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21.6 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
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. The
EDYSI sensor is fitted with a pressure compensating diaphragm, so
pressure changes will also cause no change. The EDYSI can be
immersed up to 60 metres.