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As mentioned earlier, DO sensors measure the dissolved oxygen pressure in the water (or air), not the absolute concentration. This pressure depends not only on the oxygen concentration, but also on the atmospheric (barometric) pressure, which varies according to elevation and weather. The atmospheric pressure is not, however, needed to be known to obtain correct concentration values. Proper calibration of the sensor is enough to ensure proper measurements.
When the sensor is calibrated, known atmospheric pressure is used. After calibration, the measurements are correct, even though the pressure would change.
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
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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