Siemens SIPROCESS GA700 Operating Manual page 23

Measuring principle
Oxygen is highly paramagnetic compared to other gases. 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. A higher concentration or partial pressure of oxygen is generated at this point.
If two gases with differing oxygen content are combined in a magnetic field, a difference in
partial pressure results between them.
The measuring effect is therefore always based on the difference in the oxygen content of
the two gases, the sample and reference gases.
How it works
To measure oxygen in the OXYMAT 7, the reference gas (air, oxygen, or nitrogen) flows
through two reference gas channels into the sample chamber. One of these partial flows
enters the sample chamber in the area of the magnetic field.
If the sample gas contains no oxygen, the reference gas flows freely into the sample
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 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 voltage signal by a
microflow sensor located between the reference gas channels. The alternating flow results in
a change in resistance which is a measure for the oxygen concentration in the sample gas.
Any vibrations occurring at the installation site may impair the measured signal. Two
additional optional microflow sensors serve as vibration sensors and thus compensate for
the undesirable behavior.
Operating with the Local User Interface
Operating Manual, 06/2017, A5E31930478-05
Description
3.2 OXYMAT 7
23