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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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Some of the DO sensors do the pressure compensations automatically; see for example Hanna Instruments's model HI 98186.
http://www.hannainst.com/usa/prods2.cfm?ProdCode=HI%2098186&id=004002
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.
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- Winkler titration
- Air-saturated water
- Water-saturated air
Cleaning and Maintenance
[need to be paraphrased]
The component of the sensor that is sensitive to contamination is the membrane. Contamination results in lower readings when measuring or lesser slopes when calibrating because a portion of the membrane surface is not available for the diffusion of oxygen. The attempt to compensate for the contamination by adjusting the instrument does not agree with the water principle. It is preferable to clean the membrane. Acetic or citric acid with a concentration of 5--10% (percent in weight!) is used for calcium and iron oxide deposits and warm (<50C) household detergent is used for fats and oils.
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Display of the concentration: The instrument requires the appropriate data of the calibration curve and uses them to calculate the concentration in mg/L (ppm is identical in this case), allowing for the temperature dependency of the individual parameters
Display of the percentage of oxygen saturation: The instrument measures the sensor current and calculates the partial pressure of oxygen according to the calibration. The current air pressure is measured for the calculation of the saturation partial pressure. The display corresponds to the quotient, converted into a percentage.
References
Erlich Industrial Development. D. O. theory. http://www.eidusa.com/Theory_DO.htm
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