Appendix A – DO Measurement
84
Membrane Correction
The membrane correction term is defined as follows :
M = e
A([1/T]-[1/To])
Where : A = Membrane Correction Factor
T = Temperature (in °K)
To = Temperature at calibration (in °K)
The membrane correction factor is specific to each make of probe and
characterises the type and thickness of the membrane material in terms of how its
permeability to Oxygen varies with temperature. From this, it can be seen that the
membrane correction term can contribute a variation in the saturation value of as
much as 3% for each degree of change in temperature (for a typical membrane
correction factor of 2220).
The above equations demonstrate the benefits of having active temperature and
pressure measurement when an accurate reading is required. For systems where
active pressure or temperature measurement is not available, manual
compensation is available.
Oxygen Solubility
The Oxygen solubility is easily defined as : % Saturation X Maximum Theoretical
Solubility of Oxygen in water. The maximum theoretical solubility is heavily
dependant on the temperature, pressure and salinity of the measured liquid.
Tables of data for Oxygen solubility are readily available from a number of sources
such as BS EN 25814, ISO5814 , and see solubility table.
Partial Pressure of Oxygen (pO
2
)
The concentration of a gas dissolved in a solution at equilibrium is proportional to
the partial pressure of the gas in contact with the solution (Henry’s Law). The
partial pressure of the gaseous component of the air in contact with the solution
remains proportional to the total pressure of the air sample.
The partial pressure of Oxygen in air at atmospheric pressure of 1 Bar (1000mBar)
is 210mBar (air is 21% Oxygen), so if a solution of pure water were 100% saturated
with Oxygen at atmospheric pressure the partial pressure of Oxygen in solution
would be 210mBar. e.g. 20% saturation at a pressure of 1 Bar gives a reading of
42mBar, 50% saturation at a pressure of 3 Bar gives a reading of 315mBar.
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