a hole in it. A hook attaches to the weighing mechanism via a hole in the bottom of the balance and extends through the hole in the platform. Objects can be attached to the hook and weighed below the balance. This setup is ideal and simple for doing specific gravity.

Well, the first go round, I compromised to save some money and lowered the reliability to +/-.002 g and gave up “weigh below”. I could still do specific gravities, but not as conveniently. There were no-name balances on eBay for $40 or so that supposedly could meet the reduced criteria, but I was leery of those. Jennings balances were recommended as inexpensive balances that could meet the reduced criteria. Jennings has its own web site with lots of info and downloadable manuals. So I ordered a Jennings JSVG20 for about $90. When it arrived, I calibrated it with the included 20 g weight. Then I got out my ASTM class 2, highly accurate weights. The 1 g class 2 weight is accurate within 0.054 mg (i.e. 0.000054 g), and the 100 mg class 2 is accurate within 0.025 mg. The Jennings did not do well with these weights. With the 1 g wt, it was off by 0.012 g, and with the 100 mg weight, it was off by 0.014 g (a 14% error). Calibrating again did not improve matters; so, I sent it back.

Second time around, I decided to spend more money and go with Ohaus, a brand I have used for decades at work. A 200 x 0.01 g Ohaus balance in my lab that was bought in 1981 never needed repairs and was still accurate when I retired in 2008. There were three balances with 0.001 g readability in their Carat Series: the YJ103, TAJ203, and SPJ303, which have maximum capacities of 20, 40, and 60 g, respectively. I ruled out the YJ103 because of its capacity, the draft shield is a little too close to the pan (limits height of objects that can be weighed), and it doesn’t have “weigh below” capability. I ruled out the SPJ303 because it costs more, and I didn’t need its extra capacity. So, I ordered a TAJ203 which cost me about $230.

When the balance arrived, I calibrated it with the included 20 g weight and got out the class 2 weights. It was accurate within 0.001 g as per specs with both the 100 mg and 1 g weights. I haven’t tried the “weigh below” feature yet; it comes with its own hook that screws into the bottom of the balance and I expect no problems. Both the TAJ203 and SPJ303 have an option to install a USB computer interface, but I doubt I will want it. Curiously, I can’t find the Carat Series on Ohaus’s US web site (ohaus.com), but you can find descriptions and manuals on some of their international sites and some dealer’s sites (affordablescales.com, scalesgalore.com, oldwillknottscales.com). The Carat Series is widely available from US dealers. After purchase, mine was shipped directly from Ohaus’s NJ warehouse, and the warranty card address is in NJ. For those who don’t need more than 20 g capacity, “weigh below”, high draft shield, or computer interfacing, the YJ103 can be bought for as little as $140 (plus tax, S & H).

I think there are a couple of lessons here. If often pays to go with proven quality even if it costs more. The Ohaus cost more than twice the Jennings, but is a much better balance. Secondly, check your balance with accurate weights that are close to the weights you will commonly be weighing. 100 mg (1/2 carat) and 1 g (5 carats) are good choices for gems. Do not rely solely on the calibration weight supplied with the balance. You don’t need class 2 weights; class 4 weights, which are much cheaper, will suffice. A class 4 100 mg weight is accurate to 0.10 mg, and a class 4 1 g weight is accurate to 0.20 mg. The pair will cost about $50. If you want only one weight, a 500 mg class 4 weight, accurate to 0.16 mg, will cost about $20. Scalesgalore.com is one source of weights (I have not done business with them yet, though). I would not buy used weights on eBay or elsewhere under any circumstances, and don’t buy any weights without knowing their class. If you want to learn more about weight standards, troemner.com is an excellent web site (go to reference guides, then mass standards handbook, then to the tolerance table).

High accuracy weights should be handled only with plastic- or fiber-tipped tweezers. Fingers will leave dirt, skin cells and oils; metal tweezers will gouge; tissue paper will leave fibers. Store each weight in its own small baggie which is contained within a glass or rigid plastic container. Always use the draft shield, make sure the balance is level, and avoid vibration when weighing. Check your balance with accurate weights every few months or whenever the balance has been moved.

