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DissolvedOxygen (DO)
What is
?
It IS elemental oxygen, O2, physically absorbed in a liquid.
It IS NOT the chemically boundO of H2O.
It IS NOT physically adsorbed orentrained bubbles.
Dissolved Oxygen - the amount of O2 gas dissolved in a sample of liquid water.
Units of measure:• ppb
• ppm
• % saturation
• mg/liter( ppm )
What is Dissolved Oxygen?
How does DO get there? Chemical, Electrochemical orBiochemical processes such as Photosynthesis
Exposing the liquid to air or O2Shaking helps
Why is Dissolved Oxygen measured?
“Dissolved Oxygen is one of the mostimportant measurements in the determination of the health of a body of water”
In ppm Applications, DO must stay above a minimal value to support life.
Who makes this measurement?
Fish & Shrimp Farms Wastewater Treatment Plants
In ppb Applications, DO needs to be below some maximum value to inhibit Corrosion or Food Spoilage.
Semiconductor
Power Utilities
Food & Beverage
POWER PLANT APPLICATIONS--
DISSOLVED OXYGEN-- DO
Blowdown
Chemical Feed
Condenser
L-P TurbineH-P Turbine
I-P Turbine
Boiler
H-P Heaters L-P Heaters
Deaerator
DO
8.25
DO
8.25
DO
8.25
DO
8.25
Chemical Feed
Economizer
SaturatedSteam
Cooling WaterDrum
Super
Reheater
heater
Hotwell
Raw water
CondensatePolishers
Make-up Water Treatment
Demineralizer
Condensate Storage Tank
DO
8.25
Feed Water
How Much gets there?
Air Saturation in water [25C, 1 ATM]: 8.24 mg O2/L [8.24 ppm]
Compare to O2 in air: PV = nRTn/V = P/RT = 1 atm/0.082[L-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 = 33.3
• 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
Reasons to use On-line versus Off-line Measurements
sells.
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.
Electrochemical
Measurement of
DissolvedOxygen [DO]
Dissolved O2: Typical Diffusion Cell
ReplaceableElectrolyte
Pb Anode
InertCathode
Replaceable Membrane
Cathode:
O2 + 4 H+ + 4 e- = 2H2O
Anode:
2 Pb + 2 H2O =
2 PbO + 4H+ + 4 e-
OxygenTransport
Diffusion Rate OC [DO] / path length
Dissolved O2: Typical Diffusion Cell,
Fouled
Fouling
Pb Anode
ReplaceableElectrolyte
InertCathode
Replaceable Membrane Oxygen
Transport. DiminishedDiffusion Rate OC [DO] / path length
in Still WaterDissolved O2: Typical Diffusion Cell,
Pb Anode
ReplaceableElectrolyte
InertCathode
Replaceable Membrane
Stagnant [still] water Oxygen Transport
DiminishedDiffusion Rate OC [DO] / path length
Diffusion Probe:
Fouling DependentFlow Dependent
Requires:Membrane ChangingElectrolyte ReplenishmentElectrode Treatment
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.
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
Dissolved
Oxygen [DO]
Equilibrium Sensor
Desired are..
Reliable Results
Long Life
Low Maintenance
Low Cost of Ownership
To Achieve this...
we invented….
The Equilibrium DO Sensor
At the end of a support probe,
is the sensor consisting of ...
A Ceramic Mandrel
encoiled by...
inert, closely-paired bifiler electrodes
sealed in ...
a tough, permanent, gas-permeablemembrane
and bathed in ...
a permanentinternal electrolytesolution.
When the sensor is turned on,
+-+-Oxygen
immediately adjacent to the CATHODE[the negativeelectrode] is consumedaccording to
O2 + 4H+ + 4 e- = 2 H2O
…and, uniquely for the equilibrium sensor,
oxygen is generated
-
-
+
+O2 + 4H+ + 4 e- = 2 H2Oat the ANODE
[positive electrode]
2 H2O = O2 + 4H+ + 4 e-
in an amountequal to thatconsumed at theCATHODE . The rest of the
reaction balances, too.
