YSI ADV6600 - Manual (Page 196)

Appendix F. Percent Air Saturation
ADV6600
Y S I Environmental
Page 186
F-2. “DOsat % Local” Convention
The second convention has been used for many years by other multiparameter instrument companies
and by many European handheld instrument manufacturers. The "%saturation" value is always set
to exactly 100 % at the time of calibration regardless of the barometer value or altitude that was
input. The software of the instrument “remembers” the barometric pressure input at the time of
calibration for use in later calculation of the DO concentration in mg/L. Effectively, this convention
provides a value of water oxygenation that can be carried out by air exposure with the value
referenced to the local air pressure. The 100 % value at calibration for this convention basically
reflects the fact that this is the most oxygen that can be dissolved in water for this location at the
time of calibration. In late 2001, this dissolved oxygen convention was added to the YSI 6-series
software through the use of the parameter “DOsat %Local”.
For example, when performing an air calibration in the mountains at a typical barometric pressure of
630 mmHg, the “DOsat %Local” value at calibration would read 100.0 % and the value of 630
mmHg would be stored in instrument memory. Assume the sensor did not drift electrochemically
and the sonde was taken to a location where the atmospheric pressure was exactly 760 mm Hg (e.g.,
sea level). The “DOsat %Local” reading in air (or air-saturated water) would change to 120.6 %
(760/630 * 100) because there is more total oxygen in the air at the lower elevation.
The mg/L value for water saturated air or air-saturated water, assuming a temperature of 20 C at
both sites, would change from 7.54 mg/L (1.00 * 630/760 * 9.09) in the mountains to 9.09 mg/L
(1.206 * 630/760 * 9.09) at sea level. Thus, to calculate mg/L values at any site after calibration
using this convention, the "DOsat %Local" value is multiplied by the ratio of the barometric
pressure at calibration to 1 atm (table conditions) and then by the table value as shown above.
Observe that the same mg/L values are obtained at both sites for the “DOsat %” and “DOsat
%Local” conventions even though the "percent saturation" values are significantly different.
It is important to note that the convention specified for "%saturation" by the British (and EU)
standard requires that the local barometric pressure be read for every DO data point taken after
calibration. Under this convention, a sensor should always read 100 % saturation when in water-
saturated air or air-saturated water. Since the software of most instruments does not “know” the
barometer at any time after calibration, neither of the conventions described above complies with
the British standard.