YSI Pro2030 Operating Manual

The Dissolved Oxygen
Handbook
a practical guide to dissolved oxygen measurements
YSI.com/weknowDO
CONTENTS
Introduction .......................................................................................................1
Dissolved Oxygen Sensors ................................................................................ 3
Optical Sensors ..............................................................................................5
YSI Optical Dissolved Oxygen Instruments ............................................6
Optical Sensing Element ............................................................................ 7
How an Optical Sensor Measures Dissolved Oxygen ............................. 9
Electrochemical Sensors .............................................................................10
YSI Electrochemical Instruments ........................................................... 10
Electrochemical Membranes ................................................................... 12
How an Electrochemical Sensor Measures Dissolved Oxygen ............ 15
Advancements in Steady-state Electrochemical Sensors ...................... 20
Comparing Steady-state Polarographic and Galvanic Sensors ............ 21
Comparing Optical and Electrochemical Sensing Technologies ................ 25
Measurement Accuracy ..............................................................................25
Approved Methodology .............................................................................. 28
Response Time ............................................................................................. 29
Flow Dependence ........................................................................................32
Warm-Up Time ............................................................................................ 34
Calibration Frequency ................................................................................ 34
Measurement Interferences ........................................................................ 35
Maintenance Requirements ........................................................................ 35
Power Consumption....................................................................................35
Summary .....................................................................................................35
Measuring Dissolved Oxygen with Either Sensor Type ............................... 37
Variables that Affect Dissolved Oxygen Measurements .......................... 38
Temperature .................................................................................................39
Salinity .......................................................................................................... 40
Electrode Maintenance ............................................................................67
Correcting for Salinity .............................................................................42
Barometric Pressure .................................................................................... 43
Using Barometric Pressure for DO Calibration ....................................44
Local DO % Measurements ..................................................................... 46
Calibration ........................................................................................................ 46
Calibration Frequency ................................................................................ 46
Calibration Methods ..................................................................................47
Winkler Titration Calibration .................................................................48
Air-saturated Water Calibration ............................................................. 49
Water-saturated Air Calibration ............................................................49
Two Point Calibration .............................................................................. 54
Calibration Musts .......................................................................................55
Calibration Musts for Electrochemical Sensors ....................................55
Calibration Musts for Optical Sensors ..................................................56
Errors During Calibration .......................................................................... 57
Probe Care and Maintenance for Optical Sensors ...................................69
Storage ..........................................................................................................70
Final Words .......................................................................................................70
Appendix A - Oxygen Solubility Table ..........................................................72
Appendix B – Calibration Values for Various Atmospheric Pressures and
Altitudes ............................................................................................................74
References ......................................................................................................... 76
GLP (Good Laboratory Practices) File ...................................................... 58
Taking Measurements ...................................................................................... 59
BOD Measurements .................................................................................... 60
Measurement Precautions and Interferences ...........................................61
Biofouling .................................................................................................. 61
Coating Materials .....................................................................................61
Probe Attacking Liquids .......................................................................... 62
Interfering Gases .....................................................................................63
Membrane and Sensing Element Integrity ............................................ 63
Probe Care and Maintenance .......................................................................... 64
Probe Care and Maintenance for Electrochemical Sensors ....................64
Changing a Membrane ............................................................................. 64
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INTRODUCTION

