In-Situ TROLL 9500 Operators Manual - Manual (Page 86)

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TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
THE POLAROGRAPHIC DISSOLVED OXYGEN SENSOR
THEORY OF OPERATION
The sensor is a Clark-type polarographic sensor consisting of two
metallic electrodes in contact with an electrolyte and separated from
the measurement water by a polymeric membrane. Dissolved oxygen
and other gases diffuse through the membrane into the electrolyte.
An electric potential is applied to the electrodes, which causes an
electrochemical reaction. Oxygen is reduced at the cathode:
O
2
+ 2H
2
O + 4e – œ 4(OH) –
while silver is oxidized at the anode:
4Ag + 4Cl – œ 4AgCl + 4e –
The resulting current is proportional to the oxygen crossing the
membrane. The electric potential is carefully selected so that only the
dissolved oxygen is reduced.
The concentration of dissolved oxygen is usually reported in mil-
ligrams of oxygen per liter of water (mg/L), but the sensor actually
measures the partial pressure of dissolved oxygen. Other gases such
as nitrogen, carbon dioxide, and water vapor are also dissolved in
the water. The partial pressure of the oxygen is the fraction of the
oxygen multiplied by the total pressure of all the gases. This value is
also a function of water temperature and water salinity. The maximum
amount of oxygen that can be dissolved in water at a given atmo-
spheric pressure, water temperature, and salinity (100% D.O.) can be
calculated from first principles. D.O. measurements taken in the field
are then compared to the 100% D.O. value. D.O. measurements of
surface water are typically less than the 100% D.O. value due to the
presence of biological and chemical processes that consume oxygen.
Field measurements are corrected for changes in temperature, air
pressure, and salinity.
During the electrochemical process dissolved oxygen is consumed
while silver chloride (AgCl) is deposited on the anode. In time, both
processes will adversely affect the stability and accuracy of the D.O.
measurements. Depletion of oxygen near the membrane will cause
readings to decrease when measuring D.O. in stagnant water. The
use of a stirrer, or similar mechanism to increase water movement,
alleviates this problem.
SENSOR CONDITIONING
As soon as the software “recognizes” and displays the D.O. sensor
in port 2, powering of the D.O. channel begins. A low-level current
is applied continuously to the D.O. circuitry, resulting in continuous
polarization. This is similar to laboratory instruments for measur-
ing dissolved oxygen, which are always “on.” A certain amount of
conditioning or “warm-up” time is necessary for the sensor to return
accurate readings during calibration and use.
Before calibrating a new D.O. sensor, or a sensor with a new mem-
brane, we recommend that you allow a minimum of two hours for
conditioning. For stable long-term performance and faster stability
during calibration, we recommend 10 hours of conditioning. If the
D.O. sensor is installed when you receive the instrument, it will be
conditioned and ready to calibrate. If you remove the sensor, be sure
to allow for another period of conditioning before you calibrate.
TIP: Testing has shown that 10 hours of conditioning yields
very stable long-term performance.
SENSOR INSTALLATION
The MP TROLL 9500 may be shipped with a polarographic D.O. sen-
sor installed in port 2. When the sensor is shipped in the instrument, it
is pre-conditioned and ready for calibration.
If installation is necessary, unpack, fill, install, and condition a polaro-
graphic D.O. sensor as follows.
FILL THE MEMBRANE MODULE
New sensors are shipped with a dry membrane module loosely at-
tached.
1. Remove the soft protective caps from the
membrane end and the connector end of
the sensor.
2. Remove the membrane module from the
sensor body and fill with electrolyte as follows:
Holding the membrane module open-end up,
position the electrolyte dispenser against the side
of the module without touching the membrane. Fill
slowly.
TIP: To eliminate air bubbles, tap the side of the
module briskly with your fingernail.
3. Insert the sensor into the open end of the membrane
module. To minimize air, some of the electrolyte
SHOULD
POLAROGRAPHIC MEASUREMENT OF DISSOLVED OXYGEN
SECTION 13: DISSOLVED OXYGEN—POLAROGRAPHIC
Membrane
Cathode
Membrane
module
Anode
o-rings
Cap
Cap