YSI The Dissolved Oxygen Handbook
TheDissolved OxygenHandbooka practical guide to dissolved oxygen measurementsYSI.com/weknowDO
ContentsIntroduction. 1Dissolved Oxygen Sensors. 3Optical Sensors. 5YSI Optical Dissolved Oxygen Instruments. 6Optical Sensing Element. 7How an Optical Sensor Measures Dissolved Oxygen. 9Electrochemical Sensors. 10YSI Electrochemical Instruments. 10Electrochemical Membranes. 12How an Electrochemical Sensor Measures Dissolved Oxygen. 15Advancements in Steady-state Electrochemical Sensors. 20Comparing Steady-state Polarographic and Galvanic Sensors. 21Comparing Optical and Electrochemical Sensing Technologies. 25Measurement Accuracy. 25Approved Methodology. 28Response Time. 29Flow Dependence. 32Warm-Up Time. 34Calibration Frequency. 34Measurement Interferences. 35Maintenance Requirements. 35Power Consumption. 35Summary . 35Measuring Dissolved Oxygen with Either Sensor Type. 37Variables that Affect Dissolved Oxygen Measurements. 38Temperature. 39
Salinity. 40Electrode Maintenance. 67Correcting for Salinity. 42Probe Care and Maintenance for Optical Sensors. 69Barometric Pressure. 43Storage. 70Using Barometric Pressure for DO Calibration. 44Final Words. 70Local DO % Measurements. 46Calibration. 46Appendix A - Oxygen Solubility Table. 72Calibration Frequency. 46Appendix B – Calibration Values for Various Atmospheric Pressures andAltitudes. 74Calibration Methods . 47References. 76Winkler Titration Calibration. 48Air-saturated Water Calibration. 49Water-saturated Air Calibration . 49Two Point Calibration. 54Calibration Musts . 55Calibration Musts for Electrochemical Sensors. 55Calibration Musts for Optical Sensors . 56Errors During Calibration. 57GLP (Good Laboratory Practices) File. 58Taking Measurements. 59BOD Measurements. 60Measurement Precautions and Interferences . 61Biofouling. 61Coating Materials. 61Probe Attacking Liquids. 62Interfering Gases . 63Membrane and Sensing Element Integrity. 63Probe Care and Maintenance. 64Probe Care and Maintenance for Electrochemical Sensors. 64Changing a Membrane. 64
I n t r o duc t i o nThi s pag e l e f t i n t e n t i o n ally bla n kYSI has a long history in developing and manufacturing sensors thatmeasure dissolved oxygen in aqueous solutions and has had many firstsover the years including the invention and commercialization of the firstportable dissolved oxygen instrument in 1963. This instrument utilized amembrane-covered Clark Polarographic sensor, commonly referred to as aClark electrode, which was developed in 1956 by Dr. Leland Clark (figure1), a researcher at Antioch College who was working in collaboration withYSI scientists. Before the introduction of the Clark electrode, methods formeasuring dissolved oxygen were laborious, time-consuming and highlysusceptible to interference. Today the world continues to benefit from Dr.Clark’s invention as the Clark electrode is still used by many manufacturersand in several YSI instruments. In addition to the variety of Clark electrodesoffered, YSI also manufactures optical based dissolved oxygen sensorsfor laboratory, spot sampling and long term monitoring applications. Seefigure 2 for a brief overview of other YSI milestones in dissolved oxygenmeasurement technologies over its 60 year history.This booklet describes in detail the different types of dissolved oxygen sensingtechnologies available. It also covers, in general terms, recommendedcalibration methods, regular maintenance procedures that can be performedby the user and how to take a measurement in order to obtain accuratedata. For instrument specific instructions and recommendations, pleaserefer to the instrument’s instruction manual.YSI offers seminars on the topic of dissolved oxygen measurementtechnologies which may apply to continuing education units depending onthe certifying agency. If you would like to schedule a seminar for your groupor organization, please contact YSI at environmental@ysi.com, 1-800-8974151 or 1 937-767-7241.1
2006 – YSI releases the ROX optical dissolved oxygen sensor. Thesensor has a dedicated wiper for long term monitoring on multi-parametersondes.2007 – YSI releases a galvanic electrochemical sensor for use on the ProSeries handheld product family.2008 – YSI releases the ProODO optical dissolved oxygen instrument forspot sampling and laboratory applications.Figure 2. YSI’s Dissolved Oxygen Time line.D i s s o lv e d O x yg e n S e n s o r sFigure 1. Dr. Leland Clark, inventor of the Clark polarographic electrode.Notable Events in YSI’s History of Developing SensingTechnologies for Measuring Dissolved Oxygen1956 – Dr. Leland Clark invents the membrane covered Polarographicelectrode while working with YSI Scientists.1965 – YSI develops the first biological oxygen monitor. Considered abreakthrough for modern medicine and surgery, this instrument enabledphysicians to perform open-heart surgery for the first time because immediateblood oxygen measurements could be taken real-time in the operating roomrather than having a sample drawn and taken to a lab for analysis.There are two primary types of dissolved oxygen sensing technologiesavailable: the optical based sensing method which is commonly referredto as luminescent and the Clark electrochemical or membrane-coveredelectrode. Within these two types of technologies, there are slight variationsavailable. For example, there are two types of optical sensors. Both typesof optical sensors measure luminescence as it is affected by the presenceof oxygen; however, one sensor measures the lifetime of the luminescencewhile the other sensor measures the intensity of the luminescence.The two types of Clark electrochemical sensors available are Polarographicand Galvanic. Additionally, YSI manufacturers two types of Polarographicsensors: Steady-state and the patented Rapid Pulse sensor. Refer to figure 3for a diagram of the various sensor types.1993 – YSI patents first long-term, in-situ, stirring independent oxygen sensorTM(Rapid Pulse DO) and packages it with multiparameter instruments.1993 – YSI patents first stirring independent micro-electrode oxygen sensor(Micro-Electrode Array or MEA) for spot sampling applications.2002 – YSI releases polyethylene membranes for use on polarographicdissolved oxygen sensors. This advancement in membrane materiallowered the stirring dependence and quickened the sensor’s response timeover traditional Teflon membranes.23
