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ISOGEOCHEM  January 2001

ISOGEOCHEM January 2001

Subject:

Re: Micro-Balance Experience

From:

Paul Brooks <[log in to unmask]>

Reply-To:

Stable Isotope Geochemistry <[log in to unmask]>

Date:

Wed, 10 Jan 2001 11:19:49 -0800

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (169 lines)

Len,

Just over a year ago I tested all three of the 6 place balances that we
have. I weighed out the same tin capsule on each one 12 or 13 times and
recorded the displayed weight, time to zero before putting the tin in, and
the time before the reading steadied. Please note that I was only able to
test one balance of each type, and they are of different ages and
condition. I have pasted in the results of the full test below, but many
email programs may not be able to get the columns correct so that they can
be read easily. If anyone wants all the data email me personally and I
will send them an attached file with the actual data.

They are:

1) Sartorius XM 1000P, 10 years old rebuilt about 2 years before test.

average weight, one outlier removed, n=12, average 9.678 mg, stdevp= 0.003,
average time to zero 20 sec, time to weigh 20 secs.


2) Metter Toledo AT21 Comparator, 1 month old when tested.

n=13, average weight 9.672 mg, stdevp 0.003mg, average 14 seconds to zero,
15 seconds to steady.


3) Cahn C-30 microbalance, 9 years old when tested having just been
salvaged from a junk pile in the building. Had been repaired at least once
in its life time prior to testing.

n=12, average weight 9.673 mg, stdevp 0.001 mg, average 14 seconds to zero,
16 seconds to steady.


I would not hesitate to use any of these balances. Each has advantages and
disadvantages.

   The Sartorius can often be bought new (I think around US$6,000), or
sometimes used and rebuilt at a very good price (I think around US$6,000),
try Costech Analytical, [log in to unmask] However, the
weighing chamber is very small and somewhat difficult to put samples in,
students often bang the tin against the sides of the chamber when putting
the tin capsules in and out. As does the Mettler, this balance will change
its reading if placed on a normal wooden lab bench, even when it is on top
of a stone slab on the bench. This results in the reading changing if the
user rests their elbow or arm on the lab bench while waiting for the
balance to steady. However, if the user is careful not to lean on the
bench, then the balance is quite accurate.

The Mettler is more costly but very nice to use, the cover slides back and
forth automatically, and it can go up to 21 g. If you are buying the CE
analyzer new you can get a very good price on the balance from the CE
dealer, and possibly buy one if it is a replacement for and old one bought
with a CE analyzer, try [log in to unmask] Both the Sartorius and
Mettler are very robust and I do not hesitate to have our undergraduates in
the lab use them

Surprisingly, the Cahn that we salvaged out a junk pile (and it looks it!)
has the best reproducibility, and is my personal favorite to use. It also
does not suffer from any effect of putting it on a wooden lab bench and
moving it from one room to another. However, it only goes up to 200mg, is
difficult to put samples on the weighing trays, and very easy to damages if
the user removes their forceps quickly and drags the weighing tray with
them. This happened about six months ago and the balance cost US$850 to
repair and was gone for over 2 months. For this reason I prefer that our
undergraduates do not use this balance.

Hope this helps, Paul



Test of 6 place balances performed 99\10\22 at UC berkeley by Paul Brooks.
The same tin capsule was weighed on all balances.
Note that balances are not all the same age, a comparison of new balances
might yield
a different results. Also only one balance of each type was tested, so
this is not a
very scientific test.
reportedtime totime to
#weightzeroweigh
Sartorius XM 1000P19.6793060
10 years old, 29.6783020
rebuilt 2 years 39.6791010
before test49.6732620
59.6821710
69.6781920
79.6791010
*89.6534525
99.6801330
109.6791512
119.6881311
129.6822510
139.6811020
average=9.6782020
stdevp=0.0081013
* This one was a major outlier, removing it resulted in the results below
average=9.6782020
stdevp=0.003714
Mettler Toledo AT21 Comparator19.6792018
1 month old when tested29.6731620
39.6751320
49.6721614
59.6751713
69.6721318
79.6711316
89.6711314
99.6661112
109.6711513
119.6761215
129.6701315
139.6701312
average=9.6721415
stdevp=0.00323
Cahn C-30 microbalance19.6742412
9 years old when tested, 29.6711520
salvaged from junk pile, 39.6731520
had been repaired at least once 49.6751515
in its life time.59.6762020
69.6731120
79.6741520
89.6741015
99.6721015
109.6731010
119.67385
129.6731015
average=9.6731416
stdevp=0.00155

At 10:34 AM 1/10/01 -0600, you wrote:
>Dear Listmembers,
>
>We are planning to purchase a new micro-balance for weighing of EA-IRMS
>samples and other purposes. We will require readability of about 1-2 ug
>(+/-2-4 ug). As these are quite costly (e.g. Mettler AX Series - >$14k
>USD), and sometimes tricky instruments to maintain, I am wondering if any
>Listmembers have recently researched new micro-balances and then made a
>purchase decision. And if so, were you pleased with the performance,
>stability, and reliability of the micro-balance you purchased? Was it
>robust enough for a multi-user environment? Was the price negotiable from
>the list price? Any models to avoid?
>
>Any suggestions and advice are greatly appreciated.
>
>
>Len Wassenaar
>
>
>Leonard I. Wassenaar, Ph.D.
>Stable Isotope Hydrology and Ecology Laboratory
>National Water Research Institute
>Environment Canada
>11 Innovation Blvd.
>Saskatoon, SK
>S7N 3H5
>
>tel: (306) 975-5747
>fax: (306) 975-5143




Paul D. Brooks,
Dept. ESPM-ES,
147 Higard Hall MC 3110,
UC Berkeley, Ca. 94720-3110.

phone (510)643-1748,
FAX (510)643-5098.

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