NAVI D-25 Instruction Manual page 209

7 Reference
7.5 Dissolved oxygen measurement
● Salinity concentration correction
● Air pressure correction
198
The DO measurement method that is based on this
principle is called the "Membrane electrode method."
This is a much simpler and more convenient way of
measuring DO than using chemical analysis, which
requires complex pretreatment in order to eliminate the
effects of reductants and oxidants in the sample.
When a solution is in contact with air and is in a state of
perfect equilibrium
relationship between the DO contained within the
solution (C; expressed in mg/L) and the partial pressure
of the oxygen in the air (Ps; expressed in Mpa) is shown
by the following equation:
C = Ps / H
The H (expressed as MPa/[mg/L]) in this equation is
referred to as the "Henry constant" and has a different
value depending on the composition of the solution.
Generally, the higher the salinity concentration within a
solution, the larger H becomes, and, consequently, the
smaller C becomes.
DO tips actually detect the "Ps" that occurs in the above
equation. This means that even if a DO tip is immersed
in pure water that is saturated with air or in an aqueous
solution containing salt, the output current will not
change, which gives rise to a problem.
For this reason, it is necessary to correct the salinity
concentration, to enable the correct DO to maintain a
current, even in an aqueous solution containing salt, and
resolve the problem.
The amount of DO in a solution is proportional to the
partial pressure of the oxygen contained within the air in
which the solution is in contact.
At 25ºC, for example, when water is saturated by air that
has an atmospheric pressure of 1013 hPa (1
atmosphere), the DO is 8.11 mg/L. As the elevation at
which measurement takes place increases, however, the
atmospheric pressure caused by the air decreases. So,
(a state of saturation), the
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