Optical SensorsDissolved Oxygen SensorsElectrochemical SensorsOptical SensorsIntensity-basedOptical SensorsLifetime-basedOptical SensorsPolarographicSensorsGalvanicSensorsLifetime and intensity optical measurement methods detect dissolved oxygenbased on the well documented principle that dissolved oxygen quenchesboth the lifetime and intensity of the luminescence associated with carefullychosen chemical dyes. When there is no oxygen present, the lifetime andintensity of the signal are at their maximum. As oxygen is introducedto the sensing element, both the lifetime and intensity of the luminescencebecome shorter. Therefore, the lifetime and intensity of the luminescenceare inversely proportional to the amount of oxygen present. The relationshipbetween the oxygen pressure outside the sensor and the lifetime or intensityof the luminescence in the dye layer of the sensing element can be generallyquantified by the Stern-Volmer equation (figure 4). However, the SternVolmer equation implies an inversely linear relationship which is not strictlytrue especially at higher oxygen concentrations; therefore, YSI employs theuse of a 3rd order polynomial to correct for this non-linearity and to obtainthe desired range of dissolved oxygen readings.The Stern-Volmer RelationshipROX - availableon most 6-seriessondesIo/I 1 kqt0 * O2Available onPro20 andProPlusProODOWhere:Io Intensity or lifetime of luminescence without the quenching molecule(O2).I Intensity or lifetime of luminescence with the quenching molecule (O2).Rapid Pulse available on some6-series sondesSteady-statekq Is the quencher rate coefficient.t0 Is the luminescence lifetime of the chemical (the dye) to be quenched.O2 The concentration of oxygen.Available on severalinstruments including:ProPlus, Pro20, 550A,DO200 and 5100Figure 3. Diagram of Dissolved Oxygen Sensors.4Figure 4. Stern-Volmer equation.Given that the sensing elements of the two optical sensor types are identical,the primary advantage of the lifetime method over the intensity method isthat a lifetime sensor will be more stable in the long term. This is because the5
degradation of the dye in the sensing element has less effect on the lifetimebased measurement than the intensity based measurement. Therefore, theintensity method will require more frequent calibrations - particularly at zerooxygen.YSI Optical Dissolved Oxygen InstrumentsYSI offers two lifetime optical sensors: the ProODO (figure 5) samplinginstrument and the 6150 ROX (figure 6) sensor which can be used onmost 6-series sondes that have an optical port. In addition, there will be anoptical BOD-style sensor available for use on the ProODO in early 2010.ROXSensorFigure 6. The ROX sensor’s dedicated wiper and anti-fouling accessories help extenddeployment times while protecting data integrity making it ideal for unattended,remote and real time monitoring applications.Optical Sensing ElementYSI’s two optical dissolved oxygen sensors utilize sensing elements that aresimilar in function but slightly different in design. The ProODO’s sensingelement is referred to as a Sensor Cap due to its screw on cap design (figure7). The ROX’s sensing element is referred to as a ROX Membrane (figure 8)and is held in place by 3 screws.Sensor CapFigure 5. The ProODO is a compact, handheld instrument designed to withstandthe harshest field conditions yet is accurate enough for use in a laboratory.Figure 7. ProODO Sensor Cap.67
Oxygen is constantly diffusingthrough the paint layer, affecting theluminescence of the sensing layer.Figure 8. ROX membrane.Each sensing element has two layers. The outer layer is a paint that actsas an oxygen permeable diffusion layer which allows oxygen moleculesto pass through while protecting the dye layer. The sensing layer is animmobilized polystyrene dye layer that luminesces when excited with lightof a proper wavelength (figure 9). The degradation of this dye layer overtime is what causes the sensor cap to need replacement and all lifetimebased optical sensors require that this dye layer be replaced periodically.YSI sensing elements are warranted for 1 year but may last much longer.The working life of a sensing element may be extended by keeping it cleanand properly stored between uses. See the Probe Care and Maintenancesection of this booklet for more information on cleaning and storage.The sensing elements are factory calibrated at YSI and a calibrationcode specific to each individual sensing element is determined during themanufacturing process. The calibration code consists of coefficients thatare preloaded into the sensor at the factory for increased measurementaccuracy. Replacement sensing elements are supplied with their uniquecalibration codes which can easily be entered into the instrument and probewithout the need to return it to the factory. The