Dissolved oxygen concentration is an important parameter in water quality measurements, since it has a direct impact on organisms living within the water. Over a very short space of time, dissolved oxygen (D.O.) can change from optimal to lethal levels. It is therefore essential that corrective action be taken as quickly as possible, making it critical to have a rapid and reliable method of measuring dissolved oxygen concentrations. Electrochemical oxygen sensors were the first generation of sensors to be adopted for dissolved oxygen measurement, replacing the traditional titration methods. However, they characteristically lack accuracy and stability and require lengthy calibration. The past year has seen the introduction of new optical dissolved oxygen sensors that have been designed to achieve the highest levels of accuracy throughout the lifetime of the sensor. This article discusses the limitations associated with conventional electrochemical oxygen sensors and introduces a new technology that provides a viable solution for fast, accurate, and reliable measurement of dissolved oxygen in various applications in the biotechnology, beverage, and environmental industries.
An optode is a sensor used for optical measurement, functioning with the use of special dyes embedded in membranes that have come into contact with the surface of a sample. Unlike the traditional Clark cell method for D.O. measurement, when using an optode there is no chemical reaction with mass and energy exchange. This type of reaction is called fluorescence and involves the molecules of the dye being excited by light. When the molecules fall back into a ground state, the energy received is dispersed as light with an increased wavelength.
Depending on their concentration, specific agents, also known as quenchers or oxygen molecules, influence the use of the fluorescent reaction method. Oxygen molecules take over the energy of the excited state, preventing the dye from sending out fluorescent light; as a consequence, the signal is quenched. The intensity of fluorescence is lowered when the concentration of the quencher increases and the lifetime of the excited state is reduced. The reflected light undergoes a phase shift that enables the concentration to be calculated.
Challenges for effective dissolved oxygen measurement
Although dissolved oxygen measurement offers a wealth of benefits in comparison to conventional electrochemical sensors, it is also associated with various challenges. One of the most recognized benefits of D.O. measurement is that the need for calibration is eliminated, i.e., the membrane and electrolytes never have to be changed, and consistent flow is not required. In addition, optical sensors are unaffected by hydrogen sulfide (H2S) contaminants and demonstrate a fast response time.
One of the most significant challenges faced in this process is that the dye used in the sensors has a tendency to degrade over time due to bleaching from light exposure. Thus, it is vital that users manually replace the sensor cap periodically. Additionally, optical components such as light-emitting diodes (LEDs) also deteriorate with age. For this reason, frequent calibration is necessary. This lack of accuracy and lack of stability is a major flaw in first-generation optical oxygen sensors.
New-generation optical dissolved oxygen sensors provide a solution to these challenges. These technologies, such as the IQ SensorNet with the FDO™ 700 IQ optical D.O. sensor from Xylem Analytics (White Plains, NY) (available in the U.S. through YSI [Yellow Springs, OH]), achieve the best possible accuracy from initial implementation while maintaining this precision throughout the entire product lifespan.
Wastewater treatment methods and environmental applications
Dissolved oxygen measurement is often used in wastewater treatment, river monitoring, and fish farming, as well as in the profiling of lakes, brackish water, and specific consumption tests. In addition, D.O. measurement ensures the safety of groundwater and drinking water. The latest advancements in the technology fulfill the needs of each individual application within a single sensor system.
Dissolved oxygen sensors must be active and kept outside year round, with only short interruptions for maintenance work. To ensure reliable and continuous operation in all weather conditions, the new sensors possess important characteristics such as mechanical ruggedness, water resistance, long service and calibration intervals, insensitivity to electromagnetic interferences, and ability to withstand atmospheric exposure.
Unlike in wastewater treatment applications, the emphasis of D.O. measurements in the laboratory is on precision and measurement conditions. One of the most important parameters when conducting measurements in a laboratory setting is the flow of the sample on the sensor. In contrast to their electrochemical counterparts, which require the continued consumption of oxygen in order to start a reaction, optical sensors do not require oxygen.
