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.
Background
Dissolved Oxygen is really a physical distribution of oxygen molecules in water. There are two main sources of DO in water: atmosphere and photosynthesis.
Ambient air contains about 20% oxygen and is essential for breathing, fish and other aquatic organisms need oxygen to breathe as well. 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 already 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” (http://www.eidusa.com/Theory_DO.htm).
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.
Figure 1. Diagram of sensor types. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
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: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
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.
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
The probe emits a blue light of the proper wavelength that causes the dye in the sensing element to luminesce or glow red. Oxygen constantly diffuses through the paint layer, affecting the luminescence of the sensing layer. The amount of oxygen passing through to the sensing layer is inversely proportional to the lifetime of the luminescence in the sensing layer.
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)), so the lifetime of luminescence of the blue light is compared to it, and the stable oxygen concentration is calculated by the probe.
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: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
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
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. Instead of measuring the DO concentration, the electrode senses DO activity (or tension).
Reduction reaction at the cathode: O2 + 4e + 4H+ → 2H2O
As in the case for the polarographic electrodes, a voltage is applied externally while an internal potential is generated as in the galvanic electrodes.
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.
A reduction reaction will occur when a suitable potential is applied to the electrode: ox + ne → red
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:
ired = - nFAJox ↔ Jox = - ired / nFA
Fick's law of diffusion: ......
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Polarographic Electrode
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.
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Limitations
- Response time is described as the time required for the electrode to reach >90% of the output. Typical response time for polarographic sensors are is 30 sec, which makes them not compatible to be used for dynamic measurements.
- 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.
- Chloride ions in the electrolyte will be eventually consumed resulting in gradual drift in the electrode signal. The electrolyte must be replaced.
- Since the electrode consumes oxygen, readings are affected by flow across the sensor tip. Thus enough flow rate at the membrane (or sample renewal rate) must be ensured for accurate results.
Galvanic Electrode
Measuring dissolved oxygen with either sensor type
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).
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Temperature
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:
- Diffusion of oxygen through the membrane (electrochemical) or sensing element (optical) on the probe increases/decreases with higher/lower temperature due to change in molecular activity (up to 4% difference per °C).
With digital sensors, the effect of temperature can be compensated with software, as the temperature is known; with analog sensors, compensation is done by adding a thermistor (a temperature-sensitive resistor) into the circuit. - Ability of water to dissolve oxygen is directly proportional to temperature. Warmer water dissolves less oxygen than colder water. Consequently, with same saturation rate, warmer water contains less oxygen in absolute terms. The absolute (mg/L) concentration must be therefore compensated according to the temperature of the sample.
Salinity
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
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
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.
Optical sensors are more stable than traditional electrochemical sensors. It has been shown that optical sensors can hold their calibration for months. It is still recommended to calibrate the sensor regularly to obtain most correct measurements. The calibration is done by measuring known sample and comparing the measured value to the known real value.
Three main methods for calibrating DO sensor are
- Winkler titration
- Air-saturated water
- Water-saturated air
References
Contributors:
Eila Myllylä, Chi-En Shan, Olga Gerasimenko, Judit Nyári & Nhat Nguyen