User Guide

Instruction Manual DO 100
50
12.1.4 Air Calibration
Understanding the principle of air calibration is easy, once you know that it is partial pressure
that the probe is responding to. When the probe is in air, it is measuring the partial pressure
of oxygen in air. If water is air saturated, then the partial pressure of oxygen in the water will
be the same as it is in air. Therefore, all you need to know is the temperature of the air in
which the probe is placed. By consulting solubility tables for oxygen at the particular
barometric pressure and salinity of the water being measured, the corresponding
concentration (mg/L or ppm) can be found for air saturated water at the air calibration
temperature, and the meter can be set accordingly. Because most meters are temperature
compensated, they will still give correct readings in mg/L even though the actual water
temperature may be different to the air calibration temperature.
NOTE: The closer the air calibration temperature to the water temperature, the
more accurate is the calibration.
12.1.5 Applications
Oxygen is essential for fish, invertebrate, plant, and aerobic bacteria respiration. DO levels
below 3 ppm are stressful to most aquatic organisms. Levels below 2 or 1 ppm will not
support fish. Fish growth and activity usually require 5 to 6 ppm of DO, an important
consideration for Aqua-culture industry.
Low DO indicates a demand on the oxygen of the system. Natural organic material such as
leaves accumulate in the stream and create an oxygen demand as it is decomposed. Organic
material from human activities also create an oxygen demand in the system. Micro-organisms
consume oxygen as they decompose sewage, urban and agricultural run-off, and discharge
from food-processing plants, meat-packing plants and diaries. There is an optimum DO level
for this process and if DO level falls too low, the micro-organisms die and the decomposition
ceases. If DO level is too high, more power is used than necessary for aeration and the
process becomes costly.
In boiler water application, presence of oxygen in the water will increase corrosion and helps
build up boiler scale that inhibits heat transfer. In such instance it is critical to keep DO
concentration to a minimum.
Some pollutants such as acid mine drainage produce direct chemical demands on oxygen in
the water. DO is consumed in the oxidation-reduction reactions of introduced chemical
compounds such as nitrate (NO
3
1-
) and ammonia (NH
4
1+
), sulfate (SO
4
2-
), and sulfite (SO
3
2-
)
and ferrous (Fe
2
+
) and ferric (Fe
3
+
) ions.
These are important consideration for water and wastewater treatment industry.