Dependence On Flow - Mettler Toledo InPro6900 Bedienungsanleitung

34

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 sur-
face of the membrane not only by diffusion but addi-
tionally by the flow (convection). In that case no oxy-
gen impoverishment occurs at the membrane surface.
A high degree of flow dependence occurs mainly with
large cathodes, thin and highly permeable mem-
branes, 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 6900 electrodes, the thin
PTFE membrane determining the electrode current (i.e.
the actual measuring signal) is separated from the
sample solution by a relatively thick silicone mem-
brane. This latter is highly permeable to oxygen mole-
cules and thus acts as an oxygen reservoir. The diffu-
sion 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 PTFE/silicone
membrane forms an effective buffer against distur-
bances due to hydrodynamic flow.
This membrane coupled with the guard ring and the
special electrolyte ensures excellent signal stability
even when the hydrodynamic flow stops (as on a beer
filling line for example)
InPro 6900
52 201 029
InPro 6900 Series O
© 12 / 15 Mettler-Toledo GmbH
Sensor 12 mm
2
Printed in Switzerland