YSI has a long history in developing and manufacturing sensors that measure dissolved oxygen in aqueous solutions and has had many firsts over the years including the invention and commercialization of the first portable dissolved oxygen instrument in 1963. This instrument utilized a 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 the Clark electrode, methods for measuring dissolved oxygen were laborious, time-consuming and highly susceptible to interference. Today the world continues to benefit from Dr. Clark’s invention as the Clark electrode is still used by many manufacturers and in several YSI instruments. In addition to the variety of Clark electrodes offered, YSI also manufactures optical based dissolved oxygen sensors for laboratory, spot sampling and long term monitoring applications. See figure 2 for a brief overview of other YSI milestones in dissolved oxygen measurement technologies over its 60 year history.
This booklet describes in detail the different types of dissolved oxygen sensing technologies available. It also covers, in general terms, recommended calibration methods, regular maintenance procedures that can be performed by the user and how to take a measurement in order to obtain accurate data. For instrument specific instructions and recommendations, please refer to the instrument’s instruction manual.
YSI offers seminars on the topic of dissolved oxygen measurement technologies which may apply to continuing education units depending on the certifying agency. If you would like to schedule a seminar for your group or organization, please contact YSI at environmental@ysi.com, 1-800-897­4151 or +1 937-767-7241.
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Figure 1. Dr. Leland Clark, inventor of the Clark polarographic electrode.
Notable Events in YSI’s History of Developing Sensing Technologies for Measuring Dissolved Oxygen
1956 – Dr. Leland Clark invents the membrane covered Polarographic electrode while working with YSI Scientists.
2006 – YSI releases the ROX® optical dissolved oxygen sensor. The sensor has a dedicated wiper for long term monitoring on multi-parameter sondes.
2007 – YSI releases a galvanic electrochemical sensor for use on the Pro Series handheld product family.
2008 – YSI releases the ProODO® optical dissolved oxygen instrument for spot sampling and laboratory applications.
Figure 2. YSI’s Dissolved Oxygen Time line.

DISSOLVED OXYGEN SENSORS

There are two primary types of dissolved oxygen sensing technologies available: the optical based sensing method which is commonly referred to as luminescent and the Clark electrochemical or membrane-covered electrode. Within these two types of technologies, there are slight variations available. For example, there are two types of optical sensors. Both types of optical sensors measure luminescence as it is affected by the presence of oxygen; however, one sensor measures the lifetime of the luminescence while the other sensor measures the intensity of the luminescence.
1965 – YSI develops the first biological oxygen monitor. Considered a breakthrough for modern medicine and surgery, this instrument enabled physicians to perform open-heart surgery for the first time because immediate blood oxygen measurements could be taken real-time in the operating room rather than having a sample drawn and taken to a lab for analysis.
1993 – YSI patents first long-term, in-situ, stirring independent oxygen sensor (Rapid PulseTM 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 polarographic dissolved oxygen sensors. This advancement in membrane material lowered the stirring dependence and quickened the sensor’s response time over traditional Teflon® membranes.
The two types of Clark electrochemical sensors available are Polarographic and Galvanic. Additionally, YSI manufacturers two types of Polarographic sensors: Steady-state and the patented Rapid Pulse sensor. Refer to figure 3 for a diagram of the various sensor types.
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5
Dissolved Oxygen Sensors

Optical Sensors

Electrochemical Sensors
Intensity-based Optical Sensors
Lifetime-based Optical Sensors
Polarographic
Sensors
Galvanic
Sensors
ROX - available
on most 6-series
sondes
ProODO
Rapid Pulse -
available on some
6-series sondes
Steady-state
Available on
Pro20 and
ProPlus
Available on several
instruments including:
ProPlus, Pro20, 550A,
DO200 and 5100
OPTICAL SENSORS
Lifetime and intensity optical measurement methods detect dissolved oxygen based on the well documented principle that dissolved oxygen quenches both the lifetime and intensity of the luminescence associated with carefully­chosen chemical dyes. When there is no oxygen present, the lifetime and intensity of the signal are at their maximum. As oxygen is introduced to the sensing element, both the lifetime and intensity of the luminescence become shorter. Therefore, the lifetime and intensity of the luminescence are inversely proportional to the amount of oxygen present. The relationship between the oxygen pressure outside the sensor and the lifetime or intensity of the luminescence in the dye layer of the sensing element can be generally quantified by the Stern-Volmer equation (figure 4). However, the Stern­Volmer equation implies an inversely linear relationship which is not strictly true especially at higher oxygen concentrations; therefore, YSI employs the use of a 3rd order polynomial to correct for this non-linearity and to obtain the desired range of dissolved oxygen readings.
Figure 3. Diagram of Dissolved Oxygen Sensors.
The Stern-Volmer Relationship
Io/I = 1 + kqt0 * O2
Where:
Io =Intensity or lifetime of luminescence without the quenching molecule (O2).
I = Intensity or lifetime of luminescence with the quenching molecule (O2).
kq = Is the quencher rate coefficient.
t0 = Is the luminescence lifetime of the chemical (the dye) to be quenched.
O2 = The concentration of oxygen.
Figure 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 is that a lifetime sensor will be more stable in the long term. This is because the
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degradation of the dye in the sensing element has less effect on the lifetime based measurement than the intensity based measurement. Therefore, the intensity method will require more frequent calibrations - particularly at zero oxygen.

