Page 184 - Gas Adsorption Equilibria

170 Chapter 3

5.2 Disadvantages

1. Complexity
Modern microbalances, especially the magnetic suspension balances
are fairly complex systems which often are sensitive to electro-
magnetic or – to lesser extend – mechanical disturbances from
outside. Oscillations of the basement caused by nearby subways or
trucks may cause disturbances, though balances recently (2002) have
become less sensitive to these. However, electromagnetic fields
radiating from electric heating systems or other wires inside a lab can
lead to serious problems in the performance of either the balance
itself or the data recording system.

Also fine grained sorbent materials, especially activated carbon fibers
or powder may cause problems as they may change their position
within a vessel mounted to the balance or simply are blown out of it
due to the sorptive gas flow.

We mention this here to indicate to the reader that microbalances are
fairly sensitive and – by the way – expensive instruments and require
some experience and – foremost – patience in handling to get accurate
and reliable results.

2. Measuring Techniques
Gravimetric measurements of gas adsorption equilibria are laborious
as they require
detection of several thermophysical quantities like pressure,
temperature, and sometimes concentrations of sorptive gas
mixtures,
various calibration experiments,
careful handling of sorbent material, and
sophisticated software for balance controlling and data handling,
cp. also Chap. 4.
It also should be mentioned that measurement of sorbent material’s
temperature is not as easy as in volumetric experiments as – for
example – a thermocouple only can be placed in the vicinity of the
sample but not inside it.

Automation of gravimetric sorption instruments is not an easy matter
and for precision measurements permanent supervision of the
adsorption / desorption process by an experienced co-worker is
strongly recommended.