Re: mass vs. volume measurement

Old Man wrote:


Old Man is amazed at the trust that microbiologists put
into their instruments.  A lab balance is easily calibrated
with a set of standard weights, but do they ever think to
check their micro-pipettes ?

Oh, come now... the set of standard weights in our lab is subject to the same abuse as everything else. Eppendorf and every other major manufacturer routines come around to perform calibrations of pipettors. Glass syringes need very little recalibration, as well.


What of temperature effects ?  What of abuse and of
improper technique ?  They know not and care even less.

Yes, yes, yes... that's why for a few bucks (like $50), one buys a pycometer, which was temperature calibrated at the factory. Or buys a few to cover the lab ambient temp.


Old Man learned proper technique via checking micro-
pipetted samples with a calibrated lab balance.  One
quickly learns the effects of technique and abuse,  as
well as those of  time and evaporation rate.


Care is always taken for accurate measurements, regardless of the type of measurement! I don't dispute this. My comment was simply this- for $138, one may accurately dispense sub-microliter volumes of fluid. I challenge anyone to buy a balance (and weights) that can approach the 0.1 mg level for that price. And my question is simply why is this so?

--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
.