Oxygen Partial Pressure - Oxygen Concentration - Mettler Toledo InPro 6850i Bedienungsanleitung

38
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
Teflon 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
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).
Measurement in liquids
When measuring in liquids, the concentration of oxy-
gen also depends on the solubility of the oxygen in
the measurement medium. However, since this is not
measured by the sensor current, the concentration of
oxygen must be calculated in the transmitter. To do
this, Henry's law is applied which states that the con-
centration of oxygen is proportional to the partial
pressure of oxygen (pO
Cl = pO
a = Solubility factor
If "a" is constant, the oxygen concentration can be
determined by means of the electrode. This applies at
constant temperature and with dilute aqueous solu-
tions such as drinking-water.
InPro 6850i, 6900i, 6950i
52 206 349
InPro 6850i / 6900i / 6950i O
a
2
2
).
2
© 02 / 09 Mettler-Toledo AG
Printed in Switzerland
Sensor 12 / 25 mm