YSI OPTICAL DISSOLVED OXYGEN INSTRUMENTS

YSI offers two lifetime optical sensors: the ProODO (figure 5) sampling instrument and the 6150 ROX® (figure 6) sensor which can be used on most 6-series sondes that have an optical port. In addition, there will be an optical BOD-style sensor available for use on the ProODO in early 2010.
ROX Sensor
Figure 5. The ProODO is a compact, handheld instrument designed to withstand the harshest field conditions yet is accurate enough for use in a laboratory.
Figure 6. The ROX sensor’s dedicated wiper and anti-fouling accessories help extend deployment times while protecting data integrity making it ideal for unattended, remote and real time monitoring applications.

OPTICAL SENSING ELEMENT

YSI’s two optical dissolved oxygen sensors utilize sensing elements that are similar in function but slightly different in design. The ProODO’s sensing element is referred to as a Sensor Cap due to its screw on cap design (figure
7). The ROX’s sensing element is referred to as a ROX Membrane (figure 8) and is held in place by 3 screws.
Figure 7. ProODO Sensor Cap.
Sensor Cap
8
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Figure 8. ROX membrane.
Oxygen is constantly diffusing through the paint layer, affecting the luminescence of the sensing layer.
Each sensing element has two layers. The outer layer is a paint that acts as an oxygen permeable diffusion layer which allows oxygen molecules to pass through while protecting the dye layer. The sensing layer is an immobilized polystyrene dye layer that luminesces when excited with light of a proper wavelength (figure 9). The degradation of this dye layer over time is what causes the sensor cap to need replacement and all lifetime based 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 clean and properly stored between uses. See the Probe Care and Maintenance section of this booklet for more information on cleaning and storage.
The sensing elements are factory calibrated at YSI and a calibration code specific to each individual sensing element is determined during the manufacturing process. The calibration code consists of coefficients that are preloaded into the sensor at the factory for increased measurement accuracy. Replacement sensing elements are supplied with their unique calibration codes which can easily be entered into the instrument and probe without the need to return it to the factory. The unique codes and instructions for entering them into the instrument can be found on the instruction sheet provided with the replacement sensing element.
The lifetime of the luminescence is measured by the sensor and compared against a reference.
The amount of oxygen passing through to the sensing layer is inversely proportional to the lifetime of the luminescence in the sensing layer.
Figure 9. Illustration of how a YSI optical sensor measures oxygen.

HOW AN OPTICAL SENSOR MEASURES DISSOLVED OXYGEN

The probe measures dissolved oxygen by emitting a blue light of the proper wavelength that causes the dye in the sensing element to luminesce or glow red. Oxygen dissolved in the sample continually passes through the diffusion layer to the dye layer, affecting the luminescence of the dye both in intensity and lifetime. The YSI sensor measures the lifetime of the dye’s luminescence as 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 also emits a red light that is reflected by the dye layer back to the photodiode in the sensor. The sensor measures the reflected light and uses that reading as the reference value for comparison to the previously measured lifetime luminescent value. The lifetime of the luminescence from excitation by the blue light is compared to that of the reference value (red light) and a stable dissolved oxygen concentration is calculated by the probe.
Although the accuracy of an optical sensor’s measurement is not dependent on flow, it is dependent on temperature. This temperature dependence
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is removed by proprietary algorithms in the system software. As for any oxygen probe, the mg/L concentration is calculated from the sensor’s % saturation reading (temperature compensated), temperature, and salinity after the calibration of the system using barometric pressure. The effects of these factors on dissolved oxygen readings are described in the Measuring Dissolved Oxygen with Either Sensor Type section of this booklet.

