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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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Dissolved Oxygen
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. [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 lethal to fish: the amount of oxygen level below 2 mg/l is deadly and the amount between 2 and 5 mg/l affects fish health.
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” [1].
Types of DO sensors
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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The oxygen concentration is determined with the Stern-Volmer equation , that sets the relationship between luminescence lifetime (intensity) and oxygen concentration see Figure 4.
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A reduction reaction will occur when a suitable potential is applied to the electrode: ox + ne- → red
Where ox is the oxidized species, red is reduced species, n is the number of electrons transferred and e- is an electron.
A concentration gradient of ox caused by its depletion at the electrode surface leads to mass transport by diffusion. This leads to a flux of ox, Jox (mol/m2s) that related to the reduction current, ired, through the electrode with an area A according to Faraday’s law [5]:
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∂C/∂x = concentration gradient (x = 0 => C = 0, means ; the concentration of ox at the electrode drops to 0)
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- Fast response time therefore best suited where fastest response time is necessary or for huge amount of measurmentsmeasurements.
Galvanic Electrode
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.
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- The sensor continuously consumes the anode, even when truned turned off. Therefore the lifetime of the sensor is much sorther shorter than of the polarographic sensor and the warranty is usually for 6 months only [2].
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- No warm up time, it can be sed set immediately after turn-off
- The electrolyte is never used up, ; in theory it can be used forever.
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Measuring dissolved oxygen with either sensor type
For both Neither electrochemical and nor optical dissolved oxygen sensor, they sensors do not measure the concentration of dissolved oxygen in mg/L or ppm (parts per million which is equivalent to mg/L). Instead, they measure 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 -saturation. The instrument converts the dissolved oxygen pressure value from the sensor to % Saturation -saturation by dividing the sensor output in mmHg by 160*** (the pressure of oxygen in air at 760 mmHg) and then multiplying 38 39by by 100. For example, a measured oxygen pressure of 150 mmHg would be displayed by a sensor as 93.8 % Saturation -saturation (150/160 * 100). Source: [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 can be illustrated by two water samples: one of fresh water and the other of sea water. The 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 Figure 9 for an example of this concept.
Figure 9. DO sensors measure %-saturation. Source: [6]
Variables that affect DO measurements
There are several parameters that affect the DO measurement accuracy and reliability, they are temperature, salinity, atmospheric (barometric) pressure and flow (stirring). Temperature, salinity and pressure are discussed below.
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Temperature is the most significant variable for the measurement accuracy. Therefore, it should be ensured that the temperature sensor on the probe is working correctly. Temperature can influence the DO measurement in two ways [6]:
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Electrochemical sensors are more prone to drift and require more frequent calibrations than optical sensors. In principle, teadysteady-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. [6]
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Three main methods for calibrating DO sensor are
- Winkler titrationtitration (See, for example http://web.colby.edu/colbyatsea/2011/02/11/winkler-titrations-measuring-dissolved-oxygen/ or http://water.usgs.gov/owq/FieldManual/Chapter6/Archive/Section6.2.pdf)
- Air-saturated water (See, for example http://water.usgs.gov/owq/FieldManual/Chapter6/Archive/Section6.2.pdf)
- Water-saturated air (Hanna Instruments's video of calibration: http://www.youtube.com/watch?v=sxI_PS7b8XI) [6]
Cleaning and Maintenance
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- 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 dondoesn't fit either!
Note:
- The spacing lattice is clearly visible when the membrane head is held up against the light.
- Always re-calibrate an instrument after changing a membrane.
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B. Aquaculture (Fish Farming)
Dissolved oxygen sensors, such as multi-channel dissolved oxygen meters, are needed for fish farmers. It is essential to have such instrument to measure and control the dissolved oxygen level in the water body. Dissolved oxygen monitoring and logger are encompass encompassing alert units with both high dissolved oxygen alarm and low dissolved oxygen alarm.
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You can find different DO sensors with price in the link for the two company companies below.
In general, optical sensors are much more expensive than electrochemical sensors.
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