55
19.1.6 Equilibrium Conditions
Whilst Saline Water has a lower ppM than does Fresh Water, it does not mean it necessarily has
less biologically available oxygen. Both have 100% Saturation (presuming no Chemical Oxygen
Demand (C.O.D.), Biological Oxygen Demand (B.O.D.), etc.) because both are in partial pressure
equilibrium with air. Any usage of oxygen is immediately replenished by the dissolving of more
from air to meet partial pressure equilibrium requirements. This is so for both saline and fresh
water. The reporting of oxygen at a lower level (in ppM units) in the Salt Water is therefore QUITE
MISLEADING !
In closed systems, such as tanks, pipes and deep waters, equilibrium is not so readily available and
the Salinity Effect gains the importance in the reporting of Dissolved Oxygen. It is suggested,
unless such closed (or deep, low diffusion) systems are encountered, that Oxygen should be
reported in % Saturation or ppM of equivalent Fresh Water.
19.1.7 Velocity Past The Membrane
Workers have shown that the relationship between the diffusion current (oxygen current) and the
external velocity of the liquid is expotential. Some workers using thicker membranes have shown
even less dependence of the diffusion current on liquid velocity. Because of the expotential nature
of the relationship, considerable changes in velocity have to be made before noticing any change in
the diffusing current once the flow is sufficiently high. Tests with this sensor have shown that flow
rates above 0.2 litres/minute past the membrane give results indistinguishable from those with
appreciably higher flow rates (5 litres/minute). Fluctuations in readings due to air bubbles passing
through the membrane are a different matter, however.
With the type of sensor to be used with this instrument, very little change in diffusion current is
caused by altering the pH of the external environment. Pressure changes over a moderate range
exerted on the membrane also cause no change. The EDYSI has a pressure compensation
diaphragm to allow submersion to 60 metres.
