Xylem 6-Series Multiparameter - Manual (Page 338)

Percent Air Saturation Appendix J
YSI Incorporated Environmental Monitoring Systems Operations Manual J-1
APPENDIX J PERCENT AIR SATURATION
The term “Percent Air Saturation” (abbreviated in many applications to “Percent Saturation”) is a common
parameter for expressing the state of the oxygenation of environmental water and is widely used by YSI
and other manufacturers during the calibration of oxygen sensors. Basically the "Percent Saturation" value
is simply a surrogate for the partial pressure of oxygen in the medium (air or water) being measured. In
this light, it is important to understand that, to our knowledge, all environmental oxygen sensors (either
electrochemical or optical) directly measure the pressure of oxygen – not the concentration. Thus, the
“Percent Saturation” value from an environmental instrument reflects the directly measured parameter for
the system, with the concentration in mg/L being easily calculated from known equations involving Percent
Saturation, Temperature, and Salinity.
Because the “Percent Saturation” parameter can be expressed by two different conventions, there can be
confusion around the use of the term. This section is designed to minimize that confusion by defining each
of these conventions and to provide instructions for configuring your 6-series sonde so that you will have a
choice as to which of the conventions is used. It is very important to note, however, that no matter
which of the conventions is employed (in YSI or competitive instrumentation), the values of dissolved
oxygen in mg/L, the units usually reported, will be identical and unaffected by the choice of DO %
convention.
Note also that the discussion below applies equally to data obtained from both the YSI Rapid Pulse
membrane-covered polarographic oxygen sensor and the YSI ROX optical dissolved oxygen sensor.
The “DOsat %” Convention
In the first convention, used for many years in YSI handheld and laboratory instruments and in 6-series
sondes, the "% saturation" value at the time of calibration in air reflects the value of the barometer that was
input in the calibration protocol. If the parameter “DOsat %” is active in your 6-series Report menu, then
this convention is being used. Effectively, this convention provides a value of water oxygenation that can
be carried out by air exposure, with the value referenced to having the air at exactly 1 atmosphere (760 mm
Hg or 101.3 kPa). A “DOsat %” value of 89 means that the water contains 89 % of the oxygen that could
be dissolved if the water was sparged with air that had a total pressure of 1 atm. For example, for an air
calibration in the mountains at a typical barometric pressure of 630 mm Hg, the YSI “DOsat %" value at
calibration would read 82.9 % (630/760 * 100). 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 %” reading in air (or air-saturated water) would change to 100 % (760/760 *100) because there is
more total oxygen in the air at the lower elevation. (Note that the ratio of oxygen to other gases in the air is
effectively independent of the barometer or altitude, but the absolute amount of oxygen changes with
barometer or altitude.) If the water at both sites is assumed to have a temperature of 20 C (where water
exposed to water-saturated air at exactly 760 mm Hg is 9.09 mg/L), then the mg/L value would be 7.54
mg/L (0.829 * 9.09) in the mountains and 9.09 mg/L (1.00 * 9.09) at sea level. Thus, to calculate mg/L
values at any site after calibration using this convention, the observed “DOsat %” value is simply
multiplied by the Standard Methods or ISO table values that correspond to 100 % at various temperatures
and salinities.