1. 7ENT1112 Smart Embedded Systems Engineering
Answer:
Introduction
The methods used in the modern measurement of dissolved oxygen (DO) in the field or lab
involves a meter connected DO sensor which is responsible for the recording of
calibration and data measurement. The design of the DO sensors can be for biological
oxygen demand (BOD) tests, discrete sampling, as well as long-term applications for
monitoring. Generally, there are two types of Dissolved Oxygen sensors including Optical
Sensors and Electrochemical sensors.
Working Principles.
Galvanic DO Sensor
When this type of DO sensor is immersed in the sample of water, there will be reduction in
the diffusion of oxygen across the membrane which is oxygen-permeable. Usually this rate
of diffusion should be at a proportional value with the water oxygen pressure. The
reduction is occurring at the cathode section. Through this kind of the reaction, there is
production of electrical current which is related directly to the concentration of oxygen. The
ions in the electrolytes will carry this current then run to anode from the cathode.
Anode (Pb) – lead oxidation reaction: 2Pb ? 2Pb2+ + 4e-
Cathode (Ag) – oxygen reduction reaction: O2 + 4e- + 2H2O ? 4OH-
Overall reaction: O2 + 2H2O + 2Pb ? 2Pb(OH)2
The produced current is directly proportional to the consumed oxygen and by extension
to the partial pressure of the quantity of oxygen found within the sample.
There is precipitation out of the white solid known as Pb(OH)2 which is a result of these
reactions into the solution of electrolyte. It neither consumes the electrolytes nor coats the
anode and therefore it does not have any impact on the performance of the sensor until
there is an excess of the quantity. In case there is occurrence of the same, then the ability of
the ion to carry the current will be interfered with. It is important to note that galvanic DO
sensor is characterized by the processes of self-polarizing. This implies that there is
2. continuous consumption of the anode even during that period when the sensor is not put in
use. As a result, it is recommended to disconnect the tip of DO sensor when there is
measurement for a long period of time. The storage after disconnection should be as per the
guidelines in the manual.
Optical DO Sensor
When this type of the DO sensor is immersed into the sample of water, there will be
interaction of oxygen with dye after crossing membrane. This reduces or quenches the
lifetime and intensity of dye’s luminescence. This will be measured through the use of
photodetector and the result used in the calculation of DO concentrations.
The lifetime and intensity of luminescence when there is exposure of dye to blue light is
indirectly proportional to the sample’s contained quantity of oxygen. There will be
monitoring of lifetime as an oxygen concentration function through the use of technique of
phase fluorometry. In this particular technique, there measurement of phase difference for
oxygen sensitivity between a modulated reference signal and modulated luminescence
signal.
Figure 1: sinusoidal Signal excitation
If the signal of excitation is basically sinusoidal, the luminescence will equally be modulated
although there will be delay or the phase will shift relative to the signal of excitation. The
shifting of phase is as illustrated in the figure above. The red and blue LEDs are switched
alternately so as to obtain the phase difference as a result of the electronics itself (∅ref)).
There is subtraction of this phase from phase shift which is time dependent (∅sig,). This is
done in real time so as t get specific output of the sensor of phase shift.
Comparison Of Advantages And Disadvantages
Features
Galvanic DO Sensor
Optical DO Sensor
3. Stirring
This kind of the sensor requires stirring
In this type of the sensor of DO, there is no requirement of sensor
Response Time
Galvanic DO sensor has a faster time for response than the Optical DO sensor.
The response time here is fast but it would take 2-4X longer than electrochemical DO
sensors.
Time for Warm-up
It does not need this warm-up time
It does not need time for warm-up too.
Consumption of Power
4. Less power is required than what is consumed by the optical DO sensor
It consumes a lot power
Calibration
In this particular type of sensor, there is retention of the calibration data in the meter.
There is requirement of frequent calibration since it tends to occasionally drift away from
the same data of calibration.
There is retention of the data on calibration in the head of the sensor.
There is better holding of calibration with very little drift although there will still be
recommendations for regular calibration.
Lifetime
It has a lifespan which is shorter than that of the optical DO sensor
Its lifespan is comparatively longer than that of the electrochemical DO sensor
5. Membrane
It is extremely vulnerable to wear and tear and even ultimate damage
This type of the DO sensor is durable
Replacement Frequency
Depending on the handling and application, the tip replacement in this type of the DO
sensor will be at an interval of 6 months
When the reading of the sensor is unstable or unusually low, there will be need for the tip
replacement.
The replacement of the cap of this kind of the DO sensor is at an interval of 1 year
Overtime there is degradation of the dye. When there will be no calibration of the sensor,
the solution will be to have another one.
Warranty
When it is still effective and function, this DO sensor warranty period is 6 months
6. When it is still effective and function, this DO sensor warranty period is 12months
Cost
It is usually cheaper than the Optical DO sensor
In comparison with the electrochemical DO sensor, it is more expensive.
Applications
The application of this DO sensor is not preferred for those samples with hydrogen sulfide
gas and strong acids.
It is best suited for those samples which have very strong acids as well as the presence of
hydrogen sulfide gas.
