Xylem 6-Series Multiparameter - Manual (Page 339)

Percent Air Saturation Appendix J
YSI Incorporated Environmental Monitoring Systems Operations Manual J-2
The “DOsat %Local” Convention
In the second convention, 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 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 (Version 2.13 of 6-series code), this
dissolved oxygen convention was added to the YSI 6-series software through the use of the parameter
“DOsat %Local”. For example, for an air calibration in the mountains at a typical barometric pressure of
630 mm Hg, the “DOsat %Local” value at calibration would read 100.0 % and the value of 630 mm would
be stored in instrument memory. If 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), then 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. Note 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.
Calibrating Using the mg/L Mode
When the user performs a mg/L calibration, no barometer input will be required. Instead, values of “DOsat
%Local” reported after the calibration will be based on the barometer reading input by the user at the time
of the last “DO %” calibration. If the user requires the best possible accuracy for “DOsat %Local” values
after calibration then a “DO %” calibration (with correct barometer input) should be performed
immediately before the “DO mg/L” calibration. If a “DO %” calibration has never been carried out, then
the software will assume that the barometer at the last calibration was exactly 760 mm Hg.
When the user performs a mg/L calibration, values of “DOsat %” will be calculated as they are in the
current software, i.e., the input mg/L value will be divided by the theoretical mg/L value at 1 atmosphere
(found in the Std. Methods Table) to yield the reported “DOsat %” value. For example, if a value of 7
mg/L is entered at a temperature value of 21 C (where 100 % saturation at 1 atmosphere is 8.915), the
“DOsat %” value will be (7/8.915) or 78.5. DO % readings for field readings are carried out in exactly the
same way – find the Table value in mg/L which corresponds to the environmental salinity and temperature
and divide the environmental DO mg/L by it.
If the user performs a mg/L calibration and “DOsat %Local” is active in the Report, the software will, as
noted in above, assume that the barometer reading at calibration is the same as that at the last “DOsat %”
calibration. Thus, after the calibration procedure is complete “DOsat %” values calculated as described
above are then multiplied (in the software) by a factor of (760/BP) to yield the corresponding "DOsat