ELECTROCHEMICAL SENSORS

YSI offers three types of field rugged electrochemical sensors: steady-state galvanic, steady-state polarographic, and Rapid Pulse polarographic (figure
3). In addition to several different field probes, YSI offers a BOD-style laboratory polarographic probe with a built in stir bar.
Dr. Leland Clark first invented the Polarographic electrode in 1956 and variations of this electrode are still used by many manufacturers today. Figure 10 shows a picture of a YSI 5739 polarographic sensor which is currently offered by YSI and is similar in design to the original Clark electrode.
The Professional Plus and Pro20 as well as other YSI instruments can be equipped with either a field or lab BOD-style sensor (figure 11). This flexibility allows for the convenience and cost-savings of having one instrument for both applications. Additionally, these two instruments utilize a screw-on cap membrane which makes membrane changes simple and easy to perform. The third electrochemical sensor type, the Rapid Pulse sensor, can be used on several of the 6-series multiparameter sondes (figure
13).
Figure 10. Model 5739 polarographic sensor.

YSI ELECTROCHEMICAL INSTRUMENTS

YSI offers numerous instruments that utilize an electrochemical sensor. Most notably are the Professional Plus multiparameter and the Pro20 dissolved oxygen and temperature instruments (figure 11 and 12). These two models are the most versatile YSI handheld dissolved oxygen instruments since they can use either a polarographic or galvanic sensor. Deciding between a polarographic and galvanic sensor depends on the application and user preference. See the Comparing Steady-state Polarographic and Galvanic Sensors section to understand the advantages and disadvantages of using one type of sensor over the other.
Figure 11. The Professional Plus multiparameter instrument with a BOD-style sensor. The Professional Plus can be outfitted with several different single and multi-parameter cables including the Quatro which can measure dissolved oxygen, temperature, conductivity, and two ISEs at the same time and on the same cable.
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thicknesses and materials offered by YSI and their corresponding response times, flow dependences, and required flow rates. For tips on how to overcome flow dependence, see the Taking Measurements section. The topics of flow dependence and response time are discussed further in the Comparing Optical and Electrochemical Sensing Technologies section.
Membrane Comparison Table
Figure 12. The Pro 20 as well as the Pro Plus can be outfitted with a polarographic or galvanic sensor.
Figure 13. Rapid Pulse sensor for use on some 6-series sondes.

