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Optical Technology for Dissolved Oxygen reduces calibration, maintenance and
increases accuracy. |
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Fluorescence Technology for
the Measurement of Dissolved Oxygen in water |
EnviroEquip News,
August 2003
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Insite IG has developed a new type of
Dissolved Oxygen measurement system, based on fluorescent technology. The system is
designed for the harsh environment of wastewater treatment plants, but has
also found use in other applications such as environmental monitoring,
process waters and aquaculture.
Existing DO measurement technology has limitations...
Clark cell (galvanic and polarographic) based dissolved
oxygen sensors have been the predominant methods for measuring dissolved
oxygen in wastewater treatment facilities. Constant cleaning of the
sensors, the need for membrane and electrolyte replacement, probe fouling,
and re-calibrating of the instrument can be so demanding that monitoring,
and more importantly control, can be a frustrating, time consuming
exercise.
New fluorescence technology has the advantages...
With the introduction of fluorescent based oxygen
sensors designed specifically for the wastewater industry an alternate
method of making this important measurement is now available. The biggest
advantages of fluorescence based sensors are the inherent reliability and
low maintenance requirements of this technology. Low maintenance and no
replaceable membranes or electrolyte are the key features of this type of
sensor.
This in turn reduces hours of lost time for maintenance
and eliminates the cost of replacement parts. Another very important
feature is that fluorescent-based sensors do not consume oxygen and
require no flow across them to work. They also perform very well in harsh
environments that normally destroy other conventional sensors.
Fluorescence Technology
- Principal of Operation
- The emitter sends light, at ~475 nm, to the back side of the sensing
element.
- The wetted side of the sensing element consists of a thin layer of a
hydrophobic sol-gel material. A ruthenium complex is trapped in the
sol-gel matrix, effectively immobilized and protected from water.
- The light from the LED excites the ruthenium complex immobilized in
the sensing element.
- The excited ruthenium complex fluoresces, emitting energy at ~600
nm.
- If the excited ruthenium complex encounters an oxygen molecule, the
excess energy is transferred to the oxygen molecule in a non-radiative
transfer, decreasing or quenching the fluorescence signal (see
Fluorescence Quenching below). The degree of quenching correlates to the
level of oxygen concentration in contact with the sensing element.
Fluorescence Quenching
Oxygen is able to efficiently quench the fluorescence and phosphorescence
of certain luminophores. This effect (first described by Kautsky in 1939)
is called "dynamic fluorescence quenching." Collision of an oxygen
molecule with a fluorophore in its excited state leads to a non-radiative
transfer of energy. The degree of fluorescence quenching relates to the
frequency of collisions, and therefore to the concentration of the
oxygen-containing media.
FAQ's on Fluorescence Technology
Q1.) How can
dissolved oxygen be measured using fluorescence?
Q2.) How long has this
technology been in use?
Q3.) What is the expected
life of the sensor?
Q4.) How often do the
sensors need to be calibrated?
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Insite IG Links
QuipNotes Articles
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Q7.) What is
the minimum flow required for the sensor to properly measure DO?
Q10.) Can
the Insite DO systems be used in very low oxygen environments, such as
anoxic and anaerobic
zones?
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Answers |
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Q1.)
How can dissolved oxygen be measured using fluorescence?
A very specific energy wavelength is transmitted to a ruthenium compound
immobilized in a sol-gel matrix. The ruthenium will absorb this energy,
changing the outer electron’s energy level. The electron will then
collapse back to it’s original energy state, emitting the energy as a
photon with a different specific wavelength. This is called fluorescing.
If the intensity of the transmitted wavelength is tightly controlled,
the amount of fluorescing is both predictable and repeatable.
If oxygen molecules are present the amount
of fluorescing is reduced, referred to as fluorescence quenching. By
measuring the amount of quenching it is possible to determine the amount
of oxygen present. |
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Q2.) How
long has this technology been in use?
Top
Fluorescence technology has been used to measure oxygen uptake in the
medical industry for over twenty years. Within the last five years the
technology has been adapted for use in measuring dissolved oxygen in
biological reactors. The key issues in making the technology viable in
the water treatment industry were the durability of the sensing
element and cost. Another important concern was packaging the system
in a way that was easy to use and required very little maintenance.
The Insite units have accomplished all of these objectives.
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Q3.) What is the expected life of the sensor?
Top
Seven to ten years. During this time there are no consumables at all. No
spare parts, no recharging kits, no replacement films, and no membranes
or membrane cartridges. |
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Q4.) How often do the sensors need to be calibrated?
Top
We recommend that the calibration be checked at least once a year. The
sensor will drift less than one percent per year. |
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Q5.) Can the sensor be calibrated in the field?
Top
Yes, the sensor can be calibrated in the field. A simple calibration to
a reference takes about a minute. |
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Q6.) What is the accuracy of the Insite fluorescence DO system?
Top
+/- 0.05 ppm |
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Q7.) What is the minimum flow required for the sensor to properly
measure DO? Top
Unlike Clark type cells with a membrane, the Insite DO sensors do not
require any flow. Clark type DO sensors actually consume oxygen to make
the measurement so a new supply of oxygen must be continuously provided.
The Insite sensor does not consume anything so no flow is needed to
obtain a correct reading. |
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Q8.) Does the measuring area of the sensor need to remain moist,
even when not in actual use?
Top
No, the sensor can periodically become completely dry with no loss of
accuracy, response time, or calibration. No soaking caps are required
for storing the sensor. |
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Q9.) If the actual measuring area of the sensor is physically
damaged, can this be repaired?
Top
In the very unlikely event that the measuring surface is damaged it can
be repaired. The cost of this repair is under $150.00. We recently had a
customer put a system (analyzer and sensor) in the back of a golf cart
to take to the shop. The sensor did not quite make it into the cart and
was left hanging out the back on the ground. The sensor was then dragged
the entire trip back to the shop, a couple of thousand feet. The sensor
looked like it was put under a grinder but it still worked. This says
something about how hard it is to physically damage the sensor. |
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Q10.) Can the Insite DO systems be used in very low
oxygen environments, such as anoxic and anaerobic zones?
Top
Yes, the Insite method of measuring DO is very accurate at extremely low
ranges. There have been tests in which the unit performed very well in
the 0.03ppm to 0.08ppm range. |
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Q11.) What are the main reasons to monitor and control
DO in biological reactors?
Top
While there are almost as many specific reasons as there are treatment
facilities, they generally fall into one of three general areas.
- Reduce the amount of power required to run the blowers. This in
turn will significantly reduce plant costs. In general, up to 70% of
a facility’s power consumption is for aeration. An industry
association estimates that the average treatment facility could save
up to 30% by automatically controlling aeration.
- Optimize the conditions in the basins to achieve the correct
microbiological mix. This is especially important in BNR facilities.
- Reduce the overall maintenance required to operate the biological
reactors.
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