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Dissolved oxygen is a physical distribution of oxygen molecules in water. Oxygen does not react with water, but mixes with it. There are two main sources of DO in water: atmosphere and photosynthesis.http://www.eutechinst.com/techtips/tech-tips15.htm [2]
Ambient air contains about 20% oxygen and is essential for breathing, also for fish and other aquatic organism. Dissolved oxygen is the amount of free oxygen in water suitable for the breathing purpose. If there is not enough oxygen, it is letal to fish: the amount of 2 mg/l is deadly and the amount between 2 and 5 mg/l affects fish health.
is this relevant to DO? Also Yes Also dissolved oxygen data or BOD (biological oxygen demand) is needed to determine effluent water quality. It is a common environmental procedure to determine the amount of microorganisms in a sample. This measurement is used in wastewater treatment, food manufacturing and filtration facilities where this quantity is important for the process and final product. “High concentrations of DO predict that oxygen uptake by microorganisms is low along with the required break down of nutrient sources in the medium” (http://www.eidusa.com/Theory_DO.htm). [1].
Types
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.
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Figure 1. Diagram of sensor types. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf [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.
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Figure 2. Best applications for different types of sensors. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf [6]
Optical Sensors
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.
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The sensor measures the lifetime of the dye’s (sensing layer’s) luminescence, caused by the presence of oxygen, with a photodiode (light detector) in the probe. To increase the accuracy and stability of the measurement the reading is compared to a reference. The lifetime of the luminescence from excitation by the red light acts as the reference (“the sensor emits a red light that is reflected by the dye layer back to the photodiode in the sensor” https://www.fondriest.com/pdf/ysi_do_handbook.pdf[6])), so the lifetime of luminescence of the blue light is compared to it, and the stable oxygen concentration is calculated by the probe.
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Figure 4. Stern-Volmer equation. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf [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: https://www.fondriest.com/pdf/ysi_do_handbook.pdf. Advantages and disadvantages of optical sensors. Source: [6]
Electrochemical Sensors
Electrochemical DO electrodes are divided into two separate types: polarographic and galvanic. These electrodes are constructed with an anode and a cathode submerged in an electrolyte solution. An oxygen-permeable membrane is used to confine the cathode. When the cathode is polarized with a constant voltage, dissolved oxygen molecules diffusing through the membrane is reduced at the cathode. Then, an electrical signal produced by the cathode travels to the anode and then to the instrument. The oxygen tension versus the electrode current can be calibrated since the diffusive flux is a function of the partial pressure of oxygen in the flow.
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Figure 6. An illustration of an electrochemical sensor. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf [6]
Amperometry
Amperometry is a technique used to detect ions in a solution based on electrical current produced by electrochemical reaction of an electro-active species.
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Figure 7. A simplified diagram of a polarographic sensor. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf [6]
Limitations
- Warm-up time for this type is approximately 10 minutes. Wrong readings will occur if measurements are made when the required amount of time has not been attained.
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Figure 8. A simplified diagram of a galvanic sensor and circuit. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf[6]
Limitations
- The sensor continuously consumes the anode, even when truned off. Therefore the lifetime of the sensor is much sorther than of the polarographic sensor and the warranty is usually for 6 months only.
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For both electrochemical and optical dissolved oxygen sensor, they do not measure the concentration of dissolved oxygen in mg/L or ppm (parts per million which is equivalent to mg/L). Instead, the pressure of oxygen that is dissolved in the sample is being measured. To interpret the readings from the measurement, the pressure of the dissolved oxygen is expressed as DO % Saturation. To explain this in detail, the instrument converts the dissolved oxygen pressure value from the sensor to % Saturation by dividing the sensor output in mmHg by 160*** (the pressure of oxygen in air at 760 mmHg) and then multiplying 38 39by 100. For example, a measured oxygen pressure of 150 mmHg would be displayed by a sensor as 93.8 % Saturation (150/160 * 100). Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf[6]
***The pressure of oxygen at sea level is 160 mmHg because oxygen is about 21% of the earth’s atmosphere and 21% of 760 (average sea level barometric pressure) is about 160 mmHg.
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: https://www.fondriest.com/pdf/ysi_do_handbook.pdf. DO sensors measure % saturation. Source: [6]
Variables that affect DO measurements
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Similarly with temperature, increasing water salinity decreases its ability to dissolve oxygen. Some of the DO sensors measure also conductivity, and the value is used for calculating salinity and, based on that, oxygen concentration. If built-in conductivity sensor is available, it is important to ensure that it is calibrated and working correctly. If the conductivity is measured with separate sensor, the salinity value must be entered by the user.
https://www.fondriest.com/pdf/ysi_do_handbook.pdf [6]
Pressure
As mentioned earlier, DO sensors measure the dissolved oxygen pressure in the water (or air), not the absolute concentration. This pressure depends not only on the oxygen concentration, but also on the atmospheric (barometric) pressure, which varies according to elevation and weather. The atmospheric pressure is not, however, needed to be known to obtain correct concentration values. Proper calibration of the sensor is enough to ensure proper measurements.
