Xylem 6-Series Multiparameter - Manual (Page 250)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-12
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) If air-saturated water is used as the calibrating medium, make certain that the water is truly air-saturated
by sparging with an aquarium air-stone for at least 1 hour prior to implementing the calibration procedure.
(3) If calibrating in water-saturated air, insure that the calibration cup being used is vented or pressure
released.
(4) When not in use, you MUST keep the probe moist, either by immersing in water or by covering the
membrane on the probe tip with the plastic cap/moist sponge which was in place on the sensor when it was
received.
(5) If you inadvertently leave your sensor exposed to ambient air for a period of more than approximately 2
hours, you can rehydrate the membrane by the following method: (1) Place approximately 400 mL of water
in a 600 mL beaker or other similar glass vessel – do NOT use plastic vessels – and heat the water on a
thermostatted hotplate or in an oven so that a consistent temperature of 50+/- 5 C is realized. Place the
probe tip containing the sensor membrane in the warm water and leave it at the elevated temperature for
approximately 24 hours. Cover the vessel if possible to minimize evaporation. After rehydration is
complete, store the probe in either water or water-saturated air prior to calibration and deployment.
CAUTION: MAKE CERTAIN THAT THE WATER IN THE VESSEL DOES NOT
COMPLETELY EVAPORATE DURING THE REHYDRATION STEP.
(6) To assure that the best possible accuracy for the ROX Optical DO sensor, YSI recommends that the
membrane assembly be replaced after 1 year. The 6155 Optical DO Membrane Replacement Kit can be
ordered from YSI Technical Support.
For more information on the practical aspects of using the 6150 ROX dissolved oxygen sensor, see
Appendix M of this manual.
5.10 NITRATE
The sonde nitrate probe consists of a silver/silver chloride wire electrode in a custom filling solution. The
internal solution is separated from the sample medium by a polymer membrane, which selectively interacts
with nitrate ions. When the probe is immersed in water, a potential is established across the membrane that
depends on the relative amounts of nitrate in the sample and the internal filling solution. This potential is
read relative to the Ag/AgCl reference electrode of the sonde pH probe. As for all ISEs, the linear
relationship between the logarithm of the nitrate activity (or concentration in dilute solution) and the
observed voltage, as predicted by the Nernst equation, is the basis for the determination.
Under ideal conditions, the Nernst equation predicts a response of 59 mV for every 10-fold rise in nitrate
activity at 25°C. However, in practice, empirical calibration of the electrode is necessary to establish the
slope of the response. Typical slopes are 53-58 mV per decade for YSI sensors. This slope value is
determined by calibration with two solutions of known nitrate concentration (typically 1 mg/L and 100
mg/L NO
3
-N). The slope of the plot of log (nitrate) vs. voltage is also a function of temperature, changing
from its value at calibration by a factor of the ratio of the absolute temperatures at calibration to that at
measurement. The point where this new plot of log (nitrate) vs. voltage intersects the calibration plot is
called the isopotential point, that is, the nitrate concentration at which changes in temperature cause no