Hach BODTrak, Инструкция по эксплуатации инструмента, Страница: 18 / 63

7
SECTION 2 INSTRUMENT OPERATION
2.1 Principles of Operation
2.1.1 Oxygen Transfer to Sample
A measured water sample is placed in each of the six amber
sample bottles. The bottles are placed on the instrument
and connected by caps and tubing to the instrument’s
pressure sensors.
Bacteria in the sample uses oxygen (O
2
) to oxidize organic matter
within the sample bottles. The air in the bottle above the sample
contains 21% oxygen and replenishes the oxygen used by the
bacteria. During the test period, stir bars continually rotate within
each bottle. Stirring helps transfer oxygen from the air to the
sample and helps simulate natural conditions.
2.1.2 Pressure Sensor Function
The BODTrak is sealed to prevent external atmospheric pressure
changes from affecting the BOD reading. Pressure sensors
monitor air pressure within the sample bottles, and when the air
pressure drops, the pressure change is converted to mg/L BOD.
2.1.3 Removing Carbon Dioxide
Carbon dioxide (CO
2
) is produced when microorganisms oxidize
organic matter within the sample. The CO
2
must be removed
from the system so the measured pressure difference is
proportional only to the amount of O
2
used. Lithium hydroxide
crystals, placed in the seal cup of each sample bottle before
testing, remove the carbon dioxide.
Do not substitute any other compound for lithium hydroxide.
Other materials could absorb moisture, or less CO
2
, throughout
the test.
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