Temperature; Dependence On Flow; Oxygen Partial Pressure - Oxygen Concentration - Mettler Toledo InPro 6850 i Bedienungsanleitung

InPro 6850 i / 6900 i / 6950 i O

10.5 Temperature

The temperature dependence of the current pass-
ing through an oxygen electrode when referred to a
constant O
the 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 electrode current is strong, the solution cannot
fully restore 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 6950 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
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 PTFE/
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).
Measurement in liquids
When measuring in liquids, the concentration of oxy-
gen also depends on the solubility of the oxygen in
© 04 / 13 Mettler-Toledo AG
Printed in Switzerland
Sensor 12 / 25 mm
2
partial pressure is determined mainly by
2
InPro 6850 i / 6900 i / 6950 i
37
52 206 349