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Introduction
Dissolved oxygen (DO) is an essential measurement parameter in aerobic bioreactors. The growth of all cells is heavily dependent on DO because it acts as a terminal electron acceptor in aerobic respiration. However, if excessive amount of DO is added to the process, it may limit the growth of the culture and promote undesirable organisms. Consequently, the measurement of DO is critical to effective operation of systems. Today, a variety of sensors are available in the market, each with its own advantages and disadvantages.
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There are two main types of dissolved oxygen sensors: optical (luminescent) and Clark electrochemical (membrane covered electrode or amperometric). These main types have subtypes, slightly differing from each other, see figure 1.
Figure 1. Diagram of sensor types. Source: [6]
Different sensor types suit some applications better that the others. These properties will be discussed later on the page, meanwhile the applications can be found from Figure 2.
Figure 2. Best applications for different types of sensors. Source: [6]
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Optical sensing of oxygen is based on the measurement of the red fluorescence of a dye/indicator illuminated with a modulated blue light as shown in Figure 3.
Figure 3. Principal of oxygen detection using fluorescent dye. Source: A comparison of amperometric and optical dissolved oxygen sensors in power and industrial water applications
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The oxygen concentration is determined with the Stern-Volmer equation, that sets the relationship between luminescence lifetime (intensity) and oxygen concentration see Figure 4.
Figure 4. Stern-Volmer equation. Source: [6]
The most significant advantage of an optical dissolved oxygen sensor is low maintenance cost and the possibility of less frequent calibration. Other advantages and disadvantages can be found from Figure 5.
Figure 5. Advantages and disadvantages of optical sensors. Source: [6]
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As in the case for the polarographic electrodes, a voltage is applied externally while an internal potential is generated as in the galvanic electrodes.
Figure 6. An illustration of an electrochemical sensor. Source: [6]
Working Principle - Amperometry
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A typical polarographic electrode consists of a silver anode, a gold or platinum cathode and an electrolyte solution (KCl or AgCl). In order to create a sensor, a constant voltage of 0.8 volts is applied to the probe, and a digital meter is installed to read the DO response measured by the sensor [6].
Figure 7. A simplified diagram of a polarographic sensor. Source: [6]
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A typical galvanic electrode consists of a zinc or lead anode, a gold or silver cathode and an electrolyte solution (KCl or KBr). The principle of the galvanic sensor is that it uses 2 different types of metal and the difference in them with the electrolyte results in an electromotive voltage. This electromotive voltage is around 0.8 V which is enough to operate the sensor. Therefore the biggest advantage of the galvanic sensor is that there is no need for outside voltage source and there is no warm-up time.
Figure 8. A simplified diagram of a galvanic sensor and circuit. Source: [6]
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The fact that the sensor measures the pressure instead of the concentration for dissolved oxygen is known to be true because a sample of fresh water can dissolve more oxygen than a sample of sea water at the same temperature and at the same altitude (or under the same barometric pressure). However, the sensor’s output signal is identical in both samples since the oxygen pressure is identical in both media. See the following figure for an example of this concept.
Figure 9. DO sensors measure % saturation. Source: [6]
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