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4.6
Dissolved Oxygen
The principle of operation of the ProDSS optical dissolved oxygen sensor is based on the well-documented concept that dissolved
oxygen quenches both the intensity and the lifetime of the luminescence associated with a carefully chosen chemical dye. The DO
sensor operates by shining a blue light of the proper wavelength on this luminescent dye which is immobilized in a matrix and
formed into a disk. The blue light causes the immobilized dye to luminesce and the lifetime of this dye luminescence is measured
via a photodiode in the probe. To increase the accuracy and stability of the technique, the dye is also irradiated with red light during
part of the measurement cycle to act as a reference in the determination of the luminescence lifetime.
When there is no oxygen present, the lifetime of the signal is maximal; as oxygen is introduced to the membrane surface of the
sensor, the lifetime becomes shorter. Thus, the lifetime of the luminescence is inversely proportional to the amount of oxygen pres-
ent and the relationship between the oxygen pressure outside the sensor and the lifetime can be quantified by the Stern-Volmer
equation:
((Tzero/T) – 1) versus O2 pressure
For most lifetime-based optical DO sensors, this Stern-Volmer relationship is not strictly linear (particularly at higher oxygen pres-
sures) and the data must be processed using analysis by polynomial non-linear regression. Fortunately, the non-linearity does not
change significantly with time so that, as long as each sensor is characterized with regard to its response to changing oxygen pres-
sure, the curvature in the relationship does not affect the ability of the sensor to accurately measure oxygen for an extended period
of time.
Sensors, Calibration, and Maintenance
46
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