Dissolved Oxygen and

measurement possibilitiesBerno Lüpkes, 15th March 2017

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Content

1. Introduction to Dissolved Oxygen

2. Amperometric measurement principle

3. Optical measurement principle

4. Optical vs. Amperometric Sensor Features

5. Key Applications

3

Oxygen is necessary for life

The reaction of organic material with oxygen delivers energy which is necessary for life.

Our atmosphere consists roughly of 21%

oxygen O2 and 79% nitrogen N2.

Oxygen is easily soluble in water and makes

life possible in rivers, lakes and oceans.

Oxygen is essential in cellular respiration of

all aerobic organisms.

Most of the molecules necessary for life

contain oxygen such as proteins, fats and

sugars.

4

Oxygen properties

At standard temperature and pressure, oxygen is a colorless, odorless gas with the molecular formula O2

Oxygen is a powerful oxidizing agent.

Oxygen dissolves in water.

Dissolved oxygen is a measure of the

amount of oxygen that is dissolved or

carried in a given medium.

5

DO values - Examples

DO value

Arterial Blood:11-13 kPa

Beer:50-200 ppb

Corrosion:0-10 ppb

Aquarium:5-8 ppm

Many processes need control of dissolved oxygen

low high

6

Definition of the partial pressure

= +

Ptotal = p1 + p2 p1 p2

The total pressure of a gas mixture is the sum of the partial pressures of each individual gas (Dalton’s Law)

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Partial pressure and activity

A gas reacts according to its partial pressure.

The partial pressure is a measure of

the thermodynamic activity of a gas.

Gases dissolve, diffuse, and react

according to their partial pressure.

DO sensors measure the partial

pressure of dissolved oxygen.

Example: At an atmospheric pressure of

101,3 kPa at sea level (1 atm)

oxygen exerts POXYGEN = 21,22 kPa

nitrogen exerts PNITROGEN = 79,12 kPa

pO2air

pO2water =

pO2air

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Partial pressure and concentration

Different solutions dissolve varying amounts of oxygen at the same partial pressure of oxygen

The concentration of oxygen (ppb, mg/L) depends on and solubility of

oxygen in the sample:

CO2= pO2

. k Henry’s Law

Conversion of partial pressure to concentration requires accurate

information on oxygen solubility in the experimental media.

Patm = 101.3 kPaT = 25°CpO2

= 20.73 kPa

8 mg/L 2 mg/L 5 mg/L 58 mg/L

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Example: Closed Bioreactor

Air, 25°C100% relative humidityptot = 1 barpO2

= 0.203 bar

pH2O = 32 mbar

Water, 25°CpO2

= 0.203 bar8.0 mg/l or ppm

Air, 25°C100% relative humidity ptot = 2 barpO2

= 0.413 bar

pH2O = 32 mbar

Water, 25°CpO2

= 0.413 bar16.3 mg/l or ppm

The partial pressure of water (water vapor) does not depend on the total pressure !!

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Content

1. Introduction to Dissolved Oxygen

2. Amperometric measurement principle

3. Optical measurement principle

4. Optical vs. Amperometric Sensor Features

5. Key Applications

11

Principle of Amperometric DO Sensor

Oxygen reduction at the cathode causes the electrical current to flow proportionally to pO2 in the sample.

1 O2 diffuses through the gas permeable membrane

2 Suitable polarization voltage is applied

3 O2 is completely reduced at the cathode:O2 + 2H2O + 4e-

4OH-

4 The needed electrons are released from the anode, creating the measuring signal4 Ag 4 Ag+ + 4 e-

O2

1

O2

4

Ag/AgClanode wire

3

Platinum cathode

Gas permeable membrane

2

Glass insulation

Electrolyte

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Sensor Response to Oxygen

Current (I) [nA]

Potential E[mV]

-670 mV

400 8000 200 600 1000

Current (I)[nA]

pO2

[vol%]

10 150 5 20

pO2 = 21%

pO2 = 17%

pO2 = 12%

pO2 = 7%

pO2 = 0%

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Oxygen Diffusion

The measured signal (I) is directly proportional to the oxygen partial pressure of the sample

i = k PO2

Pm A

b

i = electrode current

k = constant

PO2= partial pressure of oxygen

Pm = permeability of membrane

A = area of cathode tip

b = membrane thickness

Ag/AgClanode

Platiniumcathode

Gas permeable membrane

A

Pm pO2

b

Electrolyte

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Content

1. Introduction to Dissolved Oxygen

2. Amperometric measurement principle

3. Optical measurement principle

4. Optical vs. Amperometric Sensor Features

5. Key Applications

15

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed

light or other electromagnetic radiation. It is a type of luminescence.

The emitted light has a longer wave length

(smaller energy) than the absorbed light.

