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Re: Mass



"On the other hand there are two ways height can come into play. One
really bad way would be to calibrate a microbalance on one floor of a
building and then move it to another floor of the building and use it
without recalibrating it. A height change of 3 meters (typical for a
building) would definitely affect a microgram balance calibration. But
you also wouldn't want to move the balance to another place on the same
floor without recalibrating because it would be difficult to achieve the
same degree of leveling if the balance is moved."

Unless, I'm confused, this means those balances are not balances. i.e. e.g. use an electric current.

W/o much thought, I think both an equal arm balance and a substitution balance are not sensitive to the value of the G field. Incidentally, the cheap (relatively) analytic balances assume sine (displacement angle) = angle.

bc



Michael Edmiston wrote:

Do we worry about height when we use a micro balance?

Yes and no.

The typical microbalance that can read to the nearest tenth microgram
has a total load capacity of several grams (some are about 2 grams and
some are about 5 grams). Your typical 2 or 5 gram masses are not going
to be different enough in physical size to be different enough in height
for height differences to matter. Indeed, these balances have weighing
chambers only 20 cm high or so and therefore the objects placed inside
have to be fairly small.

There are some very expensive vacuum mass comparators available that can
compare kilogram-sized masses to within 0.1 microgram. Again, these
chambers are small, so you can't have two objects that differ too much
in size.

On the other hand there are two ways height can come into play. One
really bad way would be to calibrate a microbalance on one floor of a
building and then move it to another floor of the building and use it
without recalibrating it. A height change of 3 meters (typical for a
building) would definitely affect a microgram balance calibration. But
you also wouldn't want to move the balance to another place on the same
floor without recalibrating because it would be difficult to achieve the
same degree of leveling if the balance is moved.

The other way height matters is in the sensitivity. Even if you
recalibrate a balance on a higher floor, the reduced gravity of the
higher floor means the detector system is working over a smaller dynamic
range, and that reduces the sensitivity. A paper you can download from
the Mettler-Toledo site claims that a micro balance sensitivity is
reduced by about 1 ppm for every altitude change of about one floor.

This Mettler-Toledo document is informative and can be found at...

http://www.mt.com/mt/resourcedetail/handbooksGuides.jsp?m=t&key=A0Mjg4NjM1_D

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu