If mass units are used for measurement of Dissolved Oxygen, the
temperature problem of relating the linear partial pressure reading of the
sensor to the mass (ppM or mg/L) at different temperatures becomes
more involved. As well, there is the mass variation due to dissolved salts
(salinity correction). Therefore, the fully corrected instrument would need
3 correction systems.
(a) Membrane correction for temperature permeability effects ;
(b) Solubility correction of Dissolved Oxygen with temperature and ;
(c) Salinity correction of Dissolved Oxygen by weight (Salinity has no
effect on pressure units readout).
In the
Aqua-DY
instrument,
(a) Membrane correction is achieved AUTOMATICALLY ;
(b) To provide the mass units (ppM) readout (so popular due to the
Winkler process used in the past), the
Aqua-DY
Meter has Solubility
Correction via an additional temperature sensor in the sensor ;
(c) Salinity correction is performed automatically once the Salinity value
of the solution is entered (section 5).
10.5
Equilibrium Conditions
Whilst Saline Water has a lower ppM than does Fresh Water, it does not
mean it necessarily has less biologically available oxygen. Both have
100% Saturation (presuming no Chemical Oxygen Demand (C.O.D.),
Biological Oxygen Demand (B.O.D.), etc.) because both are in partial
pressure equilibrium with air. Any usage of oxygen is immediately
replenished by the dissolving of more from air to meet partial pressure
equilibrium requirements. This is so for both saline and fresh water. The
reporting of oxygen at a lower level (in ppM units) in Salt Water is
therefore QUITE MISLEADING !
In closed systems, such as tanks, pipes and deep waters, equilibrium is
not so readily available and the Salinity Effect gains importance in the
reporting of Dissolved Oxygen. It is suggested, unless such closed (or
deep, low diffusion) systems are encountered, that Oxygen should be
reported in % Saturation or ppM of equivalent Fresh Water.
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