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18.3 Notes On Units Of Dissolved Oxygen
The terms "Oxygen Concentration" and "Oxygen Partial Pressure"
frequently give rise to some confusion.
Oxygen Concentration is the absolute quantity of oxygen present per
unit mass of the liquid.
Oxygen Partial Pressure is the oxygen fraction of the total pressure of
all of the gases present.
For any one liquid system, Oxygen Concentration and Oxygen Partial
Pressure are proportional. However, if the solubility of oxygen in the liquid
should change owing to increased quantities of solutes, etc., then the
ratio of the Concentration to the Partial Pressure must change. Thus, if
one saturates distilled water and a 25% solution of Sodium Chloride with
air at atmospheric pressure (25
o
C) both solutions will have almost exactly
the same Oxygen Partial Pressure, namely 15.5 cm of mercury.
However, the dissolved Oxygen Concentration parts per million
(milligrams per litre) will be 8.2 in the distilled water and 2.01 in the salt
solution. This is a rather extreme example, as ocean water is only 3.6%
saline. It does however stress the importance of correct interpretation of
the salinity, etc.
The Clark Sensor measures the partial pressure of oxygen diffusing
through a membrane. The current is a linear measure of this partial
pressure, assuming liquid flow conditions are met.
With air, at sea level, the 20.9% oxygen exerts about 15.5 cm (mercury
standard) pressure. Water in equilibrium with air and with no C.O.D. or
B.O.D., etc., is saturated and has this dissolved oxygen partial pressure.
If we define 100% Saturation in Partial Pressure terms, then 15.5 cm. Hg
= 100% Saturation. This is a practical unit to use. The sensor linear
readout is then a linear function of % Saturation. Organic cell walls
behave like the sensor and pressure units are valuable.
% Saturation is the best unit for industrial control and not ppM, contrary to
popular beliefs. The partial pressure (and consequently the pressure
defined % Saturation) varies only slightly with temperature. (Note that the
permeability of the membrane has a temperature coefficient, but the
electronics has scaled this out by the operation of the Automatic
Membrane Temperature Compensator Thermistor incorporated in the
D.O. sensor).