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19. Dissolved Oxygen Theory

Dissolved Oxygen (DO) refers to the volume of oxygen that is contained in water.

There are two main sources of DO in water; atmosphere and photosynthesis.

Waves and tumbling water mix air into the water where oxygen readily dissolves

until saturation occurs. Oxygen is also produced by aquatic plants and algae

during photosynthesis.

The amount of DO that can be held by

water depends on 3 factors:

1. TEMPERATURE:

DO in

creases with decreasing temperature

(colder water holds more oxygen)

2. SALINITY:

DO in

creases with decreasing salinity

(freshwater holds more oxygen than saltwater does)

3. ATMOSPHERIC PRESSURE:

DO de

creases with decreasing atmospheric pressure

(amount of DO absorbed in water decreases as altitude increases)

Solubility of oxygen in water contact with water saturated air

at standard atmospheric pressure

0 5 10 15 20 25 30 35 40 45 50

Temperature °C

Solubility mg/L

DO Solubility in Water vs. Temperature

Measurement Units

One measure of DO in water is parts per million (ppm) which is the number of

oxygen mole

cules (O

) per million total molecules in a sample. The “mg/L” unit is

equivalent to ppm (a liter of water weighs 1 million milligrams—one part in a

million is similar to one milligram in a liter). Calculating the % saturation is

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EUTECH INSTRUMENTS 2700 Bedienerhandbuch Seite 43