horiba U-51, Инструкция по эксплуатации

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6 Reference
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6.4.3 Salinity calibration
When the solution and air come into contact and form an equilibrium (i.e. saturation), the
relationship between the concentration of dissolved oxygen in the solution, C, [mol/L], and the
partial pressure of oxygen in the air, Ps, [MPa/(mg/L)], can be represented by the following
formula:
C = Ps/H
Where H [MPa/(mg/L)] is the Henry constant, a value that changes according to the
composition of the solution. As H typically becomes larger as the salinity of the water
increases, C becomes smaller.
The DO sensor detects the partial pressure of oxygen (Ps) in the above formula. Accordingly,
if the DO sensor is immersed in deionized water saturated with air, or in an aqueous solution
containing salt, the output current does not change, resulting in an erroneous measurement.
For example, when salt is added to a sample, the amount of oxygen that can be dissolved in
the solution decreases, but because the partial pressure of oxygen does not change, the
value displayed by the control unit stays the same regardless of salt content. This concept is
indicated in graph form below. (Fig. 2)
Fig. 2 Relationship between chlorine ion concentration and dissolved oxygen concentration
In samples with a high salt concentration, the solubility of oxygen is lower, but as the partial
pressure of oxygen does not change, the value actually indicated on the control unit is higher
than the actual value. In order to obtain a measurement of the concentration of dissolved
oxygen in an aqueous solution that contains salt, it is therefore necessary to first perform
salinity compensation. Conventionally, dissolved oxygen sensors have performed salinity
compensation by inputting the salinity of the sample. This is fine as long as the salinity is
already known. However, in most cases salinity is unknown, so even if dissolved oxygen
sensors contained a salinity compensation function, it was of no practical use.
The U-50 Series can calculate and measure salinity in samples from electrical conductivity
values, and can thus be used to automatically compensate for salinity.
C
oncen
tra
tio
n of
DO (mg/L)
Concentration of Chlorine Ions (mg/L)
Values indicated by DO sensor
Actual concentration
of DO in sample