ELECTROCHEMICAL MEMBRANES

An electrochemical membrane is a thin semi-permeable material, stretched over the sensor that isolates the electrodes from the environment while allowing gases to enter. YSI offers several different types of dissolved oxygen membranes of varying design, thickness and material.
The two types of membrane designs include the traditional, sheet-like stretch membrane that is held in place by an o-ring and the plastic screw-on cap membrane that has the membrane material pre-stretched at the factory.
The membrane’s material and thickness affect the sensor’s flow dependence and response time. The table in figure 14 shows the various membrane
Sensor Membrane
Material
Color of Cap
Optical Diffusion layer
and sensing element
Response Time 100 to 0% (T-95)
40 seconds*
Flow Dependence after 4 minutes
0 (zero)
Non-
Required Flow Rate
0 (zero)
consumptive
Steady-state Galvanic or Polarographic
Steady-state Galvanic or Polarographic
Steady-state Galvanic or Polarographic
1.0 mil Teflon
Black Cap
1.25 mil PE
Yellow Cap
2.0 mil PE
Blue Cap
18 seconds 45% 12 inches
per second
8 seconds 25% 6 inches
per second
17 seconds 18% 3 inches
per second
Figure 14. Newer membrane caps are made of polyethylene notated as PE in the table. The response time in this table is listed as T-95 which is the amount of time it takes for the sensor to get to 95% of the desired reading when moved from a 100% saturated sample to a 0 oxygen environment.
*YSI studies have shown that stirring an optical sensor can lower its response time. For example, using a magnetic stirrer or stir bar could result in an optical response time of 22 seconds or less for T-95.
The type of membrane that can be used with a particular instrument is dictated by the sensor type in use and by the instrument’s microprocessor. Using an incorrect membrane could result in erroneous readings so care
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should be taken when ordering replacement membranes. Refer to figure 15 for a guide to ordering the proper membrane for a specific instrument and sensor. If you have an instrument that can use more then one type of membrane, refer to figure 14 to determine which option is best for your application based on the response times and required flow rates of the membranes available for use on your instrument. For instruments that can use more than one membrane, it should be noted that the instrument must be properly configured for the membrane installed in order to obtain accurate readings.
Membrane Selection Guide
Membrane Model # (Item #)
5680 (060745)
5775 (098094)
5776 (098095)
5906 (059880)
Type, Thickness, Material Instrument and
probes it is used with
Stretch Membrane,
2.0 mil Teflon
Stretch Membrane,
1.0 mil Teflon
Stretch Membrane,
0.5 mil Teflon
Black Cap Membrane,
1.0 mil Teflon
Probes: 5719, 5739, and 5750 when used with a model 58 instrument only.
Instrument: 55 Probes: 5719, 5739, 5750, and Rapid Pulse
Probes: 5719, 5739, and 5750 when used with a model 58 instrument only.
Instrument: 85, 550, 556 MPS, and Pro Plus with a Polarographic sensor. Probes: 5239, 5905 and 5010
Membrane Model # (Item #)
5909 (059882)
5912 (605912)
5913 (605913)
5914 (605914)
Figure 15. Newer membrane caps are made of polyethylene notated as PE in the table.
Type, Thickness, Material Instrument and
probes it is used with
Blue Cap Membrane,
2.0 mil Polyethylene (PE)
Black Cap Membrane,
1.0 mil Teflon
Yellow Cap Membrane,
1.25 mil Polyethylene (PE)
Blue Cap Membrane,
2.0 mil Polyethylene (PE)
Instruments: 556 Probes: Pro Series Polarographic sensors
Pro Series Galvanic sensor when used with a Pro Plus instrument only
Pro Series Galvanic sensors
Pro Series Galvanic sensors