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Optical Measurement Principal

v

v

17

Construction of Hamilton’s VisiFerm Sensor

O2

O2O2

O2 O2O2

O2

O2O2

O2

O2

Isolation LayerLuminophoreCarrier Layer

Photodiodes

Red FilterBlue LED

O2O2

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Making the measurement

Intensity

Time (µs)

0,00

Excitation profile

Emission profile

Phase shift = f(pO2)

O2

O2O2

O2 O2O2

O2

O2 O2

O2

O2

O2O2

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Oxygen correlated to phase shift

Oxygen level in air

Oxygen free

Phase shift in relation to oxygen concentration

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Content

1. Introduction to Dissolved Oxygen

2. Amperometric measurement principle

3. Optical measurement principle

4. Optical vs. Amperometric Sensor Features

5. Key Applications

21

Common pain points for measuring oxygen

Accurate measurement of dissolved oxygen requires time consuming maintenance

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Delicate polarographic measuring system

Membrane diffusion potential changes with age

The process can change the electrolyte strength

The weak electrical signal can be influenced by external electrical devices

Installation conditions can influence signal

The Anode/Cathode can become damaged or corroded by oxidation

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Troubleshooting a polarographic sensor

80% of sensor

cost comes

from

maintenance

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Can you trust the control sensor?

Can you trust the signal from a perfectly maintained sensor?

% S

atu

rati

on

Time

Controlling PolarographicDO Sensor

CO2 FoulingVisiFerm DO SensorActual O2 Saturation

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Benefits of optical oxygen measurement

Key Benefits of optical oxygen sensors

Insensitive to CO2 fouling

Eliminates polarization

Simplifies maintenance with a single

replacement part

Additional Benefits:

No electrolyte to contaminate the

process

Fast response time

Measurement is independent of flow

Stable against pressure spikes

Can be installed inverted

Insensitive to poisoners like H2S and

Ammonia

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Polarographic installation

Transmitter PLC

PolarographicSensor

Weak nA Signal 4-20 mA

Digital

Sensor quality is not known until right before the run

May require polarization of multiple sensors

The measuring loop cannot be pre validated

• The transmitter is installed in the facility

• Polarographic sensors age when not in use

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4-20 mA

Digital

VisiFerm installation

PLC

VisiFerm’s integrated micro transmitter

Direct connection of the sensor to a PLC

In Lab, batch validation

Grab and go directly into the process with a

sensor known to be good

VisiFerm

Weak nA Signal

Transmitter

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The VisiFerm DO Sensor

Integrated Micro Transmitter

No redundant transmitters

Improved signal reliability

Reduce calibration costs

At Line In Lab

Co

st o

f C

alib

rati

on

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Easy Sensor Troubleshooting

Arc Wi Adapter

The blue LED indicates an active Bluetooth connection

Sensor has no errors or warnings

There is at least one sensor warning

Flashing: No sensor connected

Solid: There is at least one sensor error

- Status Lights

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Easy Sensor Troubleshooting

Arc Wi Adapter - ArcAir Application

Bluetooth Monitoring, Maintenance,

and Configuration

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Easy Sensor Troubleshooting

Errors

Warnings

Quick status indicator

Good

Warning text strings

81 – 100% Excellent

61 – 80% Good

46 – 60% Acceptable

36 – 45% Poor

< 35% Replace

Sensor cap quality

Operating indicators

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Content

1. Introduction to Dissolved Oxygen

2. Amperometric measurement principle

3. Optical measurement principle

4. Optical vs. Amperometric Sensor Features

5. Key Applications

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Hamilton Sensors in Biotech/Pharma

Every step of the biopharmaceutical process has specific requirements for

sensors to monitor and control it precisely.

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750 Liter Bioreactor system in production

Arc sensor while transmitting data Arc sensor operation with Arc View Handheld at main fermenter

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Use in Process Development

VisiFerm for sterile bioprocess

Process description

Accurate DO measurement

Absolute sterility

Cleaning: CIP, SIP (manufacturing),

autoclavation (R&D)

Recommended sensor: VisiFerm

Reliable measurement, not affected by flow, stirring or CO2

Stable reading even after frequent sterilization

Maintenance free

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Use in R&D

Arc View Mobile with ArcAir

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Hamilton Sensors in Breweries

Sensors are designed for in-line measurement of pH, dissolved oxygen

(DO) and conductivity at every step of the brewing process

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Hamilton Swiss Army Knife for your Process Control

Arc - Integrated

sensor intelligence

pH

Dissolved Oxygen

Conductivity

Redox

Biomass

Easy connectivity to PLC/SCADA

Buffer &

Standards Armature/Holders

Comprehensive Customer

Support Package

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Thank you!