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Measuring dissolved oxygen with either sensor type
For both electrochemical and optical dissolved oxygen sensor, they do not measure the concentration of dissolved oxygen in mg/L or ppm (parts per million which is equivalent to mg/L). Instead, the pressure of oxygen that is dissolved in the sample is being measured. To interpret the readings from the measurement, the pressure of the dissolved oxygen is expressed as DO % Saturation. To explain this in detail, the instrument converts the dissolved oxygen pressure value from the sensor to % Saturation by dividing the sensor output in mmHg by 160*** (the pressure of oxygen in air at 760 mmHg) and then multiplying 38 39by 100. For example, a measured oxygen pressure of 150 mmHg would be displayed by a sensor as 93.8 % Saturation (150/160 * 100). Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
***The pressure of oxygen at sea level is 160 mmHg because oxygen is about 21% of the earth’s atmosphere and 21% of 760 (average sea level barometric pressure) is about 160 mmHg.
The fact that the sensor measures the pressure instead of the concentration for dissolved oxygen is known to be true because a sample of fresh water can dissolve more oxygen than a sample of sea water at the same temperature and at the same altitude (or under the same barometric pressure). However, the sensor’s output signal is identical in both samples since the oxygen pressure is identical in both media. See the following figure for an example of this concept.
Figure [?]. DO sensors measure % saturation. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Variables that affect DO measurements
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