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.
1
2
3
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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7
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
9
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.
12
13
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
SensorMembrane
Material
Color of Cap
OpticalDiffusion 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 seconds45%12 inches
per
second
8 seconds 25%6 inches
per
second
17 seconds18%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
14
15
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, MaterialInstrument 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, MaterialInstrument 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
16
17
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:
Figure 16. An illustration of an electrochemical sensor.
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19
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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