Dissolved

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Dissolved Oxygen (DO)
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It IS elemental oxygen, O2, physically absorbed
in a liquid.
It IS NOT the chemically bound O of H2O.
It IS NOT physically adsorbed or entrained
bubbles.
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What is Dissolved Oxygen?

• Dissolved Oxygen - the amount of O2 gas dissolved
  in a sample of liquid water.
• Units of measure
• ppb
• ppm
• saturation
• mg/liter( ppm )

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How does DO get there?
Chemical, Electrochemical or Biochemical
processes such as Photosynthesis
Exposing the liquid to air or O2 Shaking helps
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Why is Dissolved Oxygen measured?
Dissolved Oxygen is one of the most important
measurements in the determination of the health
of a body of water
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Who makes this measurement?
In ppm Applications, DO must stay above a
minimal value to support life.

Fish Shrimp Farms
Wastewater Treatment Plants
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In ppb Applications, DO needs to be below some
maximum value to inhibit Corrosion or Food
Spoilage.
Semiconductor
Food Beverage
Power Utilities
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POWER PLANT APPLICATIONS--DISSOLVED OXYGEN-- DO
Raw water
Saturated Steam
Make-up Water
Treatment Demineralizer
H-P Turbine
L-P Turbine
I-P Turbine
Chemical Feed
Condenser
Drum
Cooling Water
Hotwell
Condensate Storage Tank
Super
heater
Economizer
Blowdown
Reheater
Boiler
Deaerator
Condensate Polishers
Feed Water
H-P Heaters
L-P Heaters
Chemical Feed
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How Much gets there?
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Air Saturation in water 25C, 1 ATM
8.24 mg O2/L 8.24 ppm
Compare to O2 in air
PV nRT n/V P/RT 1 atm/0.082L-atm/moleK
x 298K 0.0406 moles air x 0.209 moles O2/mole
air x 32,000mg O2/mole O2 274 mg O2/L
oxygen in air/ Oxygen in air-sat. water
274/8.24
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Reasons to use On-line versus Off-line
Measurements

• Off-line are too slow to correct sudden upsets
• Off-line cannot catch momentary fluctuations
• With Off-line, it is easy to contaminate the
  sample
• Have to manually record other influences at time
  of sample collection
• temperature - weather behavior
• time of day - day of week
• Currently, no reference standard available for
  the ppb industry

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Continuous On-line Measurement Membrane
Technology - the membrane separates the internal
electrolyte and electrodes from the sample,
allowing only the gases to penetrate the membrane
for measurement. Two Types Diffusion - based on
the rate of diffusion of oxygen across a
membrane. Example Polarographic or
Galvanic Equilibrium - based on the partial
pressure of oxygen.
sells.
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Electrochemical Measurement of
Dissolved Oxygen DO
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Dissolved O2 Typical Diffusion Cell
Anode 2 Pb 2 H2O 2 PbO 4H 4 e-
Pb Anode
Replaceable Electrolyte
Inert Cathode
Cathode O2 4 H 4 e- 2H2O
Replaceable Membrane
Oxygen Transport
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Dissolved O2 Typical Diffusion Cell,
Fouled
Pb Anode Replaceable Electrolyte Inert Cathode
Replaceable Membrane
Oxygen Transport. Diminished
Fouling
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Dissolved O2 Typical Diffusion Cell,
in Still Water
Diffusion Probe Fouling Dependent Flow
Dependent Requires Membrane Changing Electrolyte
Replenishment Electrode Treatment
Pb Anode Replaceable Electrolyte Inert Cathode
Replaceable Membrane
Stagnant still water
Oxygen Transport
Diminished
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Special Requirements of Diffusion Types Minimum
Flow - since oxygen is continuously consumed at
the cathode, a deficiency of oxygen may result if
a steady flow of oxygen is interrupted.
Therefore, a continuous minimum supply of oxygen
must diffuse through the membrane for an accurate
reading. No Membrane Fouling - Fouling reduces
the surface area for oxygen transport, less
oxygen available for reduction results in lower
readings.
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• Advantages
• Electrolyte can be restored
• Lower initial cost
• Disadvantages
• Anode needs to be cleaned, recharged or replaced
  as oxide build-up occurs (electrolyte must be
  replaced at this time also)
• Membrane is delicate - needs to be replaced
  periodically
• Probe is flow sensitive

