In-Situ TROLL 9500 Operators Manual - Manual (Page 93)

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TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
USAGE RECOMMENDATIONS AND CAUTIONS
The amount of oxygen that can be dissolved in water decreases at
higher temperatures and decreases with increasing altitude (i.e., as
barometric pressure drops) and salinity. In other words, as water be-
comes warmer and saltier, it can hold less and less dissolved oxygen.
During tests that include the D.O. channel, the MP TROLL 9500
automatically measures barometric pressure and temperature for
compensation of D.O. readings.
The barometric pressure value at the time of calibration is stored in
the sensor and will be used to correct D.O. readings for weather-
INDUCED
Be sure to supply a barometric pressure value to correctly
calculate dissolved oxygen measurements if the sensor will
be attached to suspension wire or non-vented cable. This
can be done during calibration or prior to setting up a test.
To assure you are able to take fast D.O. measurements on demand, it
is best to leave the D.O. sensor installed in the MP TROLL. This takes
full advantage of the conditioning and polarization that was accom-
plished during calibration. After replacing the TROLL 9500 batteries,
be sure to power the sensor for an hour or two before use, especially
if the batteries were out for a while.
During the first day, some drift is to be expected. After 24 hours or so,
the D.O. values can be expected to stabilize.
The D.O. sensor, like the other water-quality sensors, has been tested
to 350 psi pressure (246 m, 807 ft). We recommend gradual submer-
sion and retrieval—no faster than 4 ft per second.
STIRRING
Polarographic D.O. measurements drop in very stagnant water due to
depletion of oxygen next to the membrane. A slight perturbation to the
system will cause the D.O. measurements to return to normal.
Stirring is not necessary for a hand-held instrument, an instrument
ATTACHED
Stirring is recommended if the instrument is anchored to a fixed struc-
ture in stagnant conditions—for example, attached to a pier in a calm
lake that has no underwater currents. If the wind is blowing and waves
are slightly moving the cable, then stirring is probably not necessary.
Attaching the Stirrer
Install the stirrer accessory on the MP TROLL
9000 as follows:
1. Remove the nose cone from the MP TROLL
9500 and set it aside.
2. Screw the top of the stirrer (propeller end) to
the stainless steel restrictor in place of the
nose cone.
Starting the Stirrer
The stirrer is powered by two alkaline D-
cells (installed). To start the motor, tighten
the end cap.
The magnetic stir bar in the propeller
compartment will start to spin. The stir bar
is protected by a guard plate that may be
removed for cleaning if necessary.
TIP: If the stir bar does not start spinning, try giving it a
gentle nudge by sliding a narrow tool such as a screwdriver
or key between the protective bars of the guard plate.
To turn the stirrer off, back off the end cap until the stir bar stops spin-
ning.
Stirrer
Propeller
compartment
Battery
compartment
Motor
compartment
End cap
j
off on
When do I need to stir and when can I skip it?
Use a stirrer for best results when measuring D.O. in still or
stagnant water.
If the water (or the person holding the instrument) is mov-
ing at all, you can probably safely skip the stirring. Any water
movement, as for example when the instrument is hand-held,
eliminates the need for the stirrer.
SECTION 13: DISSOLVED OXYGEN—POLAROGRAPHIC
Body
Restrictor
Stirrer installed
in place of nose
cone
A:
Q: