Mettler Toledo InPro 6000G Serie Instructions D'utilisation page 40

40
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.
The solubility factor is strongly influenced not only by
the temperature but also by the composition of the
solution:
Medium, saturated with air
Water
4 mol/l KCI
50 % Methanol-water
Although the solubilities vary widely, the oxygen elec-
trode gives the same reading in all three solutions.
Thus, determination of the oxygen concentration is
only possible with constant and known solubility fac-
tors "a".
Solubility may be determined by a Winkler titration or
the method developed by Käppeli and Fiechter.
Measurement in gases
The concentration of oxygen when measuring in gas-
es is always given as a proportion by volume of the
gas composition. Common units are % (by vol.)
ppm (by vol.).
They can simply be converted from one unit of mea-
surement to the other.
Example:
The percent by volume of the composition of air is
generally known. For example, air contains 20.95 %
oxygen. This corresponds to 209,500 ppm (by vol.).
(Conversion: ppm value = 10,000
InPro 6000 G – Series
52 206 477
InPro 6800 G / 6850i G / 6900i G / 6950i G O
a
2
).
2
Solubility at 20°C (68°F)
and 760 mm Hg
9.2 mg O
2
2 mg O /l
2
21.9 mg O
© 09 / 09 Mettler-Toledo AG
12 mm
2
/l
/l
2
value in %)
Printed in Switzerland