Another factor with regard to potential affects in your dissolved oxygen calibration and measurement is barometric pressure.
You might want to grab a coffee, or a beer, we're going to get into some math right away here. Let's do this!
Barometric pressure affects the pressure of oxygen in a sample of air or water. For example, the percentage of oxygen in air is always 21%, but the actual pressure of oxygen varies with changes in barometric pressure. At sea level, the pressure of oxygen is 160 mmHg (.21 x 760 mmHg).
In a fully aerated sample, under these conditions, the % saturation measured by a sensor would be 100%(160/160 x 100%). If the temperature of the sample is 25ºC, the instrument would calculate the dissolved oxygen concentration as 8.26 mg/L based on the Oxygen Solubility Table. As the sample is moved up in altitude and kept air-saturated, the barometric pressure would decrease and so would the pressure of oxygen in the sample. At 1126 ft of elevation, the pressure of oxygen would be 153 mmHg (.21 x 730 mmHg) and the % saturation relative to sea level read by the probe would be 95.6% (153/160 x 100%) in the fully aerated sample. If the temperature of the sample is 25ºC, the instrument would calculate a dissolved oxygen concentration of 7.92 mg/L or 96% of 8.26 based on the Oxygen Solubility Table.
The effect of barometric pressure is overcome by proper sensor calibration. Barometric pressure is used in the majority of dissolved oxygen sensor calibrations as described in the Calibration section of The Dissolved Oxygen Handbook since it determines the absolute pressure of oxygen in a sample of air or water at the time of calibration and it is this pressure which is measured by all oxygen sensors. When calibrating oxygen sensors, the sensor’s output is set to this known pressure of oxygen. If the sensor output changes after calibration, then the instrument would calculate a % saturation based on a simple linear regression calculation.
Thus, as long as the system does not drift, the sensor’s output can always be used to define the oxygen pressure in any medium after performing a proper calibration and the use of the barometric pressure (or altitude) at the time of calibration is the key factor in setting the proper calibration coefficient. Therefore, it is not necessary to correct for changes in barometric pressure after performing a proper calibration in order to obtain accurate readings in the field. OK, we're going to say this one again - seriously - we're repeating it and underlining it. It is not necessary to correct for changes in barometric pressure after performing a proper calibration in order to obtain accurate readings in the field.
In summary, as barometric pressure changes due to a change in altitude or local weather front, the pressure of oxygen changes. However, there is never any reason to compensate for this change if a proper calibration has already been performed and the sensor has not drifted. As always, YSI recommends a proper daily calibration of your DO meter or, at the very least, a quick check to see if the DO % value is reading within +/-2% of what it should read at your altitude/barometric pressure.
Note: If DO% Local is being measured with a YSI 6-series sonde or 556 it may be necessary to recalibrate the instrument after extreme changes in barometric pressure or altitude in order to keep the DO% Local value at 100% in a fully saturated environment. This is not a requirement if only mg/L values are being recorded since these values will remain accurate without recalibrating Local DO %. If reporting DO% Local with newer instruments such as the Professional Plus, ProODO, or Pro20, it is not necessary to recalibrate after a significant barometric pressure change in order to report an accurate DO% Local since these instruments have an on-board barometer that is read by the instrument continuously.
A subsequent blog post will delve into the issue of using barometric pressure for dissolved oxygen calibrations.
Additional Blog Posts of Interest:
What is Affecting Your Dissolved Oxygen Measurements? Part 4 of 4
What is Affecting Your Dissolved Oxygen Measurements? Part 2 of 4
What is Affecting Your Dissolved Oxygen Measurements? Part 1 of 4