Mike


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PostPosted: Thu Apr 09, 2009 3:02 pm 
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Welcome to the forum Mike! :D

Thank you for sharing your experiences. There are a large number of companies selling scales and the accuracy and precision is all over the map.

If one is not worth wasting time with it is good to have that information.

I recently had the occasion to work on three Acculab (owned by Sartorius) scales that supposedly could read to a milligram. But the stated accuracy was something like plus or minus seven digits in the milligram digit. So I think they should not have displayed that third digit. The particular acculab units are discontinued but the replacements are in the two to three hundred dollar range.

Interestingly my institution replaced them with some Ohaus balances which perform much better. and were only about $150.

The old conventional wisdom was only buy balance made by Mettler or Sartorius. But there are more choices now. Ohaus has been around forever but now have legal for trade units that read a milligram or even a tenth of a milligram and the new list prices for these are very low indeed.
Under one thousand dollars and probably available on the street for three or four hundred dollars.

When you buy a better product you are often paying for the calibration or for the ability to be calibrated and for stability and repeatability.
No matter how cheap labor is in the far east you can't do a good job on those things and retail the product for under $100. When the price gets that low the sellers are letting the customer do the QC.

Glad to see another science lab type on the forum.

Gene


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PostPosted: Thu Apr 09, 2009 3:59 pm 
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Welcome Mike!

Gene, can you specify which Acculab scale models you found wanting? As well as the Ohaus models you like? I may be in the market for a new scale soon and find this thread helpful.

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PostPosted: Thu Apr 09, 2009 5:10 pm 
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I think I would avoid all Acculab scales. The ones I worked on are discontinued models that read to 0.001 gram (1mg) but if you look at the specs they have +/- 7 digits. I would run in the other direction from that.

Also since I had that occasion to work on the scales I also have talked with my local Sartorius repair guy. He said that Acculab was essentially disposable. The ones I worked on needed cleaning but had no adjustments on them whatsoever.

My recommendation would be to stick with either Sartorius or Mettler branded products OR if looking at some other brand stick with Legal for Trade.
This requires a balance to be able to hold a decent calibration.

If you want me to look at your candidates I am sure I could "point and grunt"

I received a Tanita pocket scale in a lot of gear I bought once from the heirs of a deceased jeweler. It is the kind that runs on two little silver oxide batteries. I decided to check it out. And darn if that little thing isn't right on the money after heaven knows what kind of abuse it had in its previous life. (the guy had a stall at a flea market) But I was impressed with how it read exactly as expected.

And I have seen inexpensive instruments of other types work surprisingly well. But you can't count on it I think.


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PostPosted: Thu Apr 09, 2009 8:18 pm 
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I found a gentleman years back who is a former field service engineer for Mettler who comletely refubs the units including new displays, and sells them on ebay. I purchased a Mettler AE 500C which is the carat scale and have been very happy with the seller. He did warn me however that the carat scales don't come on the market very often though. Below is his explaination as to why from an email exchange:

Quote:
The difference between this AE carat balance and those which have replaced it is simply QUALITY. The AE was a very, very expensive balance to be aimed at the jewelery trade as it was a reprogrammed top of the range analytical balance and as it turned out the best selling analytical of all time and the most reliable by far! AE500c's rarely come on the market because very often when diamond traders upgraded to the later and very much inferior models, they quickly decided to refurbish their AE and sold the newer unit.


I can't tell you much more about the seller as I have only had the one dealing with him, but based on that experience I will contact him next time a need a balance.

It doesn't look like he has any carat scales right now, but he does have a few gram scales listed. Here is his ebay seller link if anyone is interested:

http://shop.ebay.com/merchant/flyingweightech

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PostPosted: Thu Apr 09, 2009 10:08 pm 
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The only problem with this guy is that he is not auctioning balances he is selling them. There is a reserve on every one of them. I hardly ever post auctions that have reserves to this list unless it is a very unusual or rare item. Balances are not either.

He is right about analytical balances. They are usually much to preferred
to the ones that are aimed at the jewelry trades. However they often are not legal for trade even though they are more accurate than ones that are legal for trade. Whether or not that matters to you depends on what you are doing.


