Page 123 - Gas Adsorption Equilibria

2. Volumetry / Manometry 109

6. PROS AND CONS OF VOLUMETRY /
MANOMETRY

In this section we will discuss some of the advantages and disadvantages
of the volumetric or manometric method of measuring gas adsorption
equilibria as they have appeared to us after more than 20 years of
experimental work in this field.

6.1 Advantages

1. Simplicity
Volumetric gas adsorption instruments are fairly simple and do not
require sophisticated high tech equipment, cp. Figs. 2.1, 2.6. Also the
experiment in itself is simple - opening of a valve between gas storage
and adsorption chamber. Therefore, volumetric instruments easily can be
automated and run without permanent supervision for hours and days
[2.3, 2.4]. To achieve this it is recommended that every instrument is
equipped with its own personal computer (PC) for data handling, storage
and transmission. For screen based operation and control the program
“Lab View” (Version 6.1) has proved to be very useful. Combined with
the program “PC Anywhere” it could allow remote operation of the
measuring instrument from nearly any place around the world.

2. Measuring techniques
Volumetric measurements of gas adsorption equilibria reduce, if the
mass of the sorbent sample used has been determined, to measurements
of pressures and temperatures in gas phases. For this a variety of high
precision measuring instruments operating in a fairly wide range and
partly also in corrosive environment are available today. Of course these
instruments prior to measurement have to be calibrated with meticulous
care which may be laborious and even cumbersome. For pressure
measuring devices, calibration with pressure maintaining valves of
Desgranges & Huot has proved to be successful.

6.2 Disadvantages

1. Amount of sorbent material
For volumetric measurements of technical gas-sorbent equilibria a
certain amount of sorbent material is needed, usually several grams. This
is caused by the fact that only then considerable changes in the gas
pressure which are due to adsorption can be observed. If only tiny