dissolved oxygen (do) what is ?. it is elemental oxygen, o 2, physically absorbed in a liquid. it is...

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)

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