The current supporting these reactions
O2 + 4H+ + 4 e- = 2 H2O
2 H2O = O2 + 4H+ + 4 e- is directlyproportional
-
-
+
+to the DOconcentration.
In Detail...
Let’s look at just3 windings….
Oxygen, , starts uniformly distributed
SensorSample
and then we turn the sensor
ON
SensorSample
At the Cathode,O2 is consumed
SensorSample
At the Anode,O2 is generated
SensorSample
providing high concentrationat the anode
SensorSample
and zero concentrationat the cathode
SensorSample
But there is zero concentration gradientbetween the sample and the outer portion of the membrane:
DO equilibrium exists betweenSensor and Sample
The oxygen flux is all within the membrane.
Fouling, Still Water do not disturb equilibrium
SensorSample
Caveat
Only when sample DO Changes
does O2 diffuse…..
INTO or...
OUT OF the sensor
SensorSample
Until equilibrium is reestablished
SensorSample
Fouling or still water only slowreaching equilibrium
And... since the reactions are balanced,
O2 + 4H+ + 4 e- = 2 H2O
2 H2O = O2 + 4H+ + 4 e-
No chemical changes occurin the sensor and
a permanentsensor ispossible.
-
-
+
+
Summary:
with close-spaced inert electrodes and internal oxygen generation,
the equilibrium DO sensor is...
Independent of fouling
Independent of flow…and...
Permanent,
Requiring:NO membrane changing
NO electrolyte replacing
NO electrode treating
For….
Reliable Results
Long Life
Low Maintenance
Low Cost of Ownership
7020 Analyzer
DL5000 Equilibrium Probe
DirectLine®
or+
Honeywell DO Instrumentation Honeywell DO Instrumentation
7020 Analyzer
Honeywell DO Instrumentation Honeywell DO Instrumentation
Model :Model : 7022 ppm/ 7021 ppb 7022 ppm/ 7021 ppb
Case :Case : Aluminum NEMA 4X/IP65 Aluminum NEMA 4X/IP65
Display :Display : Backlit dot matrix LCD Backlit dot matrix LCD
Operating Condition :Operating Condition : -20 to +40 degC -20 to +40 degC
Accuracy :Accuracy : ppm= +/-0.2 ppm ; ppb=2 ppb or 5 ppm= +/-0.2 ppm ; ppb=2 ppb or 5%of reading %of reading
Advance feature :Advance feature : Auto Ranging of Input, Auto Auto Ranging of Input, Auto Calibration & Cleaning relays & software, ContCalibration & Cleaning relays & software, Control function rol function
Operating Voltage :Operating Voltage : 16-24 Vdc power looped 16-24 Vdc power looped
Output Signal :Output Signal : 4-20 mA 4-20 mA
DirectLine®
Honeywell DO Instrumentation Honeywell DO Instrumentation
Model :Model : DL424 ppm/DL425 ppb DL424 ppm/DL425 ppb
Case :Case : Plastic Plastic
Display :Display : Backlit dot matrix LCD Backlit dot matrix LCD
Operating Condition :Operating Condition : -20 to +60 degC -20 to +60 degC
Accuracy :Accuracy : ppm= +/-0.2 ppm ; ppb=2 ppb or 5 ppm= +/-0.2 ppm ; ppb=2 ppb or 5%of reading %of reading
Advance feature :Advance feature : Integral electronics/sensor Integral electronics/sensor
Operating Voltage :Operating Voltage : 16-24 Vdc power looped 16-24 Vdc power looped
Output Signal :Output Signal : 4-20 mA 4-20 mA
7020 With Automatic Clean and Calibration 7020 With Automatic Clean and Calibration
Example Application Example Application
DirectLine®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)
DirectLine® DirectLine® Remote TypeRemote Type
Electronic ModuleElectronic Module
Pre-Amp ModulePre-Amp Module
DO ElectrodeDO ElectrodeRemote Cable Shield Remote Cable Shield Twist Pair 20’/100’Twist Pair 20’/100’
DissolvedOxygen (DO)
Thank You