Endress+Hauser Memosens COS51E, Operating Instructions Manual

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Memosens COS51E Product description
Hauser 9
3.2 Measuring principle
3.2.1 Amperometric measuring principle
During amperometric oxygen measurement, oxygen molecules diffuse through the
membrane and are reduced to hydroxide ions (OH-) at the working electrode. At the
counter-electrode, silver is oxidized to silver ions (Ag+) (this forms a silver halide layer).
The associated release of electrons at the working electrode and absorption of electrons at
the counter-electrode causes a current to flow. Under constant conditions, this current
flow is proportional to the oxygen content of the medium. The current is converted in the
transmitter and indicated on the display as an oxygen concentration in mg/l, µg/l, ppm,
ppb or Vol%, ppmVol, raw value nA, as a saturation index in % SAT or as an oxygen partial
pressure in hPa.
3.3 Potentiostatic three-electrode system
The high-impedance, current-free reference electrode plays an important role. The
formation of the silver bromide or silver chloride layer at the anode causes the electrolyte's
bromide or chloride ions to be depleted. With conventional membrane-covered sensors
with a two-electrode system, this results in increased signal drift.
Not so with the three-electrode system:
The change in the bromide or chloride concentration is recorded by the reference electrode
and an internal regulator circuit keeps the working electrode at a constant potential. The
advantages are a much higher signal accuracy and significantly extended calibration
intervals.
3.4 Membrane body
The oxygen dissolved in the medium is transported to the membrane by the necessary
flow. The membrane is permeable for dissolved gases only. Other substances dissolved in
the liquid phase, e.g. ionic substances, will not penetrate through the membrane.
Therefore, medium conductivity has no impact on the measuring signal.
The sensor is shipped with a membrane body, which can be used for both measuring
ranges. The membrane is pretensioned at the factory and can be used immediately.
Electrolytes are measuring range-specific and cannot be mixed in a single application!
Also pay attention to the safety datasheet of the electrolyte
www.endress.com/downloads .
3.5 Polarization
When the sensor is connected to the transmitter, a fixed voltage is applied between the
working electrode and the counter-electrode. The resulting polarization current can be
identified on the transmitter by a reading that is initially high, but decreases over time.
The reading must be stable before the sensor can be calibrated and a reliable
measurement is possible.
3.6 Memosens technology
Sensors with Memosens protocol have an integrated electronics unit that stores calibration
data and other information. Once the sensor has been connected, the sensor data are
transferred automatically to the transmitter and used to calculate the measured value and
for Heartbeat Technology functions.
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Call up the sensor data via the corresponding DIAG menu.