Optical Dissolved-Oxygen - Measurement Specialties, Inc.

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Eureka 2 Optical Dissolved-Oxygen Sensor (HDO) How is DO measured? There are two types of sensors commonly used to measure DO. The traditional Clark Cell consists of two electrodes surrounded by a water-based electrolyte solution and covered with an oxygen-permeable membrane. As oxygen crosses the membrane to dissolve in the electrolyte, it is consumed in a chemical reaction which generates a small electrical current between the two electrodes. That current is directly proportional to the amount of oxygen in the water sample. This method is further described in Standard Methods 4500-O G. Eureka’s Clark Cell DO Sensor A weakness of the Clark Cell is that it consumes oxygen from the water surrounding the sensor; this means that the water must be replaced either by natural flow or the use of a circulator to prevent the readings from drifting downward. The Clark Cell has performed will in the field for decades, but is slowly being replaced by optical DO sensors. The latter have little calibration drift in the field, are not flow-sensitive (no circulator needed), and do not require the occasional membrane changes that annoy Clark Cell users. Clark Cell’s circulator Clark Cell DO sensor The second type of DO sensor is the optical DO sensor, in which a blue light is directed to an oxygen-active compound that has been stabilized in an oxygen-permeable polymer. The blue light causes the oxygen-active compound to fluoresce – i.e. it absorbs energy in the form of blue light and then emits energy as red light. The fluorescence is quenched by oxygen – that is, the emission of red light is reduced if oxygen molecules are present to interfere with the oxygen-active compound. The more oxygen present, the smaller the amount of red light produced. When the polymer sensing surface is exposed to water, oxygen diffuses into the sensing surface according to the amount (“partial pressure”) of oxygen in the water. Thus, the amount of red light received by the sensor is directly relatable to the amount of oxygen in the water. The red light signal is calibrated to the proper DO units. Despite its considerably higher cost, the optical sensor is often favored over the Clark Cell because the optical sensors have little calibration drift in the field, are not flow-sensitive (no circulator needed), and do not require the occasional membrane changes that annoy Clark sensor users. Eureka 2 HDO Sensor www.meas-spec.com September 2012 2113 Wells Branch, Suite 4400, Austin TX 78728 512-302-4333 WQ.sales@meas-spec.com
Eureka 2 Optical Dissolved-Oxygen Sensor (HDO) The amount of oxygen dissolved in, for instance, a lake or river, depends on several variables. The higher the barometric pressure, the more oxygen can dissolve in water. And the higher the water temperature, the less oxygen can dissolve in water. Eureka’s Intensity-Based Optical DO Sensor If the water has absorbed as much oxygen as it can for a particular combination of temperature and barometric pressure, the water is said to be saturated with oxygen. If, on average, no oxygen is moving into the water or out of the water, the oxygen in the water is said to be in equilibrium with the oxygen in the atmosphere. Dissolved oxygen (DO) is commonly reported in two units. DO concentration is the weight of oxygen dissolved in water, and is reported in mg/l or ppm. DO percent saturation is the ratio of oxygen actually in the water to the maximum amount of oxygen that can dissolve in a water sample under the same conditions, and is reported in % saturation. Clark Cells were traditionally calibrated in water-saturated air, but calibration in air-saturated water is growing in favor. The latter is done by shaking a half-liter of water in a one-liter container for one minute, and then waiting one minute for the bubbles to rise to the surface and disappear. The Clark Cell is immersed in that water and given time to stabilize. With knowledge of the water temperature and the barometric pressure, the instrument can figure out the DO level in the water because it knows that the water is saturated with oxygen. The instrument sets the Clark Cell reading accordingly. The Clark Cell can also be calibrated in water with a DO determined by, for instance, a Winkler titration. Optical DO sensors can also be calibrated in air-saturated water, with the same process. Some optical DO sensor manufacturers recommend that the user also calibrate at zero DO, and some say that zero calibrations are rarely necessary. What should I know about DO measurement in the field? Flow - Clark Cells are sensitive to water flow, but optical sensors are not. Low water flow results in depleted DO near the active surface of a Clark Cell; this produces falsely low sensor readings. Traditionally, a sample flow rate of one ft/sec is required for proper Clark Cell operation. Eureka’s sample circulator provides adequate flow for accurate DO measurement, and also somewhat reduces biological fouling and insures the measurement is being taken on a representative sample. Temperature - Both Clark Cell and optical DO sensors are intrinsically sensitive to changes in temperature, but the correction for this sensitivity is done automatically by the instrument. The behavior of oxygen in water is also intrinsically sensitive to changes in temperature. The saturation level of oxygen in water increases with decreasing temperature. If you have oxygen-saturated water at 25°C and slowly raise the temperature of the water to 30°C, oxygen will leave the water because hot water won’t hold as much dissolved oxygen as cold water. This phenomenon doesn’t matter to DO concentration (mg/l) measurements. But it matters to DO saturation (% sat) measurements because % sat relies on knowledge of a water’s DO saturation point. Fortunately, Eureka instruments automatically compensate for this effect. Barometric pressure (BP) - If you have oxygen-saturated water at one standard atmosphere (760 mmHg) and slowly lower the BP to 740 mmHg, oxygen will leave the water because there’s just not enough air pressure to force the oxygen to stay dissolved. When you calibrate DO, you must tell the instrument the local barometric pressure so that it can calculate the proper relationship between concentration and saturation. But suppose the barometric pressure changes after calibration. The concentration readings will be correct, but the saturation readings will not be correct – you must tell the instrument the new BP so that it can make the proper calculations. Eureka 2 HDO Sensor www.meas-spec.com September 2012 2113 Wells Branch, Suite 4400, Austin TX 78728 512-302-4333 WQ.sales@meas-spec.com
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Optical Dissolved-Oxygen - Measurement Specialties, Inc.

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