Page 422 - Instrumentation Reference Book 3E

Special gas analyzers 405

of a very early component, such as hydrogen, pre- A ferro- or paramagnetic substance when
cludes the existence of earlier reference and trigger placed in a magnetic field in a vacuum or less
peaks. Operation of the various functions is then strongly paramagnetic medium tries to move
programed using the fixed-time mode. Selection of from the weaker to the stronger parts of the field.
the required mode is made using a switch on the A diamagnetic material, in a magnetic field in a
psoces sor . vacuum or medium of algebraically greater sus-
ceptibility tries-although the effect is very small
-to move from the stronger to the weaker parts of
Maizual operation This mode of operation, the field. Thus when a rod of ferromagnetic or
selected by the “autolmanual” switch on the front
panel, provides a single analysis which is followed paramagnetic substance is suspended between the
poles of a magnet it will set with its length along
by column backflushing and the normal “halt” the direction of the magnetic field. A rod of bis-
condition. Single analyses are initiated by oper- muth, on the other hand, placed between the
ation of the “action” push-button, provided that poles of a powerful electromagnet will set at right
the previous analysis has been completed. This
mode of operation is used during initial program- angles to the Geld.
It has been shown experimentally that for para-
ming or servicing.
magnetic substances the susceptibility is indepen-
dent of the strength of the magnetizing field but
decreases with increase of temperature according
to the Curie-Weiss law:
1 Special gas analyzers
relative atomic mass
atomic susceptibility =
18.5.1 Paramagnetic oxygen analyzers density
x volume susceptibility
Many process analyzers for oxygen make use of
the fact that oxygen, alone among common gases,
is paramagnetic.
where Tis the absolute temperature and C and B
18.5.1.1 Basic principles are constants.
The susceptibilities of ferromagnetic materials
The strength of a magnet is expressed as its mag- vary with the strength of the applied field, and
netic moment. When a material, such as piece of above a certain temperature (called the Curie
soft iron, is placed in a magnetic field, it becomes temperature and characteristic of the individual
magnetized by induction, and the magnetic material) ferro-magnetics lose their ability to
moment of the material divided by its volume is retain a permanent magnetic Geld and show para-
known as the intensity of magnetization. The magnetic behavior. The Curie temperature of
ratio of the intensity of magnetization to the iron is 1000 K.
intensity of the magnetic field is called the volume The susceptibility of diamagnetic substances is
susceptibility k of the material. All materials almost independent of the magnetizing field and
show some magnetic effect when placed in a mag- the temperature.
netic field, but apart from elements such as iron, The paramagnetic properties of oxygen are
nickel, and cobalt and alloys such as steel, all exploited in process analyzers in two main ways;
known as ferro-magnetics, the effect is very small, the so-called “magnetic wind” or thermal magnetic
and intense magnetic fields are required to make instruments, and magnetodynamic instruments.
it measurable.
Substances which are magnetized in the oppos-
ite direction to that of the applied field (so that k 18.5.1.2 Magnetic wind instruments
is negative) are called diamagnetics. Most sub-
stances are dizmagnetic, and the value of the The magnetic wind analyzer, originally introduced
susceptibility is usually very small. The most by Hartmann and Braun, depends on the fact that
strongly diamagnetic substance is bismuth. oxygen, as a paramagnetic substance, tends to
The magnetic properties of a substance can be move from the weaker to the stronger part of a
related to its electronic structure. In the oxygen magnetic field, and that the paramagnetism of
molecule, two of the electrons in the outer shell oxygen decreases as the temperature is raised.
are unpaired. Because of this the magnetic
moment of the molecule is not neutralized as is volume susceptibility -~ C (Curie-Weiss law)
-
the commoner case, and the permanent magnetic density (T - 0)
moment is the origin of oxygen’s paramagnetic i.e.; volume susceptibility = x density
properties. ~ (T - 0)