Siemens SIPROCESS GA700, Operating Instructions Manual, Page: 145 / 224

oxygen partial pressure). When you combine two gases with differing oxygen content 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
③
into the measuring chamber
⑥
. One of these partial flows
enters the measuring chamber in the area of the magnetic field
⑦
. 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 current signal by two
microflow sensors
④
located between the reference gas channels
③
.
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
④
are surrounded by reference gas. This arrangement protects them
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
⑩
serve as
vibration sensors and thus compensate for the undesirable behavior.
Description
3.6 Principle of operation of OXYMAT 7
Field device
Operating Instructions, 05/2015, A5E35640463-02
33