Dissolved Oxygen and measurement possibilities Berno Lüpkes, 15 th March 2017
Content 1. Introduction to Dissolved Oxygen 2. Amperometric measurement principle 3. Optical measurement principle 4. Optical vs. Amperometric Sensor Features 5. Key Applications 2
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 O 2 and 79% nitrogen N 2. 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. 3
Oxygen properties At standard temperature and pressure, oxygen is a colorless, odorless gas with the molecular formula O 2 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. 4
DO values - Examples Many processes need control of dissolved oxygen low high Corrosion: 0-10 ppb Arterial Blood: 11-13 kpa DO value Beer: 50-200 ppb Aquarium: 5-8 ppm 5
Definition of the partial pressure The total pressure of a gas mixture is the sum of the partial pressures of each individual gas (Dalton s Law) P total = p 1 + p 2 p 1 p 2 = + 6
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. p O2 air p O2 water = p O2 air 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 P OXYGEN = 21,22 kpa nitrogen exerts P NITROGEN = 79,12 kpa 7
Partial pressure and concentration Different solutions dissolve varying amounts of oxygen at the same partial pressure of oxygen P atm = 101.3 kpa T = 25 C p O2 = 20.73 kpa 8 mg/l 2 mg/l 5 mg/l 58 mg/l The concentration of oxygen (ppb, mg/l) depends on and solubility of oxygen in the sample: C O2 = p O2. k Henry s Law Conversion of partial pressure to concentration requires accurate information on oxygen solubility in the experimental media. 8
Example: Closed Bioreactor Air, 25 C 100% relative humidity ptot = 1 bar p O2 = 0.203 bar Air, 25 C 100% relative humidity ptot = 2 bar p O2 = 0.413 bar ph 2 O = 32 mbar Water, 25 C p O2 = 0.203 bar 8.0 mg/l or ppm ph 2 O = 32 mbar Water, 25 C p O2 = 0.413 bar 16.3 mg/l or ppm The partial pressure of water (water vapor) does not depend on the total pressure!! 9
Content 1. Introduction to Dissolved Oxygen 2. Amperometric measurement principle 3. Optical measurement principle 4. Optical vs. Amperometric Sensor Features 5. Key Applications 10
Principle of Amperometric DO Sensor Oxygen reduction at the cathode causes the electrical current to flow proportionally to po 2 in the sample. 2 1 O 2 diffuses through the gas permeable membrane 4 Ag/AgCl anode wire 2 Suitable polarization voltage is applied O 2 3 1 O 2 Glass insulation Platinum cathode Electrolyte Gas permeable membrane 3 O 2 is completely reduced at the cathode: O 2 + 2H 2 O + 4e - 4OH - 4 The needed electrons are released from the anode, creating the measuring signal 4 Ag 4 Ag + + 4 e - 11
Sensor Response to Oxygen Current (I) [na] Current (I) [na] -670 mv po2 = 21% po2 = 17% po2 = 12% po2 = 7% po2 = 0% 0 200 400 600 800 1000 Potential E [mv] 0 5 10 15 20 po 2 [vol%] 12
Oxygen Diffusion The measured signal (I) is directly proportional to the oxygen partial pressure of the sample Ag/AgCl anode Platinium cathode i = k P O2 P m A b i = electrode current Electrolyte k = constant b A Gas permeable membrane P O2 = partial pressure of oxygen Pm = permeability of membrane A = area of cathode tip Pm p O2 b = membrane thickness 13
Content 1. Introduction to Dissolved Oxygen 2. Amperometric measurement principle 3. Optical measurement principle 4. Optical vs. Amperometric Sensor Features 5. Key Applications 14
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. 15
Optical Measurement Principal v v 16
Construction of Hamilton s VisiFerm Sensor Photodiodes Blue LED Red Filter Luminophore O 2 O 2 O O 2 2 O 2 O O2 2 O 2 Carrier Layer O 2 O 2 O 2 O 2 O 2 Isolation Layer 17
Making the measurement Intensity Excitation profile Emission profile O 2 O 2 O O 2 2 O 2 O O2 2 O 2 O 2 O 2 O 2 O 2 O 2 0,0 0 Phase shift = f(po 2 ) Time (µs) 18
Oxygen correlated to phase shift Phase shift in relation to oxygen concentration Oxygen free Oxygen level in air 19
Content 1. Introduction to Dissolved Oxygen 2. Amperometric measurement principle 3. Optical measurement principle 4. Optical vs. Amperometric Sensor Features 5. Key Applications 20
Common pain points for measuring oxygen Accurate measurement of dissolved oxygen requires time consuming maintenance 21
Delicate polarographic measuring system Membrane diffusion potential changes with age The Anode/Cathode can become damaged or corroded by oxidation The process can change the electrolyte strength Installation conditions can influence signal The weak electrical signal can be influenced by external electrical devices 22
Troubleshooting a polarographic sensor 80% of sensor cost comes from maintenance 23
% Saturation Can you trust the control sensor? Can you trust the signal from a perfectly maintained sensor? VisiFerm Actual ODO 2 Saturation Sensor CO 2 Fouling Controlling Polarographic DO Sensor Time 24
Benefits of optical oxygen measurement Key Benefits of optical oxygen sensors Insensitive to CO 2 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 H 2 S and Ammonia 25
Polarographic installation Transmitter PLC 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 Polarographic Sensor 26
VisiFerm installation Transmitter PLC Weak na Signal 4-20 ma Digital 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 27
Cost of Calibration The VisiFerm DO Sensor Integrated Micro Transmitter At Line In Lab Improved signal reliability No redundant transmitters Reduce calibration costs 28
Easy Sensor Troubleshooting Arc Wi Adapter - Status Lights 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 29
Easy Sensor Troubleshooting Arc Wi Adapter - ArcAir Application Bluetooth Monitoring, Maintenance, and Configuration 30
Easy Sensor Troubleshooting Quick status indicator Good Warnings Errors Sensor cap quality 81 100% Excellent 61 80% Good 46 60% Acceptable 36 45% Poor < 35% Replace Operating indicators Warning text strings 31
Content 1. Introduction to Dissolved Oxygen 2. Amperometric measurement principle 3. Optical measurement principle 4. Optical vs. Amperometric Sensor Features 5. Key Applications 32
Hamilton Sensors in Biotech/Pharma Every step of the biopharmaceutical process has specific requirements for sensors to monitor and control it precisely. 33
750 Liter Bioreactor system in production Arc sensor while transmitting data Arc sensor operation with Arc View Handheld at main fermenter 34
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 35
Use in R&D Arc View Mobile with ArcAir 36
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 37
Hamilton Swiss Army Knife for your Process Control Biomass ph Dissolved Oxygen Conductivity Redox Easy connectivity to PLC/SCADA Arc - Integrated sensor intelligence Buffer & Standards Armature/Holders Comprehensive Customer Support Package 38
Thank you! 39