Methods And Working Principles Of Dissolved Oxygen Sensors

Galvanic DO Sensor

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.

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 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.

Optical DO Sensor

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.

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.

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.

Comparison of Advantages and Disadvantages

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

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