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Optical DO Sensors
Optical dissolved oxygen sensors measure how oxygen and luminescent dyes interact in the presence of blue light. The light excites the dyes, causing them to emit light. However, oxygen molecules interact with the dyes when DO is present, which alters or limits the wavelengths of the emitted light.
There is an inverse relationship between the effect measured this way and the partial dissolved oxygen pressure. More precisely, expressed by the Stern-Volmer equation, the concentration of DO, measured by its partial pressure, is inversely proportional to luminescence lifetime.
Four elements comprise an optical dissolved oxygen sensor: a blue light-emitting diode (LED), a sensing element, a semi-permeable membrane, and a photodetector. The luminescent dye is typically held immobile in xerogel, sol-gel or another matrix in the sensing element. Some sensors also have a red LED so they can ensure accuracy with a red light reference that the dye will merely reflect.
The amount of dissolved oxygen present in the sample controls the dye’s luminescence lifetime and intensity when blue light is present. Oxygen interacts with the dye as it permeates the membrane, limiting the dye’s luminescence lifetime and intensity. The photodetector then measures the lifetime or intensity of the returned luminescence, which in turn is used to determine the concentration of dissolved oxygen.
There are both benefits and drawbacks to using optical DO sensors. On the one hand, optical dissolved oxygen sensors often provide more accuracy than do electrochemical sensors, particularly at very low concentrations. Optical DO sensors are also not affected by gasses that can cross the membrane of an electrochemical DO sensor.
Furthermore, due to their minimal maintenance requirements, optical dissolved oxygen sensors are perfect for long-term monitoring programs. They show little calibration drift, holding a calibration for multiple months. They can also take measurements without any stirring or time for warming up. Sensor membrane replacement is less frequent in optical sensors, as well.
On the other hand, it takes more power to run optical dissolved oxygen sensors, and they generally require a two to four times longer period to provide you with results than do their electrochemical cousins. Temperature also has a significant impact on these sensors, as both luminescence lifetime and intensity depend on it. However, most optical DO sensors use a thermistor which automatically adjusts for temperature and corrects the data.
The bottom line: for a long-term monitoring project with a reliable power source, especially in remote areas that are difficult to service, an optical dissolved oxygen sensor is often the best bet.
Electrochemical DO Sensors
Electrochemical dissolved oxygen sensors come in two basic varieties: polarographic and galvanic. Both varieties include an electrolyte solution containing a cathode and an anode—two polarized electrodes. A thin, semi-permeable membrane separates the solution containing the electrodes and the sample.
Depending on how high the oxygen pressure in the water is, the dissolved oxygen moves past the membrane at a proportional speed. When it reaches the cathode, the oxygen is then consumed, creating an electrical current that relates directly to the concentration of oxygen. Finally, the current reaches the anode, at a rate that’s proportionate to the sample’s partial pressure of oxygen.
Electrochemical sensors can be tricky in still water or the laboratory, where DO sensors must be stirred in solution to avoid artificially low measurements of DO under no-flow scenarios. To avoid this issue, simply stir electrochemical DO sensors in the sample until the DO levels stop increasing.
Polarographic DO Sensors
Polarographic dissolved oxygen sensors can be either rapid-pulsing or steady-state sensors. A rapid-pulse polarographic dissolved oxygen sensor works similarly to a steady-state polarographic DO sensor, and both have the same electrodes and processes. However, a rapid-pulse polarographic dissolved oxygen sensor turns on and off every few seconds, reducing flow dependence, and eliminating the need to stir the sample.
Both types of polarographic DO sensor consist of a cathode made of a metal in the “noble” category such as platinum or gold, and a silver anode, both in a solution of potassium chloride. Other than that, they work similarly to the way other electrochemical sensors function.
Polarographic DO sensors have both advantages and disadvantages. Their benefits include being very cost-effective, and offering a fast response time once they’re in action. In terms of disadvantages go, this kind of sensor requires some warm-up time before it can give a reading—usually from five to 60 minutes. They also require frequent maintenance, especially since the anode’s coating can become oxidized, degrading its performance.
The bottom line: for projects on a shoestring budget that need a response time where warm-up time is available, this is the best kind of sensor.
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