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 Clark
cell (galvanic and polargraphic) based dissolved oxygen sensors
have been the predominant methods for measuring dissolved oxygen
in wastewater treatment facilities. Constant cleaning of the
sensors, the need for membrane and electrolyte replacement,
probe fouling, and re-calibrating of the instrument can be so
demanding that monitoring, and more importantly control, can
be a frustrating, time consuming exercise.
With the introduction of fluorescent based oxygen sensors designed
specifically for the wastewater industry an alternate method
of making this important measurement is now available. The biggest
advantages of fluorescence based sensors are the inherent reliability
and low maintenance requirements of this technology. Low maintenance
and no replaceable membranes or electrolyte are the key features
of this type of sensor. This in turn reduces hours of lost time
for maintenance and eliminates the cost of replacement parts.
Another very important feature is that fluorescent-based sensors
do not consume oxygen and require no flow across them to work.
They also perform very well in harsh environments that normally
destroy other conventional sensors.
PRINCIPLE OF OPERATION:
 The
emitter in the probe sends light, at ~475 nm, to the backside
of the sensing element.
The wetted side of the sensing element consists of a thin layer
of a hydrophobic sol-gel material. A ruthenium complex is trapped
in the sol-gel matrix, effectively immobilized and protected
from water.
The light from the LED excites the ruthenium complex immobilized
in the sensing element. The excited ruthenium complex fluoresces,
emitting energy at ~609 nm.
If the excited ruthenium complex encounters an oxygen molecule,
the excess energy is transferred to the oxygen molecule in a
non-radiative transfer, decreasing or quenching the fluorescence
signal. Collision of an oxygen molecule with a fluorophore in
its excited state leads to a non-radiative transfer of energy.
The degree of fluorescence quenching relates to the frequency
of collisions, and therefore to the concentration of the oxygen-containing
media. The degree of quenching correlates to the level of oxygen
concentration in contact with the sensing element.
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