PASCO PS-2169 Water Quality MultiMeasure Sensor - Manual (Page 6)

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W a te r Q u a l i t y M u l ti M e a s u r e S e n s o r D i s s o l v e d O x y g e n
6
Dissolved Oxygen
The DO
2
component of the multi-sensor is designed for use in aqueous media at tem-
peratures ranging from 0 °C to 50 °C. For greatest accuracy, the following require-
ments should be met:
• Although the unit is temperature compensated, it should be calibrated at approxi-
mately the same temperature as the test solution.
• The sensor must equilibrate for a short period when the temperature of the test
solution changes.
• The test solution should constantly flow past the membrane of the probe. This
can be accomplished by gently swirling the electrode during measurement.
The DO
2
probe consists of a platinum (Pt) cathode and a silver (Ag) anode sur-
rounded by potassium chloride (KCl
(aq)
) electrolyte filling solution with a silicon
membrane separating the filling solution from the test solution. When the probe is
placed in an aqueous medium that contains dissolved oxygen, O
2
molecules diffuse
across the membrane and react with water molecules (H
2
O) in the presence of elec-
trons from the cathode to form hydroxide ions (OH
-
). Hydroxide ions diffuse to the
anode and react with silver atoms, forming silver oxide (Ag
2
O), H
2
O, and free elec-
trons. The net movement of negative charge (electrons) from the cathode to the anode
is measured by the sensor as an electric current. Since the rate of O
2
diffusion across
the membrane is proportional DO
2
concentration, the rate of these reactions and the
resulting electric current is also proportional to DO
2
concentration. The rate at which
the probe processes oxygen also depends on temperature. To correct for temperature
variation, the probe contains a temperature-sensing thermistor, which adjusts the gain
of an amplifier in the multi-sensor.
Adding Filling Solution
Prior to its initial use, and after long-term storage or cleaning, prepare the probe by
filling it with electrolyte solution as follows:
1. Remove the storage bottle from the end of the probe.
2. Unscrew and remove the cartridge housing at the end of the probe, as illustrated
(right).
3. Pull about 10 mL of new filling solution into the syringe (included with the
probe). Be careful not to introduce air bubbles.
4. Place the tip of the syringe in the cartridge housing, very close to—but not touch-
ing—the membrane. Slowly fill the housing to about 5 mm from the top. Agitate
the housing while filling to avoid introducing air bubbles.
5. Replace the housing over the electrode and screw it into place. Excess filling
solution will spill from the housing as you do this.
6. Rinse and dry the outside of the probe.
membrane platinum
cathode
cartridge
housing
silver
anode
filling
solution
cathode
insulator
Detail of DO
2
Probe Tip
Unscrew
cartridge
housing
Replace
cartridge
housing
Fill cartridge
housing
fill to here