Endress+Hauser OxyMax W COS 31, Operating Instructions Manual

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3 Installation OxyMax W COS 41
12 Hauser
3.4.2 Functional description
Polarisation
When the sensor is connected to the transmitter, a fixed external voltage is applied
between the cathode and anode. The resulting polarisation current is indicated on a dis-
play on the transmitter. The current starts high but then drops over time. The sensor can
only be calibrated when the display is stable.
Membrane
The oxygen dissolved in the medium is conveyed to the membrane by the incoming
flow. The membrane is only permeable for dissolved gases. Other substances dis-
solved in the liquid phase e.g. ionic substances, will not penetrate through the mem-
brane. Therefore, medium conductivity has no impact on the measuring signal.
Amperometric measuring principle
The oxygen molecules diffused through the membrane are reduced to hydroxide ions
(OH
-
) at the gold cathode. Silver is oxidised to silver ions (Ag
+
) at the anode (this forms
a silver bromide layer, AgBr).
A current flows due to the connected electrode release at the gold cathode and
accepted at the anode. In equilibrium, this flow is proportional to the oxygen content of
the medium.
This current is converted in the measuring instrument and indicated on the display
as an oxygen concentration in mg/l, as a saturation index in % SAT or as an oxygen
partial pressure in hPa.
3.5 Post-installation check
After installing the sensor, carry out the following checks:
Fig. 3.14: Sensor head with membrane cap
(Side view with cut-away view of
membrane cap)
Fig. 3.15: Sensor head (top view with membrane
cap removed)
1 Screw thread for protection basket 5 Membrane
2 Sealing ring 6 Membrane cap
3 Screw thread for membrane cap 7 Anode
4 Electrolyte 8 Gold cathode
Checks Remarks
Membrane OK (visual inspection)? Replace membrane if there is a leak.
Compliance with permissible sensor installation position? see Chapter 3.3
Sensor installed in immersion assembly? Do not install the sensor suspended from the cable.
Protective cap fitted to immersion assembly? Avoid moisture by rain in the assembly.
C07-COS41xxx-16-05-07-xx-001.EPS
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C07-COS41xxx-16-05-03-xx-002.EPS
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