Nova-Tech DO 700 - Manual (Page 27)

21
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 is to the
water temperature, the more accurate the calibration.
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 materials 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 the DO level falls too
low, the micro-organisms die and the decomposition ceases. When DO
level is too high, this indicates too much power is used than necessary for
aeration resulting in an inefficient process.
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.