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12.1.6 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
exponential 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 ED1 Dissolved Oxygen sensor is
sealed by glands for total immersion to a depth of 3 metres.
