Dependence On Flow - Mettler Toledo InPro 6800 Serie Bedienungsanleitung

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10.6 Dependence on flow
With most oxygen electrodes the electrode current is
smaller in stagnant solutions than in agitated ones.
The consumption of oxygen by the electrode results
in extraction of oxygen from the solution in the close
vicinity of the cathode outside the membrane. The oxy-
gen is replaced by diffusion. If the electrode current is
strong, the solution cannot fully restore the oxygen by
diffusion. This results in an electrode current weaker
than would correspond to conditions in the solution.
In agitated solutions the oxygen is conveyed to the
surface of the membrane not only by diffusion but
additionally by the flow (convection). In that case no
oxygen impoverishment occurs at the membrane sur-
face.
A high degree of flow dependence occurs mainly with
large cathodes, thin and highly permeable membranes,
i.e. where electrode currents are large. The problem
of flow dependence is often solved by prescribing a
minimum flow rate.
In METTLER TOLEDO InPro 6800 electrodes, the
thin PTFE membrane determining the electrode cur-
rent (i.e. the actual measuring signal) is separated
from the sample solution by a relatively thick silicone
membrane. This latter is highly permeable to oxygen
molecules and thus acts as an oxygen reservoir. The
diffusion of oxygen out of the sample solution into
the silicone membrane is spread over a wide area.
Since this results in less oxygen being extracted from
the sample solution per unit area, the double PFTE/
Silicone membrane forms an effective buffer against
disturbances due to hydrodynamic flow.
InPro 6800
52 200 953 C
InPro 6800 O
© 02 / 2023 METTLER TOLEDO
Printed in Switzerland
Sensor 12/25 mm
2