YSI 6 series User Manual page 249

Principles of Operation Section 5
EFFECT OF TEMPERATURE AND SALINITY
In terms of the determination of dissolved oxygen in environmental water, the 6150 is similar to standard
membrane-covered dissolved oxygen probes in that its measured parameter is proportional to the partial
pressure (not the concentration) of oxygen in the solution being evaluated. This partial pressure value is
reported as "Percent Air Saturation". The signal from the 6150 system is affected by temperature
(approximately 1.32% per degree Celsius), and this factor must be accounted for if DO readings acquired at
temperatures different from that at calibration are to be accurate. The sonde automatically carries out this
compensation. In addition, the relationship between the measured partial pressure of oxygen (percent
saturation) and the solubility of oxygen in mg/L is very temperature dependent. For example, air-saturated
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fresh water contains 9.09 mg/L at 20 C, but only 7.65 mg/L at 30 C. The sonde compensates for both of
these temperature-related factors after instrument calibration. The temperature compensation for the
percent saturation reading is empirically derived, while the conversion from percent saturation and
temperature to solubility in mg/L is automatically carried out using formulae available in Standard
Methods for the Examination of Water and Wastewater (ed. 1989). See Appendix D, Solubility and
Pressure/Altitude Tables of the manual supplied with your 6-series sonde for dissolved oxygen solubility
tables as a function of salinity and temperature.
While the measured partial pressure of air-saturated water is not a function of the salinity (or solids content)
of the water, the concentration of oxygen at a given partial pressure changes significantly with salinity.
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Thus, air-saturated freshwater (salinity = 0) at 20 C contains 9.09 mg/L of oxygen while air-saturated sea
water (salinity = 35) contains only 7.35 mg/L. This effect is automatically accounted for when the
firmware in your 6-series sonde calculates DO mg/L, as long as the conductivity sensor is active and
properly calibrated.
YSI Incorporated Environmental Monitoring Systems Manual 5-11