Temperature; Dependence On Flow; Oxygen Partial Pressure - Oxygen Concentration - Mettler Toledo InPro 6000G Serie Instructions D'utilisation

InPro 6800 G / 6850i G / 6900i G / 6950i G O

10.5 Temperature

The temperature dependence of the current passing
through an oxygen electrode when referred to a con-
stant O
properties of the gas-permeable membrane.

10.6 Dependence on flow

With most oxygen electrodes the electrode current is
smaller in stagnant solutions than in agitated ones.
In consequence of the oxygen consumption of the
electrode, a reduction of oxygen takes place outside
the membrane in close proximity to the cathode. The
missing oxygen is replaced by diffusion. If the elec-
trode current is strong, the solution cannot fully re-
store the reduced 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 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 6950i G electrodes, the
thin Teflon 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
Teflon/silicone membrane forms an effective buffer
against disturbances 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).
10.7 Oxygen partial pressure –
oxygen concentration
The electrode current is dependent on the partial
pressure of oxygen nad the oxygen permeability of
the membrane. Conversion of partial pressure into
concentration of oxygen depends on the measure-
ment medium (measurement in liquids or gases).
© 09 / 09 Mettler-Toledo AG
Printed in Switzerland
partial pressure is determined mainly by the
2
12 mm
2
InPro 6000 G – Series
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