Xylem WQ-FDO OPTICAL DISSOLVED OXYGEN TRANSMITTER - Manual (Page 8)

Global Water
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V. Converting Partial Pressure Measurements
The calculation of dissolved oxygen concentration and saturation level is a
complicated topic. Many variables affect the result including water and air
temperature, conductivity or salinity, atmospheric pressure or altitude, and water
vapor partial pressure. Some of these variables can’t always be calculated
accurately and it is common to rely on empirical data rather than calculations.
Two common methods are used to make dissolved measurements. Electronic
methods rely on measuring the oxygen partial pressure in the sample and
mathematically converting partial pressure to concentration using the variables of
salinity, pressure and temperature. Chemical methods are the most accurate and
are generally used to check the calibration of electronic test instruments, the
Winkler method being the most common.
The concentration of oxygen is the amount of oxygen in a given volume of water
and is generally measured in mg/liter or ppm (these two units are equivalent,
1mg/L = 1 ppm). The saturation level is the percentage (%) of oxygen in the water
as measured against the maximum amount (100%) the water can theoretically hold
based on the amount of oxygen in the air above it.
There are different ways to convert oxygen partial pressure to concentration and
percent saturation, a complete discussion will not be made here. Referencing
lookup tables is common; as well as using equations, some of which are derived
from empirical data.
As stated earlier in this manual, a free software tool is available for use with
Global Water’s family of data loggers that allows the WQ-FDO sensor’s outputs to
be monitored, along with optional conductivity and barometric pressure sensors, to
determine the oxygen concentration and saturation levels. If the optional sensors
are not used, provision is made for the hand entry of barometric pressure (or
altitude) and conductivity (or salinity) into the calculation. The partial pressure,
concentration, saturation, temperature, conductivity, salinity, and barometric
pressure can all be displayed in real time, or can be downloaded for use in a
spreadsheet program.