Dissolved Oxygen Meters
Dissolved oxygen
meters are a instruments that measure the quantity of oxygen dissolved in water.
In normal condition at normal temperature and pressure in an aqueous solution
that is allowed to equilibrate with room air, oxygen constitutes about twenty point
nine percent of the total dissolved gas with nitrogen being about seventy-eight
percent(some carbon dioxide is present
as well). These proportions are similar in air and in solution equibrated with
room air. The atmospheric pressure on the water surface along with temperature
are factors that modify the concentrations of DO and therefore have to be
compensated for a correct reading.
Since
dissolved oxygen determination is based on measurement of a current that is
established between the anode and cathode, the salt content of water is another
factor that can influence readings and must also be compensated.
Applications of Dissolved Oxygen
Dissolved oxygen is an important indicator of the degree of usefulness
of a sample of water for a specific application. Dissolved oxygen can exist in
water in different concentration and is important for the respiration of a wide
variety of animals and aerobic bacteria in the aquatic environment. Other
applications include: water treatment plants, sewage treatment works, effluent
activated sludge process, river monitoring, fish farming, and generally any
other field where water quality is important.
Biotechnological processes, are another area of applications where
dissolved oxygen measurements are essential to maintain quality of the finished
product.
In water
quality applications, such as aquaculture and fish farming, the level of DO
must be kept high, if the DO level falls too low the fish will suffocate. For
waste water treatment, in sewage treatment, bacteria decompose the solids, if
the DO level is too low, the bacteria will die and decomposition stops : the DO
level is high, there is no need to spend energy in the aeration of the water.
For
industrial applications like boilers or cooling towers, low DO levels prevent
corrosion and scale build-up which inhibits heat transfer.
Polarographic and Galvanic Probes
Hanna dissolved oxygen meters utilize one
of the two common types of dissolved oxygen sensing probes: polarographic
sensors and galvanic sensor. Polarographic sonsor technology is based on the
Ross and Clark polarographic measurement method. An oxygen probe is composed of
a platinum electrode and a solid silver electrode. A concentrated potassium
chloride solution is held in place over the surfaces of the electrodes by a
PTFE membrane. An external voltage creates a different in potential between the
cathode and anode(less than zero point five volts).
The external
voltage applied to the platinum electrode cathode, silver electrode anode,
potassium chloride solution and gas-permeable membrane material establishes a
current that is proportional with the concentration of oxygen.
In contrast, a
galvanic probe requires no external voltage. The difference in potential
between the cathode and anode is greater than zero point five volts. Galvanic
DO sensors consist of two electrodes, a
Zinc or lead anode and silver cathode, both of which are immersed in
electrolyte. An oxygen permeable membrane separates the anode and cathode from
the water being measured. Oxygen diffuses across the membrane and interacts
with the probe internals to produce an electrical current.
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