Dissolved oxygen (DO) measurements tells how much oxygen is available in the water for fish and other aquatic organisms to breathe. Healthy waters generally have high levels of DO (some areas, like swamps, naturally have low levels of DO). Just like human beings, aquatic life needs oxygen to survive. A measure of the molecular oxygen dissolved in water is an important determinant of whether the water body is suitable for aerobic (oxygen-requiring) organisms, such as fish and zooplankton. Values greater than 5 or 6 parts per million generally will support diverse forms of aquatic life.
DO sensors us an oxygen-permeable membrane that sets up a current that indicates the level of oxygen present. These sensors often can read DO in the range of saturation (about 8 ppm) down to the part per billion (ppb) range.
Typical applications can easily span such a range. For example, DO in bio-oxidation systems and aquaculture applications are usually maintained in the ppm range, while it is desirable to keep DO in boiler feedwater in the low ppb range.
Several factors can affect how much DO is in the water. These include temperature, the amount and speed of flowing water, the plants and algae that produce oxygen during the day and take it back in at night, pollution in the water, and the composition of the stream bottom. (Gravelly or rocky bottoms stir up the water more than muddy ones do, creating bubbles that put more oxygen into the water.)