Siemens SIPROCESS GA700 Operating Instructions Manual page 25

Measuring principle
In contrast to other gases, oxygen is highly paramagnetic. OXYMAT 7 analyzer modules use this
physical property to determine the oxygen concentration in gases.
Oxygen molecules in an inhomogeneous magnetic field always move toward the higher field
strength. This results in a higher oxygen concentration where the field strength is higher (higher
oxygen partial pressure). If two gases with differing oxygen content come together in a magnetic
field, a partial pressure difference arises between them. The measuring effect is therefore always
based on the difference in the oxygen content of the two gases. The two gases are referred to
as the sample gas and the reference gas.
How it works
To measure oxygen in the OXYMAT 7, the reference gas (nitrogen, air or oxygen) flows through
two reference gas channels
the measuring chamber in the area of the magnetic field
oxygen, the reference gas flows freely into the sample chamber. However, if the sample gas
contains oxygen, the oxygen molecules concentrate in the area of the magnetic field. The
reference gas can then no longer flow freely into the sample chamber. There is a slight build-up
of dynamic pressure, which depends on the concentration of oxygen in the sample gas. In an
oscillating magnetic field, oscillating pressure is also generated and this results in alternating
flow. This alternating flow is converted into an alternating current signal by two microflow
sensors
The microflow sensors
with two further nickel-plated grids to form a measuring bridge. The alternating flow results in
a change in the resistance of the nickel-plated grids. The resulting offset in the bridge is a
measure of the concentration of oxygen in the sample gas.
The microflow sensors
from corrosion by the sample gas.
Vibrations at the installation site can interfere with the measured signal, for example, large
fluctuations in the output signal. Two additional optional microflow sensors
vibration sensors and thus compensate for the undesirable behavior.
Rack-mounted device
Operating Instructions, A5E31874006-12
④
into the measuring chamber
③
located between the reference gas channels
③
consist of two nickel-plated grids each, which are arranged together
③
are surrounded by reference gas. This arrangement protects them
⑥
. One of these partial flows enters
⑦
. If the sample gas contains no
④
.
⑪
Description
4.4 OXYMAT 7
serve as
25