HOW AN ELECTROCHEMICAL SENSOR MEASURES DISSOLVED OXYGEN

Electrochemical sensors, both polarographic and galvanic, consist of an anode and a cathode that are confined in electrolyte solution by an oxygen permeable membrane. Oxygen molecules that are dissolved in the sample diffuse through the membrane to the sensor at a rate proportional to the pressure difference across it. The oxygen molecules are then reduced at the cathode producing an electrical signal that travels from the cathode to the anode and then to the instrument. Since oxygen is rapidly reduced or consumed at the cathode, it can be assumed that the oxygen pressure under the membrane is zero. Therefore, the amount of oxygen diffusing through the membrane is proportional to the partial pressure of oxygen outside the membrane.
5908 (059881)
Yellow Cap Membrane,
1.25 mil Polyethylene (PE)
Instruments: DO200, 550A, and 556 Probes: Pro Series Polarographic sensors
For example, in air or air-saturated water at sea level, the oxygen partial pressure is approximately 160 mmHg (21% of 760 mmHg), while the pressure under the membrane is zero. This difference in oxygen pressures produces the current that is read by the instrument. As the oxygen pressure varies, so does the oxygen diffusion through the membrane which causes
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the probe current to change proportionally. Figure 16 is a dissection of a Clark electrode and illustrates how an electrochemical sensor, either polarographic or galvanic, works.
It is important to recognize that oxygen dissolved in the sample is consumed during the measurement with a steady-state electrochemical sensor. This results in a measurement that is dependent on flow. It is therefore essential that the sample be continuously stirred at the sensor tip. If stagnation occurs, the readings will be artificially low. The flow dependence and therefore the rate of stirring required for an accurate measurement varies by membrane, see figure 14. The topics of flow dependence and response time are discussed further in the Comparing Optical and Electrochemical Sensing Technologies section. For tips on how to overcome flow dependence, see the Taking Measurements section.
Electrochemical dissolved oxygen measurements are also affected by barometric pressure and the temperature and salinity of the sample. These three factors and how they affect dissolved oxygen readings are described in detail in Measuring Dissolved Oxygen with Either Sensor Type section.
Steady-State Polarographic Sensors
In a polarographic sensor, the cathode is gold and the anode is silver. The system is completed by a circuit in the instrument that applies a constant voltage of 0.8 volts to the probe, which polarizes the two electrodes, and a meter to read the dissolved oxygen response from the sensor.
The electrolyte held under the membrane allows the electrical signal to travel from the cathode to the anode. The signal continues down to the meter as shown by the basic circuit diagram in figure 17. The polarographic sensor operates by detecting a change in this current caused by the variable pressure of oxygen while the potential is held constant at 0.8 V. The more oxygen passing through the membrane and being reduced at the cathode, the greater the electrical signal (current) read by the probe. As oxygen increases, the signal increases and, conversely, as oxygen decreases, the signal decreases. Chemically, this is described as the oxidation of the silver and reduction of oxygen at the gold cathode as follows:
Silver Anode Reaction: 4Ag + 4Cl- 4AgCl + 4e
Gold Cathode Reaction: O2 + 2H2O +4e- 4OH
-
-
Overall reaction: O2 + 2H2O + 4Ag + 4KCl 4AgCl + 4KOH
Figure 16. An illustration of an electrochemical sensor.
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Zinc Anode reaction: 2Zn 2Zn
2
+ 4e
-
+
Sliver Cathode reaction: O2 + 2H2O + 4e- 4OH
-
Figure 17. A simplified diagram of a polarographic sensor and circuit.
Steady-State Galvanic Sensors
In the YSI galvanic sensor, the cathode is silver and the anode is zinc. The anode may be a different material, such as lead, in different manufacturers’ sensors. Figure 18 is an illustration of a galvanic sensor.
Overall reaction: O2 + 2H2O + 2Zn 2Zn(OH)
2
Figure 18. A simplified diagram of a galvanic sensor and circuit.
A circuit completes the measurement, but, unlike the polarographic sensor, the galvanic sensor does not have or need a constant voltage applied to it. In the Galvanic sensor, the electrodes are dissimilar enough to self-polarize and reduce oxygen molecules without an applied voltage. It is similar in function to a battery.
A galvanic dissolved oxygen system uses a meter to read the electrical signal coming back from the probe and this signal is proportional to the amount of oxygen passing through the membrane. Oxygen passing through the membrane and being reduced at the cathode increases the electrical signal (current) read by the probe. As oxygen increases, the signal increases and as oxygen decreases, the signal decreases. Chemically, this is described as the oxidation of the zinc and reduction of oxygen at the silver cathode as follows:
Rapid Pulse Polarographic Sensors
The third type of electrochemical sensor offered by YSI is the patented Rapid Pulse polarographic sensor. This sensor is like the steady-state polarographic sensor in that it has a gold cathode, silver anode and utilizes electrolyte solution that is held in place by an oxygen permeable membrane. Also, similar to the traditional polarographic sensor, a voltage of 1.0 V is applied to the electrodes and oxygen passing through the membrane is reduced at the cathode. This reduction causes a change in the electrical signal read by the instrument. The amount of oxygen passing through the membrane and being reduced at the cathode is directly proportional to the electrical signal sent back to the instrument.
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