Analytical balances

Tim Spahlinger txs at po.cwru.edu
Thu Jan 17 12:45:49 EST 2002


Hi Jim,

I don't pretend to be an expert at this, but QA like this
has been a responsibility for me, too, over the years.  The
fact that the bubble was not centered indicates that,
perhaps, either the balance was not properly installed and
calibrated before it began to be used, or somehow  something
happened to knock the machine out of perfect level since
it's installation.  In my limited experience, "centering the
bubble" becomes all the more important as the need for
accurate weighing of small quantities (i.e., micrograms and
less) increases.  If you're measuring grams and your bubble
is only half out of the circle, the reading is probably
decent enough to use the weighed material.  But, if you're
measuring to an accuracy of a microgram, you probably don't
want to use that weighed material.

Tim

Jim Sutton wrote:
> 
> Sir:
> 
> I am a QA Manager for an EPA research division in Research Triangle
> Park NC.  I have a question that was prompted by an audit of our
> research documentation practices last September.  Could you please
> forward this technical request to an appropriate individual?
> 
> One of our investigators was written up as being afoul of normal
> practise because he did not have a procedure for leveling the balance
> in his laboratory or for verifying that the balance was level although
> it was located on a vibration-free table and otherwise properly
> calibrated and installed.  He did not change the feet of the device
> after it was installed.  Apparently an auditor noticed that the bubble
> in the indicator was not exactly centered and wrote it up as an
> infraction.
> 
> My question is, given the proper calibration and operation of the
> instrument, would not the leveling problem be merely background noise
> that was compensated for by the fact that its attitude relative to the
> table had never been changed and that all measurements were accurate
> to the capacity of the instrument?  It seems to me that the principle
> of ceteris parabis would apply and produce results within the range of
> variability as specified by the manufacturer.  If I am in error, I'd
> like to know.
> 
> Jim Sutton
> US EPA - Neurotoxicology Division
> RTP NC




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