Xylem WQ-FDO Sensor and GL500-7-2 Data Logger - Manual (Page 26)

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XIII. DO Setup
To calculate dissolved oxygen concentration and saturation levels, several
environmental factors must be known. In a state if equilibrium; the partial pressure
of oxygen in the air and in the water are equal, as stated by Henry’s Law. The partial
pressure of oxygen in air is 20.1% of the total atmospheric pressure. The WQ-FDO
sensor, as do most or all dissolved oxygen sensors, measure the dissolved oxygen
in water by measuring the partial pressure of oxygen present. At sea level the
average atmospheric pressure is about 1013 mbar, and the partial pressure of
oxygen in the air is 20.1% of that or about 204 mbar. In a state of equilibrium
between water and air, this partial pressure would be the 100% saturation level of
oxygen in water. Since atmospheric pressure decreases with increasing altitude, the
partial pressure of oxygen in water required to reach a 100% saturation level also
decreases. Even weather activity can cause changes in barometric pressure and
effect the concentration and saturation levels in water. Besides atmospheric
pressure, temperature also changes the oxygen concentration levels. At higher
temperatures, less oxygen can be dissolved in water. Therefore; the concentration
of oxygen in water required to achieve 100% saturation decreases with increasing
temperature. Conductivity, or salinity, also decreases the ability of oxygen to
dissolve in water. So to accurately determine the oxygen concentration and
saturation levels, the oxygen partial pressure in the water, temperature, atmospheric
pressure, and conductivity must all be known. Since other biological factors also
affect oxygen levels, such as fish depleting oxygen and algae enriching it, oxygen
saturation is a dynamic and constantly changing measurement. Due to these factors
and the delay time required to reach equilibrium, the saturation levels can fall well
below 100%, and can increase well above 100% as well.
Certain assumptions can be made in cases where the salinity and atmospheric
pressure are not changing excessively. Changes in atmospheric pressure due to
storm activity does not usually make significant changes in calculations and can
sometimes be ignored. In many cases, changes in salinity can also be ignored,
especially when making measurements in fresh water. In other situations where
salinity is changing dramatically due to tidal activity, a conductivity sensor can be
added to the WQ-FDO sensor and data logger system to provide a real-time salinity
compensation. The WQ-FDO software also provides for the connection of a
barometric pressure sensor that will give real-time compensation for pressure
changes as well. This software allows the ability to either connect the optional
conductivity and barometric sensors, or enter values for these parameters manually.