JUMO 202610.0, Operating Instructions Manual, Page: 12 / 68

3 Oxygen transmitter dTRANS O2 01
12
The cathode, with which the oxygen diffused through the membrane reacts
electrochemically, is in direct contact with the membrane, via a thin electrolyte
film.
3.3 Function
of the
transmitter The transmitter operates on the 2-wire principle, i.e. the output is a
proportional modulation (4 — 20mA) of the current drawn by the transmitter.
The output signal is electrically isolated from the medium being measured.
of the
measuring cell The cell does not need an external supply, since the minimum voltage of
about 850 mV that is necessary for oxygen reduction is internally generated
through the partial reactions (1) and (2).
(1) O
2
+ 2H
2
O + 4e
-

4OH
-
E° (O
2
) = +401mV (cathodic reaction)
(2) 2Pb + 4OH
-

2PbO + 2H
2
O + 4e
-
E° (Pb) = -576mV (anodic
reaction)
Sum O
2
+ 2Pb -> 2PbO E° = 977mV
If an electrical connection is made between cathode and anode via a working
resistance, a current will flow that is linearly proportional to the reduced
oxygen and, therefore, to the oxygen dissolved in the sample liquid.
The cell has no zero current, which means that when no oxygen is present,
there will also be no voltage and, consequently, no measuring current.
3.4 Influence
on the
measurement
signal The measurement signal is influenced by the following factors
- the oxygen permeability of the membrane
- the saturation solubility of oxygen in water
- the salinity (salt content) and
- the vapor pressure of the water.
These factors are temperature-dependent.
The oxygen permeability of the membrane is design-specific. It depends on
the material, the thickness and the deformation of the membrane.
The remaining factors describe physical phenomena, such as are listed in
EN 25 814.
The factors mentioned above, and their dependency on temperature, are
taken into account by the microprocessor-controlled evaluation electronics
and compensated for.
Pressure fluctuations in the measured medium will affect the output signal!