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13. Appendix 1: Meter Theory
Dissolved oxygen levels in natural and waste waters depend on the physical,
chemical, and biochemical activities in the water body.
This meter uses a galvanic probe. It consists of a cell that contains electrolyte and
that is enclosed by a selective membrane, and two metallic electrodes. The mem-
brane is practically impermeable to water and ionic dissolved matter, but is perme-
able to oxygen and a few other gases.
The cathode consumes the oxygen passed through the membrane, and produces an
electric current in the probe. This current is proportional to the partial pressure of
oxygen in the sample.
Since the cathode consumes the oxygen in the sample, it is essential that the fluid
must flow past the sensor to maintain accurate readings.
The solubility of oxygen in water varies with barometric pressure and temperature,
and decreases as salinity increases. For the most accurate DO readings, you need
to compensate for these factors. This meter automatically compensates for tempera-
ture readings. It also allows you to enter a salinity correction factor and the baromet-
ric pressure to correct for this variability.