Xylem 6-Series Multiparameter - Manual (Page 247)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-9
protocol according to the instructions. The calibration will occur automatically at the end of the specified
warm-up time.
If the Percent Saturation method is selected, the sonde is simply placed in a calibration cup that contains a
small quantity of water or a damp sponge. The probe sensor should not be in the water for this calibration
procedure
. The sonde should be left under these conditions for 10-15 minutes to allow temperature and
humidity equilibration to occur. Then input the true barometric pressure into the sonde software and begin the
calibration protocol according to the instructions. The calibration will occur automatically at the end of the
specified warm-up time.
NOTE: Remember that control of the calibration will be manual rather than automatic if the unit is set up
properly for spot sampling applications (“Autosleep” deactivated).
The DO readings of steady state oxygen systems are greatly affected by temperature (approximately 3% per
degree Celsius) due to the effect of temperature on the diffusion of oxygen through the membrane. The Rapid
Pulse system exhibits a greatly reduced effect of temperature (approximately 1% per degree Celsius), but this
factor still must be accounted for if DO readings acquired at temperatures different from that at calibration are
to be accurate. The sonde software 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 water (DOsat % =
100) contains 9.09 mg/L at 20 o C, but only 7.65 mg/L at 30 o C. The sonde software 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 carried out automatically by the sonde software using formulae
available in Standard Methods for the Examination of Water and Wastewater (ed. 1989). See Appendix D,
Solubility and Pressure/Altitude Tables for dissolved oxygen solubility tables as a function of salinity
and temperature.
FLOW DEPENDENCE
As noted above, oxygen readings acquired using the Rapid Pulse technology are much less affected by
sample flow than steady state probes. However, there is a finite stirring dependence exhibited by the Rapid
Pulse system if measurements are taken when the probe is being pulsed continuously. Our tests indicate
that, under these sampling conditions, observed dissolved oxygen readings can be 2-3 percent lower than
the true readings in very still water. Minimal movement of the water (which occurs during most
environmental measurements) removes this effect.
This small flow dependence of the sensor is greatly reduced in longer term monitoring deployments where
the sampling interval is longer, e.g. 15 minutes. Under these conditions, the sensor is pulsed for only
approximately 40 seconds every 15 minutes, and normal diffusion of oxygen in the medium re-establishes
the oxygen which has been depleted in the previous warm-up/read sequence.
MEASUREMENT AND CALIBRATION PRECAUTIONS
(1) If water-saturated air is used as the calibrating medium, make certain that both the DO reading and the
temperature
have stabilized (10-15 minutes) before starting the calibration sequence. A wet thermistor can
indicate artificially low temperature readings due to evaporation and this situation will result in poor
temperature compensation and inaccurate readings.
(2) For calibrations in water-saturated air, ensure that the calibration cup being used is vented or pressure
released.
(3) If air-saturated water is used as the calibrating medium, make certain that the water is truly saturated by
sparging it for at least 1 hour with an aquarium air-pump and air-stone.