Xylem WQ-FDO OPTICAL DISSOLVED OXYGEN TRANSMITTER User Manual
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Oxygen Concentration:
First, calculate the oxygen concentration in mg/L (ppm) from the sensor’s partial
pressure and temperature readings assuming zero salinity (Conductivity < 1000uS).
C = PPO
2
* [(6.906334E-2) – (1.797779E-3 * t)
+ (3.108257E-5 * t^2) - (2.199777E-7 * t^3)]
Where:
C = Concentration of dissolved oxygen in mg/L
PPO
2
= Measured oxygen partial pressure in mbar
t = Measured water temperature in ºC
Next, if the conductivity of the sample is more than 1000uS, the calculated oxygen
concentration should be corrected for salinity. Based on temperature and
conductivity (or PSS, Practical Salinity Scale), find the correction value from
Tables 7.1 to 7.4. Multiply the calculated oxygen concentration by the correction
value to obtain the adjusted concentration level.
C = Cc * SCV
Where:
C = Oxygen concentration level in mg/L (or ppm) adjusted for salinity
Cc = Previously calculated concentration in mg/L based on zero salinity
SCV = Salinity correction value from Tables 7.1 to 7.4
Oxygen Solubility and Saturation:
To determine the percentage of saturation of oxygen in the water, the solubility of
oxygen must be known. Solubility is the concentration required for the water to be
100% saturated; the point where equilibrium is reached between the partial
pressure of oxygen in the water and in the air, as stated by Henry’s Law. This
saturation level is affected by temperature, atmospheric pressure, salinity, and
water vapor pressure.
One common method of getting the oxygen solubility is to find it in a lookup table,
based on the temperature and atmospheric pressure. Tables 6.1 through 6.4 list the
solubility over a wide range of conditions. These tables were developed using