Usually refurbishment from guys like this consists of vacuuming and windexing the case and glass , removing the previous owners stickers and checking the calibration with some good weights such as Mike mentions above. Probably more than half the Mettler and Sartorius balances that are surplused work perfectly and there is no reason to let someone double their money on you. Everyone on this list can use windex. And most Mettler balances of the last twenty years are self calibrating. (FACT =Fully Automatic Calibration Technology) Our young friend Tim just discovered this for him self on TWO Mettler AE100s recently

My upper price limit on an analytical balance which can read to 0.0001g (o.1mg) is around $300. There are many Mettler and Sartorius balances on ebay all the time. Some of them can be reprogrammed to readout in whatever units you prefer. (like Tola weights from India)

Here is the one I want


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PostPosted: Fri Apr 10, 2009 3:57 am 
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0,0001mg ?!!!!! :shock: Holy cow! And here I was wondering if I was going over the top with my 0,0001 g.... (which is accurate only to the milligram, the last digit is a round of one).

When it commes to scale buying: patience is bliss I've learned. I've been scouting the web for over a year now and in the last week 2 AE100's came up dirt cheap all of a sudden.

Mike, I like your idea of 'weighing below'. It's not gonna happen with that new AE100 of mine but still... an interesting thought. Makes working with a cup of water a bit safer on the scale...


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PostPosted: Fri Apr 10, 2009 4:03 am 
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Tim wrote:
0,0001mg ?!!!!! :shock:


i guess there's a mistake there...

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PostPosted: Fri Apr 10, 2009 4:09 am 
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you would think so ey? But nope...

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PostPosted: Fri Apr 10, 2009 4:11 am 
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OMG :shock: , i think it requires at least a special table, though.

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PostPosted: Fri Apr 10, 2009 5:13 am 
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Metts and Sarts that read 0.0001 gram usually are specified to flicker + or - two digits in the rightmost place. But the milligram digit is usually reliable.

The micro balance does like to be on a float table especially if you are in a large building with elevators that shake the building or near railroad tracks.
But often it isn't necessary.

The AE100 s that Tim worked on are the same balance as the one that Trace is talking about but programmed to read in grams (ie. 100grams =500 metric carats) Their AE series was probably the biggest seller of all time. That is precisely why you shouldn't pay too much for one. They come up on every lab and auction site , frequently.

Tim just enunciated one of the great principles of acquiring expensive lab equipment for the best possible price. Patience is a very important virtue.
If you get the gimmes you are going to pay more. Sometimes you need something to get a project started.

Underneath hanging weighing is not 100% necessary for SG determinations. Mettler and Sart both have kits that go on the top as does Mineralab.


Last edited by G4Lab on Sat Sep 19, 2009 6:41 pm, edited 1 time in total.

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PostPosted: Fri Apr 10, 2009 5:36 am 
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G4Lab wrote:
I was just outbid on this


Hey Massa, it would be perfect, for my mettler CB203. Sorry you've been outbidded.
i've built by myself though 8)
ciao
alberto

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PostPosted: Fri Apr 10, 2009 11:00 am 
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Just a thought.
If one is in the business of selling gems (at least in the US), they must use a scale which can be annually certified by the Bureau of Weights and Measures.
Make sure the scale you are interested in is one that CAN be certified and is meant to be "legal for trade".
It is also a good idea to check the scale of vendors that you are buying from. Is their scale affixed with a current certificate indicating that it is legal to be used for trade and certified as accurate?

I bought several gems at the last show I went to and ALL of them were sold with incorrect carat weights!


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PostPosted: Fri Apr 10, 2009 12:52 pm 
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Just to clarify... my understanding was that you need a "legal for trade" scale if you are selling stones by weight. If you are selling stones on a "price per stone" basis rather than "price per carat", you don't need one, right?


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PostPosted: Fri Apr 10, 2009 1:02 pm 
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True. as long as you are not inferring that the stone you are selling for $X. weighs X.XX carats.. The same is true if you are selling precious metal by weight. Your scale MUST be certified by Weights and Measures.

Of course if you are only using your scale for determinations of SG,etc it doesn't matter....BUT it is certainly a benefit to know that your scale is 100% accurate.


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