Biochemical oxygen demand (BOD)
Rapid measurement is vital in the laboratory in order to minimize time and costs, as well as to enable specific applications such as BOD measurement. Quick measurement ensures that the sensor avoids contact with atmospheric oxygen when being transferred from saturation into a depleted sample.
Specially adapted membranes and measuring technologies should be implemented to meet the above requirements. Modern optical laboratory sensors are versatile and, with the use of appropriate accessories, cover a multitude of laboratory applications. The sensors have been designed for their intended application and are optimized in size for the working area, have smooth surfaces for cleaning, and are easy to operate.
A D.O. measurement system designed for field applications should have the ability to operate in a broad range of conditions and environments. It is important that the system is water resistant and demonstrates a fast response time. For example, in order to measure the depth profile of a lake for detecting the thermocline, a fast-responding sensor is needed because the D.O. concentration changes within 10 cm, and the sensor must be able to trace these alterations. The latest instrumentation has been designed for this type of application.
A common application for portable systems in the wastewater process is the monitoring of on-line systems, e.g., in an aeration tank. Optical D.O. sensors for process and laboratory must feature different design and membrane qualities. It is almost impossible to increase the speed of a “slow” membrane. However, with intelligent electronics, it is simple to slow down a fast membrane electronically. This makes it possible to adjust a laboratory or field sensor so that it demonstrates exactly the same characteristic curve as a process sensor. As a result, this ensures that both curves can be easily compared and checked for correct operation.
Next-generation dissolved oxygen sensors
Recent technological breakthroughs have aided the development of new sensors that can streamline overall processes and significantly improve accuracy. The membrane cap onto which the dye is applied is a key component of D.O. measurement. Next-generation technology allows each cap to be individually calibrated at the production stage. However, even if the membrane is stable, the dye must still be considered variable. To achieve a high level of accuracy, each membrane should be examined and calibrated separately, and the value stored on the membrane. A memory chip that is permanently attached to the membrane ensures that the sensors meet this requirement.
A sensor’s optics and electronics drive its overall accuracy. The reading of a particular concentration correlates with the displacement of phase-of-light caused by changes in oxygen. Therefore, a relative time reading in the microsecond range must be performed, requiring a high level of accuracy and stability. A solution to this problem is to calibrate the running time of a light beam against the speed of light. With recent advances in technology, the running time of a light beam by a reference path can be used to align the optics and the electronics, offering a level of accuracy that exceeds other methods.
Dye plays a significant role in optical oxygen measurement, providing a support material that is activated by shortwave light. In first-generation sensors, the dye is activated by blue light and has a higher level of energy than green light. This leads to a premature bleaching of the dye, making recalibration or replacement necessary. Green light’s lower level of energy eliminates premature aging and secures a longer lifespan.
To provide a solution to the challenges posed by the aging process of optics, new reference technologies are entering the market. This technology only functions when measuring reference channels that age equally. If differing parts are used—for example, LEDs—a different energy balance is shown that leads to uneven aging. Innovative reference technology ensures that measuring channels and reference channels, including all parts, are completely identical in construction.
Optical dissolved oxygen measurement is necessary in a variety of applications, each of which has its own requirements. The use of D.O. measurement changes across these applications, with the main difference being permanent or temporary operation. As a way to optimize efficiency, it is important to have adapted sensors for specific tasks, with their uses determining the specific design of the sensor and membrane.
Various shortcomings, such as lack of accuracy and stability, are associated with first-generation optical sensors. New technological breakthroughs overcome these problems. The novel generation of sensor technology demonstrates such advantages as insensitivity to incoming flow and insensitivity in relation to certain chemical contents of the water. In contrast to conventional electrochemical products, these benefits are achieved without serious limitations in applied use.
Robert Hengel, Dipl.-Ing., is Director of Marketing, and Dr. Klaus Reithmayer is Product Manager, WTW, a Xylem brand, 600 Unicorn Park Dr., Woburn, MA 01801, U.S.A.; tel.: 781-937-4100; fax: 781-937-4101; e-mail: firstname.lastname@example.org.