unique codes and instructionsfor entering them into the instrument can be found on the instruction sheetprovided with the replacement sensing element.The lifetime of theluminescence is measuredby the sensor and comparedagainst a reference.The amount of oxygen passing throughto the sensing layer is inverselyproportional to the lifetime of theluminescence in the sensing layer.Figure 9. Illustration of how a YSI optical sensor measures oxygen.How an Optical Sensor Measures Dissolved OxygenThe probe measures dissolved oxygen by emitting a blue light of the properwavelength that causes the dye in the sensing element to luminesce or glowred. Oxygen dissolved in the sample continually passes through the diffusionlayer to the dye layer, affecting the luminescence of the dye both in intensityand lifetime. The YSI sensor measures the lifetime of the dye’s luminescenceas it is affected by the presence of oxygen with a photodiode (light detector)in the probe and compares that reading to a reference (figure 9).To increase the accuracy and stability of the measurement, the sensor alsoemits a red light that is reflected by the dye layer back to the photodiode inthe sensor. The sensor measures the reflected light and uses that readingas the reference value for comparison to the previously measured lifetimeluminescent value. The lifetime of the luminescence from excitation by theblue light is compared to that of the reference value (red light) and a stabledissolved oxygen concentration is calculated by the probe.Although the accuracy of an optical sensor’s measurement is not dependenton flow, it is dependent on temperature. This temperature dependence89
is removed by proprietary algorithms in the system software. As for anyoxygen probe, the mg/L concentration is calculated from the sensor’s %saturation reading (temperature compensated), temperature, and salinityafter the calibration of the system using barometric pressure. The effects ofthese factors on dissolved oxygen readings are described in the MeasuringDissolved Oxygen with Either Sensor Type section of this booklet.Electrochemical SensorsThe Professional Plus and Pro20 as well as other YSI instruments can beequipped with either a field or lab BOD-style sensor (figure 11). Thisflexibility allows for the convenience and cost-savings of having oneinstrument for both applications. Additionally, these two instruments utilizea screw-on cap membrane which makes membrane changes simple andeasy to perform. The third electrochemical sensor type, the Rapid Pulsesensor, can be used on several of the 6-series multiparameter sondes (figure13).YSI offers three types of field rugged electrochemical sensors: steady-stategalvanic, steady-state polarographic, and Rapid Pulse polarographic (figure3). In addition to several different field probes, YSI offers a BOD-stylelaboratory polarographic probe with a built in stir bar.Dr. Leland Clark first invented the Polarographic electrode in 1956 andvariations of this electrode are still used by many manufacturers today. Figure10 shows a picture of a YSI 5739 polarographic sensor which is currentlyoffered by YSI and is similar in design to the original Clark electrode.Figure 10. Model 5739 polarographic sensor.YSI Electrochemical InstrumentsYSI offers numerous instruments that utilize an electrochemical sensor. Mostnotably are the Professional Plus multiparameter and the Pro20 dissolvedoxygen and temperature instruments (figure 11 and 12). These two modelsare the most versatile YSI handheld dissolved oxygen instruments since theycan use either a polarographic or galvanic sensor. Deciding between apolarographic and galvanic sensor depends on the application and userpreference. See the Comparing Steady-state Polarographic and GalvanicSensors section to understand the advantages and disadvantages of usingone type of sensor over the other.10Figure 11. The Professional Plus multiparameter instrument with a BOD-stylesensor. The Professional Plus can be outfitted with several different single andmulti-parameter cables including the Quatro which can measure dissolved oxygen,temperature, conductivity, and two ISEs at the same time and on the same cable.11
thicknesses and materials offered by YSI and their corresponding responsetimes, flow dependences, and required flow rates. For tips on how toovercome flow dependence, see the Taking Measurements section. Thetopics of flow dependence and response time are discussed further in theComparing Optical and Electroch
membrane-covered Clark Polarographic sensor, commonly referred to as a Clark electrode, which was developed in 1956 by Dr. Leland Clark (figure 1), a researcher at Antioch College who was working in collaboration with YSI scientists. Before the introduction of
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The YSI Model 55 Handheld Dissolved Oxygen System is a rugged, micro-processor based, digital meter with an attached YSI dissolved oxygen probe. The YSI Model 55 is designed for field use and is available with cable lengths of 12, 25 or 50 feet. The body of the probe has been manufactured with stainless steel to add rugged durability and
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Benchtop Dissolved oxygen meter LBDO-AII is equipped With polarographic sensor to measure the dissolved oxygen content. The manua salinity Sbarometric pressure compen sation helps in accurate measurement Of dissolved oxygen. The system menu Of dissolved oxygen meter can set 9 parameters such as stability conditions, calibration points etc. This
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