In-Situ TROLL 9500 Operators Manual - Manual (Page 96)

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TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
THE RDO OPTICAL DISSOLVED OXYGEN SENSOR
The In-Situ RDO ® optical dissolved oxygen sensor measures dis-
solved oxygen using the principle of “dynamic luminescence quench-
INGv
The sensor optics include a lens, blue LED and filter, red LED and
filter, and a photodetector. When the blue LED emits light, the sensing
foil emits red photons; the presence of oxygen in the foil causes a
reduction in red light detected by the photodiode. The phase differ-
ence between the blue excitation light and the returned red light is
measured, and the result is used to compute dissolved oxygen.
This method measures the “phase” (or delay) of the returned signal,
and is thus based on the “lifetime” rather than the “intensity” of the
luminescence.
COMPARISON TO POLAROGRAPHIC D.O. SENSOR
The RDO optical dissolved oxygen sensor offers several advantages
over the more traditional electrochemical cell. Its solid-state design
does not use membranes, filling solution, or other consumables. The
SENSOR
OPTICAL MEASUREMENT OF DISSOLVED OXYGEN
The RDO Sensor and Salinity
Unlike an electrochemical cell, the RDO sensor does not respond to
changes in salinity. Since the absolute solubility of oxygen is lower
in saline water, it is advantageous to compensate DO concentration
readings (μg/L, mg/L, μmol/L) to ensure that the sensor accurately
reports the concentration of dissolved oxygen in the presence of sig-
nificant salinity. This can be accomplished by storing a salinity value in
the sensor before taking measurements. The compensation algorithm
is applied internally before concentration is reported. The degree of
compensation is minimal at very low salinities, and several percent of
reading at oceanic salinity levels.
The software prompts for a salinity value when you add a test. The
value can be changed at any time by selecting the RDO sensor in the
Navigation tree and clicking Edit, then “Edit RDO Salinity Value.”
SENSOR INSTALLATION
You will need—
s
s
Alternatively use Saft LSH-20 3.6V lithium D cells. Use of
any other lithium battery will void the warranty of the RDO
sensor and the TROLL 9500.
s
new instrument, or is available with the download of new software
at www.in-situ.com
s
s
shipped on a CD with a new TROLL 9500 instrument, or is avail-
able at www.in-situ.com.
s
s ® handheld PC
UNPACK THE RDO CABLE CONNECT SENSOR
The RDO cable connect sensor arrives installed in a 3-part adapter/
restrictor/nose cone assembly. Remove the black restrictor and nose
SECTION 13: DISSOLVED OXYGEN—OPTICAL