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CHAPTER 2 Measurement Principles and Parameters
The AADI Oxygen Optode 4330, 4831 and 4835 are based on the ability of selected substances to act as
dynamic fluorescence quenchers.
The fluorescent indicator is a special platinum porphyrin complex embedded in a gas permeable foil that
is exposed to the surrounding water. Characteristic features of these foils and sensors are exceptional
sensitivity, stability and robustness. Hundreds of examples exist of field stability for periods of 1-6 years
(see summary of scientific publications appendix 11). In addition the ability to withstand high
temperatures, to have low and fully reversible pressure effects and minimal wet-dry cycling effects are
benefits of the AADI Optodes.
The 4330 and 4831 Optodes can be fitted with the standard or faster response foils. The "standard foil" is
more robust and recommended in most applications. A black
sensing complex from influences caused by direct incoming sunlight, exciting fluorescent particles in the
water, and biofouling. The "faster response" foils are suitable if shorter response times are required
however these foils do not have any protective optical isolation layer. Normally the foils have the same
long-term stability but the fast foils are slightly noisier. If exposed to direct sunlight they will bleach and
drift towards lower responses. It is always recommended to store sensors and spare foils in the dark and
to soak sensors in water at least 24 hours prior to calibration or deployment. Sensor are delivered with a
reusable black rubber protection cap and it is recommended to add a wet piece of natural sponge to keep
the sensor foil wet at all times during storage. For more details on these and other issue please see
pages 46 & 47 along with FAQ's in
The sensing foil is fixed against a sapphire window by four screws and a plastic securing plate, providing
optical access to the foil by the measuring system from inside a watertight housing.
The foil is excited by modulated blue light, and the Optode measures the phase shift of a returned red
light, ref Appendix 2. By linearizing and temperature compensating, with an incorporated temperature
sensor located next to the sensing foil, the absolute O
The lifetime-based luminescence quenching principle, as used in AADI Oxygen Optodes, offers the
following advantages over electrochemical sensors:
•
Not stirring sensitive (does not consume oxygen; does not require pumps)
Direct Measurement of absolute oxygen concentrations without repeated calibrations
•
•
Better long-term stability (stable for years)
•
Less affected by pressure; Pressure behavior is predictable and fully reversible
o
Repeatable
o
Low Noise
o
Exceptionally Low Drift
The Optode can be logged directly by a PC (via the RS232 protocol) and by most PLC's, DCP's, I/O
devices, data loggers and systems.
2.1 Sensor Integrated Firmware
The main tasks of the sensor's integrated firmware are to control the transmitter, sample the returned
fluorescent signal, extract the phase shift of this signal, and convert it into oxygen concentration and/or
Air Saturation value.
appendix.
concentration is determined.
2
page 16 of 93
TD 269 OPERATING MANUAL
OXYGEN OPTODE 4330, 4831, 4835
June 2017
optical isolation coating protects the
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