When the sensor is calibrated, known atmospheric pressure is used. After calibration, the measurements are correct, even though the pressure would change.
https://www.fondriest.com/pdf/ysi_do_handbook.pdf even though the pressure would change. [6]
Some of the DO sensors do the pressure compensations automatically; see for example Hanna Instruments's model HI 98186.
http://www.hannainst.com/usa/prods2.cfm?ProdCode=HI%2098186&id=004002 [4]
Calibration
Electrochemical sensors are more prone to drift and require more frequent calibrations than optical sensors. In principle, teady-state galvanic and polarographic sensors need calibration daily when in use. If the measurements, however, are reliable also with less frequent calibrations, calibration frequency can be reduced.
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In general, electrode maintenance should only be necessary about once per year for steady-state polarographic sensors and about once every 3 months for steady-state galvanic sensors. Do not perform the following maintenance procedures just because a sensor ‘looks’ dirty. Only perform the following procedures if the instrument will not calibrate or if the readings are unstable and a regular membrane change did not correct either of these problems. https://www.fondriest.com/pdf/ysi_do_handbook.pdfeither of these problems. [6]
The following points should be emphasized when carrying out the cleaning and maintenance process. Source: http://www.eidusa.com/Theory_DO.htm#Cleaning of sensors [1]
- The sensor must be disconnected from the meter. When the sensor is connected and submersed in the cleaning solution, no chemical reaction takes place between the solution and the oxidized reference electrode surface; instead, the cleaning solution may become electrolyzed!
- Use the cleaning or electrolyte solution suitable for the particular sensor as stated in the operating manual! A solution that is suitable for silver electrodes cannot regenerate lead electrodes!
- Only the gold cathode should be polished; the counter electrode is merely wiped clean with a soft cloth to wipe away easily removable salt deposits! A spotty coating after regeneration of the lead or silver electrodes does not impair measurements!
- When polishing the gold electrode, only use the moistened EID abrasive film that has a special grain that polishes and do not scratch!
- It is also recommended to use a new membrane head since the used membrane cannot necessarily guarantee that the membrane fits correctly against the gold cathode which is ensured by a spacing lattice on the inside of the membrane. Baggy clothing don't fit either!
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C. Sea cages
Since it is difficult to control the dissolved oxygen content of the sea, dissolved oxygen measurement is very important because the feed uptake and dissolved oxygen levels are interconnected. Intensive feeding after fish have experienced low dissolved oxygen levels can not only be a waste of food, but can actually harm the fish. The measurement of dissolved oxygen levels enables feed to be dosed optimally and, if relayed to the shore can warn that the cage should be moved if extremely low dissolved oxygen levels should occur.
D. Waste Water Treatment
Waste water treatment is critical in these years. It is no longer enough just to filter the water and dump it into the sea directly. The larger part of the waste is mainly organic, and this must treated in sludge tanks to break it down for further filtration. Sludge tank dissolved oxygen measurement and control is kept. Flow measurement, such as suspended solids measurement, sludge blanket detection, conductivity measurement, nitrate measurement and phosphate measurement utilizing the DO sensors are also all used to enable the efficient and effective cleaning of waste water.
E. Safety Monitoring
DO sensors can be utilized for safety monitoring such as oxygen detection in flammable gas and oxygen monitoring in ambient air. Blanket gas is often used where flammable substances occur. Blanket gas is gas that cannot burn or sustain fire, i.e. it does not contain oxygen. Volumetric oxygen measurement is carried out both on the blanket gas and the surrounding air, the latter for worker safety. Special versions of the dissolved oxygen electrodes are approved for use in potentially dangerous atmospheres, i.e. in classified areas.
References
Erlich [1] Erlich Industrial Development. D. O. theory. http://www.eidusa.com/Theory_DO.htm
Eutech [2] Eutech Instruments, 1997. Introduction to Dissolved Oxygen. http://www.eutechinst.com/techtips/tech-tips15.htm
Fraden [3] Fraden J. Handbook of Modern Sensors. New York: AIP Press/Springer; 2004. p. 508-510.
Hanna [4] Hanna instruments. Dissolved Oxygen and BOD meter, HI98186. http://www.hannainst.com/usa/prods2.cfm?ProdCode=HI%2098186&id=004002
Regtien [5] Regtien P. Measurement Science For Engineers. London: Kogan Page Science; 2004. p. 257-263.
YSI[6] YSI, 2009. The Dissolved Oxygen Handbook. https://www.fondriest.com/pdf/ysi_do_handbook.pdf
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