Siemens SIPROCESS GA700 Operating Instructions Manual page 20

Description
3.4 Functional principles
In contrast to other gases, oxygen is highly paramagnetic. This physical property is used to
determine 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 are combined 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.
To measure oxygen in the OXYMAT 7, the reference gas (nitrogen, air or oxygen) flows
through two reference gas channels
flows enters the measuring chamber in the area of the magnetic field
contains no oxygen, the reference gas can flow freely into the measuring chamber. If the
sample gas does contain oxygen, the oxygen molecules concentrate in the area of the
magnetic field. The reference gas can then no longer flow freely into the measuring
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
reference gas channels
The microflow sensors consist of two nickel-plated grids each, which are arranged together
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 (e.g. large
fluctuations in the output signal). This behavior can be compensated by two further (optional)
microflow sensors
20
③
into the measuring chamber
③
.
④
are surrounded by reference gas. This arrangement protects them
⑩
, which function as a vibration sensor.
⑥
. One of these partial
⑦
. If the sample gas
④
located between the
Operating Instructions, 01/2013, A5E31930403-01
Wall-mounted device