Function
14
Hauser
5
Function
5.1
Measuring principle
5.1.1
Amperometric measuring principle
The oxygen molecules that diffuse through the membrane are reduced at the cathode into hydroxide
ions (OH
-
). At the anode, silver is oxidized into silver ions (Ag
+
) (this forms a silver halide layer).
A current flows due to the electron donation at the cathode and the electron acceptance at the anode.
Under constant conditions, this flow is proportional to the oxygen content of the medium.
This current is converted in the transmitter and indicated on the display as an oxygen concentration in
mg/l,
μ
g/l, ppm, ppb or Vol%,
as a saturation index in % SAT or as an oxygen partial pressure in hPa.
5.1.2
Sensor design
5.1.3
Polarization
When the sensor is connected to the transmitter, a fixed voltage is applied between the cathode and
anode. The polarization current this creates can be identified on the transmitter with a display that is
initially high, but decreases with time. The sensor cannot be calibrated until the display is calibrated.
Reference value for nearly complete polarization of a sensor that was previously stored for a long time:
• COS22-*1: 2 hours
• COS22-*3: 12 hours
After this time, even measurements close to the determination limit are useful.
The necessary polarization time is reduced for sensors that were in use a short time before.
a0011868
Fig. 8: Exploded
view
1
2
3
4
Plug-in head
Pressure ring
Sensor shaft
O-ring 8.5 x 1.5 mm
5
6
7
Membrane body
Shaft cover
Glass body with cathode and anode
8
9
Process sealing 10.77 x 2.62 mm
Process connection Pg 13.5