The volume of the sample required is generally less
It is accurate more to DO concentrations.
Example Of Annotated Dissolved Oxygen System
As mentioned previously, it is Galvanic Dissolved Oxygen Sensor which is recommended for
the measurement involving large volume of water sample. It would thus be most
appropriate for the depth of water more than 30m.
7. Figure: Galvanic Dissolved Oxygen Sensor
Components:
Electrolyte
Anode
Cathode
Membrane
The anode and cathode are made of dissimilar metals. This implies that they are at different
electropotentials. In order to have oxygen reduction without potential applied externally,
the potential difference between the cathode and anode should be at least 0.5V. When
placed in the solution of electrolyte, the existing potential between the metals which are
dissimilar makes them to undergo self-polarization with the travelling electrons internally
from one location(anode) to another(cathode) It is for this reason that galvanic type of DO
sensor does not need any kind of warm up duration.
The cathode which is made up of noble metal or Ag accepts electrons that originates from
the anode through internal circuit and have them passed on to the molecules of oxygen. It
does not have any interference with the reaction. Therefore anode which is primarily Pb, Zn
or another metal which is active is oxidized and oxygen is reduced at the cathode surface.
Both anode and cathode are submerged in the electrolyte e.g. NaCl and NaOH or another
electrolyte which is inert and enclosed in a cap which has been fitted with a thin membrane
that is oxygen –permeable and hydrophobic as well.
Discussion And Conclusion
In this kind of the requirement, one would recommend galvanic DO sensors. The sensor has
an advantage over optical sensor including the fact that they do not require voltage from
outside as well as time for warm up. This implies that they can be used even in the depth of
over 30metres. The electrolyte of these DO sensors can be used for longer time. This means
that they will not have to be withdrawn from the water frequently for maintenance
purposes. Depending on the handling and application, the tip replacement in this type of
the DO sensor will be at an interval of 6 months. When the reading of the sensor is unstable
or unusually low, there will be need for the tip replacement. Besides Less power is required
than what is consumed by the optical DO sensor. It is important to note that the
requirement of maintenance will be subject to the nature or properties of water where it
will be immersed.
References
[1] Alborzi, E., Flyagina, I.S., Mielczarek, D.C., Blakey, S.G. and Pourkashanian, M., 2022. A
theoretical investigation into the comparative adsorption between dissolved oxygen and
8. oxygenate species on zeolite 3.7 Å during aviation fuel treatment for thermal stability
improvement. Fuel, 317, p.123451.
[2] Blaszczak, J.R., Delesantro, J.M., Urban, D.L., Doyle, M.W. and Bernhardt, E.S., 2019.
Scoured or suffocated: Urban stream ecosystems oscillate between hydrologic and
dissolved oxygen extremes. Limnology and Oceanography, 64(3), pp.877-894.
[3] Inglev, R., Møller, E., Højgaard, J., Bang, O. and Janting, J., 2020. Optimization of All-
Polymer Optical Fiber Oxygen Sensors with Antenna Dyes and Improved Solvent Selection
Using Hansen Solubility Parameters. Sensors, 21(1), p.5.
[4] Khatri, N., Khatri, K.K. and Sharma, A., 2020. Enhanced energy saving in wastewater
treatment plant using dissolved oxygen control and hydrocyclone. Environmental
technology & innovation, 18, p.100678.
[5] Ochumba, P.B.O., 2019. Measurement of water currents, temperature, dissolved oxygen
and winds on the Kenyan Lake Victoria. In The limnology, climatology and paleoclimatology
of the East African Lakes (pp. 155-167). Routledge.
[6] Pereira, C.F., Olean-Oliveira, A., David-Parra, D.N. and Teixeira, M.F., 2018. A
chemiresistor sensor based on a cobalt (salen) metallopolymer for dissolved molecular
oxygen. Talanta, 190, pp.119-125.
[7] Roots, P., Wang, Y., Rosenthal, A.F., Griffin, J.S., Sabba, F., Petrovich, M., Yang, F., Kozak,
J.A., Zhang, H. and Wells, G.F., 2019. Comammox Nitrospira are the dominant ammonia
oxidizers in a mainstream low dissolved oxygen nitrification reactor. Water research, 157,
pp.396-405.
[8] Song, N., Yan, Z., Xu, H., Yao, Z., Wang, C., Chen, M., Zhao, Z., Peng, Z., Wang, C. and Jiang,
H.L., 2019. Development of a sediment microbial fuel cell-based biosensor for simultaneous
online monitoring of dissolved oxygen concentrations along various depths in lake
water. Science of the total environment, 673, pp.272-280.
[9] Wei, Y., Jiao, Y., An, D., Li, D., Li, W. and Wei, Q., 2019. Review of dissolved oxygen
detection technology: From laboratory analysis to online intelligent
detection. Sensors, 19(18), p.3995.
[10] Zimmermann, P., Weltin, A., Urban, G.A. and Kieninger, J., 2018. Active potentiometry
for dissolved oxygen monitoring with platinum electrodes. Sensors, 18(8), p.2404.