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Dissolved Oxygen DO
Equilibrium Sensor
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Desired are..
Reliable Results
Long Life
Low Maintenance
Low Cost of Ownership
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To Achieve this...
we invented.
The Equilibrium DO Sensor
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At the end of a support probe,
is the sensor
consisting of ...
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A Ceramic Mandrel
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encoiled by...
inert, closely-paired bifiler electrodes
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sealed in ...
a tough, permanent, gas-permeable membrane
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and bathed in ...
a permanent internal electrolyte solution.
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When the sensor is turned on,
Oxygen immediately adjacent to the
CATHODE the negative electrode is
consumed according to
O2 4H 4 e- 2 H2O
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and, uniquely for the equilibrium sensor,
-
oxygen is generated
2 H2O O2 4H 4 e-

at the ANODE positive electrode
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in an amount equal to that consumed at
the CATHODE .
The rest of the reaction
balances, too.
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The current supporting these reactions
-
is directly proportional

to the DO concentration.
-

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In Detail...
Lets look at just 3 windings.
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Sample
Sensor
Oxygen, , starts uniformly
distributed
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Sample
Sensor
and then we turn the sensor ON
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Sample
Sensor
At the Cathode,
O2 is consumed
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Sample
Sensor
At the Anode,
O2 is generated
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Sample
Sensor
providing high concentration
at the anode
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Sample
Sensor
and zero concentration
at the cathode
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But there is zero concentration gradient
between the sample and the outer portion of the
membrane
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DO equilibrium exists
between Sensor and
Sample
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The oxygen flux
is all within the
membrane.
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Sample
Sensor
Fouling,
Still Water
do not
disturb equilibrium
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Only when sample DO Changes
does O2 diffuse..
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or...
INTO
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Sample
Sensor
the sensor
OUT OF
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Sample
Sensor
Until equilibrium
is reestablished
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Fouling or still water only slow
reaching equilibrium
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And... since the reactions are balanced,
No chemical changes occur in the sensor and
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a permanent sensor is possible.
-

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Summary
with close-spaced inert electrodes and internal
oxygen generation,
the equilibrium DO sensor is...
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Independent of fouling
Independent of flowand...
Permanent,
NO membrane changing
Requiring
NO electrolyte replacing
NO electrode treating
For.
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Reliable Results
Long Life
Low Maintenance
Low Cost of Ownership
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Honeywell DO Instrumentation

or
DirectLine
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Honeywell DO Instrumentation
Model 7022 ppm/ 7021 ppb Case Aluminum
NEMA 4X/IP65 Display Backlit dot matrix
LCD Operating Condition -20 to 40
degC Accuracy ppm /-0.2 ppm ppb2 ppb or
5of reading Advance feature Auto Ranging of
Input, Auto Calibration Cleaning relays
software, Control function Operating Voltage
16-24 Vdc power looped Output Signal 4-20 mA
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Honeywell DO Instrumentation
Model DL424 ppm/DL425 ppb Case
Plastic Display Backlit dot matrix
LCD Operating Condition -20 to 60
degC Accuracy ppm /-0.2 ppm ppb2 ppb or
5of reading Advance feature Integral
electronics/sensor Operating Voltage 16-24 Vdc
power looped Output Signal 4-20 mA
DirectLine
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Example Application
7020 With Automatic Clean and Calibration
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DirectLine

• Easy to install/set-up
• DirectLine architecture i.e. integral
  sensor/electronics reduce wiring, cable runs and
  panel cut-outs
• Easy-to-use
• Local display and keypad provides easy set-up,
• calibration and operation Calibration can be done
  right at electrode, no running back and forth to
  analyzer (convenient time money savings)
• Easy Maintenance
• Plug-in sensors allow electrode/probe replacement
  in few minutes no wiring required (convenient
  time money savings)

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DirectLine Remote Type
Electronic Module
Pre-Amp Module
Remote Cable Shield Twist Pair 20/100
DO Electrode
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Thank You
Dissolved Oxygen (DO)