YSI Pro20 - Manual (Page 29)

25
EXITING DATA MODE
While in Data mode, press enter to access the bottom functions. Next, highlight
the ESC-Exit box and press enter to return to the run screen.
P R I N C I P L E S O F O P E R A T I O N
The polarographic sensor consists of a silver body as the anode and a circular gold
cathode embedded in the end. The galvanic sensor consists of a zinc anode and
silver cathode. The polarographic sensor requires an applied voltage for operation
while the galvanic sensor electrode potentials are dissimilar enough to reduce
oxygen with applied voltage.
Both sensors have a thin semi-permeable membrane, stretched over the sensor,
which isolates the electrodes from the environment, while allowing gases to enter.
In operation, this end of the sensor is filled with a solution of electrolyte
containing a small amount of surfactant to improve wetting action.
When a polarizing voltage is applied to the polarographic sensor electrodes,
oxygen that has passed through the membrane reacts at the cathode causing a
current to flow. This same reaction takes place with the galvanic sensor without
the applied voltage.
For both polarographic and galvanic DO sensors, oxygen diffuses through the
membrane at a rate proportional to the oxygen pressure difference across it. Since
oxygen is rapidly consumed at the cathode, it can be assumed that the oxygen
pressure inside the membrane is zero. Hence, the amount of oxygen diffusing
through the membrane is proportional to the absolute pressure of oxygen outside
the membrane. If the oxygen pressure increases, more oxygen diffuses through the
membrane and more current flows through the sensor. A lower pressure results in
less current.