General Principle Of Operation - Hach ORBISPHERE 3100 User Manual

Specifications

2.3 General principle of operation

Optical sensing of oxygen originates from the work of Kautsky in 1939 where he demonstrated
that oxygen can dynamically quench the fluorescence of an indicator (decrease the quantum
yield). This principle has been reported in various fields of application such as monitoring
aquatic biology in waste water, tests for blood gas analysis and cell culture monitoring. The
method is now recognized by ASTM (American Society for Testing and Materials) for the
measurement of oxygen in water. Compared to classical oxygen detection using
electrochemical sensors, luminescent technology offers several advantages such as no oxygen
consumption, independence from sample flow velocity, no electrolyte and low maintenance.
Optical sensing of oxygen is based on the measurement of the red fluorescence of a
dye/indicator illuminated with a blue light as shown below.
Figure 2 Principle of optical oxygen detection using fluorescent dye
The dye fluorescence is quenched by the presence of oxygen. The oxygen concentration can
be calculated by measuring the decay time of the fluorescence intensity. The higher the oxygen
concentration is, the shorter the decay time will be. By modulating the excitation, the decay time
is transformed into a phase-shift of the modulated fluorescence signal, which is independent of
fluorescent intensity and thus of potential aging.
The oxygen partial pressure (pO ) is then linked to the corresponding phase-shift measurement
2
( to build the sensor calibration curve. This curve is described by the Stern-Volmer equation
-1
where K is the indicator quenching constant (in mbar ) representing the quenching efficiency
sv
is a constant and 
of the oxygen and thus the sensor sensitivity, f is the phase-shift at zero
0 0
oxygen representing the unquenched fluorescence decay time of the dye.
The calibration curve thus relies on two parameters: the phase-shift at zero oxygen and the
luminescent spot sensitivity, K . The dissolved oxygen concentration is then calculated with
sv
Henry's law using the water solubility curve as a function of the temperature.
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