Water & Wastewater Treatment

In the know Technically speaking: dissolved oxygen measurement Explanation of the working principles of an optical sensor 40 | octobEr 2015 | WWt | www.wwtonline.co.uk be as accurate as possible. In most cases, this will require the sample to be stirred constantly at the sensor tip to produce the necessary oxygen levels for an accurate reading. One drawback of polarographic sensors occurs during start-up. Unlike galvanic sensors, polarographic sensors require several minutes for their probes to polarise. The requirement for a constant current means that the sensor consumes more power than other sensor types, making it comparatively less cost-effective. Although electrochemical sensors have been proven to offer similar levels of accuracy to optical devices, their requirement for a constant flow and their susceptibility to fouling or clogging make them comparatively less reliable under non-ideal monitoring conditions. Where this occurs, the risk of inaccurate measurement and inefficient blower control is greatly increased. Continued sensor dri-, coupled with fouling of the sensor membrane, also means that frequent maintenance, including calibration, is needed, ranging from once a month to once a day in extreme circumstances. Optical sensors – presenting a solution for dissolved oxygen measurement Optical sensors overcome many of the limitations associated with their electrochemical counterparts. The most advanced dissolved oxygen sensors work on the 'dynamic luminescence quenching' principle, a light-based measurement technique. These sensors are comprised of lumiphore molecules embedded in a sensing element, plus blue and red LEDs and a photodiode. When a reading is taken, the lumiphore molecules are excited by blue light from the blue LED. When excited, these molecules emit a red light, which is detected by the photodiode. Any oxygen molecules present will quench the excited lumiphore molecules, reducing the amount of red light being emitted. The shi- in the amount of red light is then measured by the red reference LED. As dissolved oxygen concentration and the amount of red light being returned are proportional, a measurement can be taken and converted into a reading based on mg/l. A key benefit of optical measurement technology is its stability and accuracy. The luminescence lifetime technique is used to measure the phase shi- between the returned red light and the red reference light. Using optical sensors for dissolved oxygen measurement can help to overcome many of the problems associated with electrochemical devices. With no requirement for sample flow or stirring to artificially raise dissolved oxygen levels, they can provide high accuracy even in low oxygen (hypoxic) conditions. They also have a low maintenance requirement compared to membrane- based sensors. ABB's ADS430 optical DO sensor, for example, features a robust design capable of withstanding the problems that can affect conventional membrane- based sensors, such as abrasion, fouling or poisoning, whilst the sensor lumiphore is not affected by photobleaching or stray light. The sensor is also immune to the effects of sulfides, sulfates, hydrogen sulfide, carbon dioxide, ammonia, pH, chloride and other interferences. This enables it to provide consistent, accurate readings over long periods of time without suffering from sensor dri-. The sensor also features a smart sensing cap, which comes pre-loaded with factory calibration coefficients, serial number, lifetime indication, and manufacture date which are automatically uploaded to the sensor, eliminating the time normally required for set-up. By automatically prompting the user when replacement is due, the cap removes the risk of unexpected sensor failure. When cleaning is necessary, the cap can also be cleaned and redeployed without calibration. Summary With aeration accounting for well over half of a plant's total energy costs, accurate control of dissolved oxygen levels presents a key step in minimising operational costs. This, coupled with the inherent maintenance benefits of optical sensors, makes the technology an attractive solution for use in aeration processes.

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