Emerson Process Management 430 DO - Manual (Page 5)

1
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LARGE ELECTROLYTE RESERVOIR allows up to twelve months and often longer
between refills.
•
OPTIONAL MOUNTING BRACKET allows easy removal of the sensor from an aeration
basin without the use of tools.
•
MODULAR PRESSURE COMPENSATOR AND MEMBRANE RETAINER simplify recharg-
ing and membrane replacement.
•
MEMBRANE RETAINER REPLACEMENT AND RECHARGING require no tools.
MODEL 430 DO SECTION 1.0
DESCRIPTION AND SPECIFICATIONS
SECTION 1.0
GENERAL DESCRIPTION AND SPECIFICATIONS
1.1 FEATURES AND APPLICATIONS
The Rosemount Analytical
® Model 430 Dissolved
Oxygen Sensor, used with the Model 1181 Dissolved
Oxygen Transmitter, measures oxygen levels in a vari-
ety of applications. The most widely used application is
the secondary phase of the waste treatment process
where a critical dissolved oxygen level must be main-
tained for the digestive process to occur. Other appli-
cations include water quality monitoring in natural bod-
ies of water such as rivers and streams, or any location
where adequate oxygen content is important to sustain
aquatic life.
The Model 430 Dissolved Oxygen Sensor is construct-
ed of PVC with an oxygen permeable Teflon 1 mem-
brane .
1.2 OPERATION
As oxygen passes through the Teflon membrane an
oxygen dependent electrochemical reaction takes
place between the gold cathode and silver anode in the
presence of the electrolyte. The resultant current flow
between these elements is proportional to the quantity
of oxygen which has entered the sensor.
Temperature and pressure compensation are both
standard features on the Model 430. Temperature com-
pensation is necessary for two reasons: First, the solu-
bility of oxygen in water decreases with an increase in
temperature; secondly, the permeability of the Teflon
membrane and the oxygen diffusion rate increase as
the temperature increases. For these reasons, it is
important for a dissolved oxygen measurement to be
accompanied by an accurate temperature measure-
ment. Temperature compensation is accomplished by
means of a thermistor located in the sensor housing,
and its associated circuitry in the analyzer.
Pressure compensation is a mechanical rather than
electronic correction. It is important for the tension on
the Teflon membrane to remain constant at all times,
even with sample pressure changes. Fluctuations in
membrane tension affect its diffusion characteristics.
By means of a pressure compensator the pressure on
the sample side of the compensator membrane is
transmitted to the electrolyte reservoir so the pressure
on both sides of the membrane remains equal. In this
manner, sample pressure changes do not affect the dif-
fusion rate of oxygen through the membrane.
The Teflon membrane is held in place by a retainer. A
water-tight seal is made with an O-ring between the
membrane and sensor reservoir. Membrane replace-
ment is rarely required, but in those situations when it
is necessary, the procedure can be performed quickly
and easily.
Since oxygen diffuses through the Teflon membrane at
a faster rate than through the sample, it is necessary to
continuously maintain a fresh sample in front of the
membrane. A stream velocity of approximately 1.5 feet
per second (0.5 m/s) is required to ensure that a fresh
sample is in contact with the membrane at all times.
1 Reg. U.S. Pat. Office for du Pont’s fluorocarbon resins.