Siemens ULTRAMAT 23, Manual, Page: 28 / 244

Description
3.3 Function
ULTRAMAT 23
28
Manual, 3/2016, A5E37100388-003
This oxygen sensor operates according to the principle of a fuel cell. The oxygen is
converted at the boundary layer between the cathode and electrolyte. An electron emission
current flows between the lead anode and cathode and via a resistor, where a measured
voltage is present. This measured voltage is proportional to the concentration of oxygen in
the sample gas.
The acidic electrolyte used is less influenced by interference influences (particularly CO
2
,
CO, H
2
, and CH
4
) than other sensor types.
Paramagnetic oxygen measurement
Figure 3-6 Operating principle of the paramagnetic O
2
sensor
In contrast to other gases, oxygen is highly paramagnetic. This property is used as the basis
for this method of measurement.
Two permanent magnets generate an inhomogeneous magnetic field in the measuring cell. If
oxygen molecules flow into the measuring cell (1), they are drawn into the magnetic field.
This results in the two diamagnetic hollow spheres (2) being displaced out of the magnetic
field. This rotary motion is recorded optically, and serves as the input variable for control of a
compensation flow. This generates a torque opposite to the rotary motion around the two
hollow spheres by means of a wire loop (3). The compensation current is proportional to the
concentration of oxygen.
The calibration point is calibrated using the AUTOCAL function by connecting oxygen
(analogous to calibration of the electrochemical O
2
sensor). In order to comply with the
technical data, the zero point of the paramagnetic measuring cell must be calibrated with
nitrogen weekly in the case of all measuring ranges < 5% or every two months in the case of
all larger measuring ranges.