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18.5 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, very 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, however, a different matter. With the type of sensor
to be used with this instrument, very little changes in diffusion current are
caused by altering the pH of the external environment. Pressure changes
over a moderate range exerted on the membrane also cause no change.
The sensor is sealed by glands for total immersion up to 3 metres.