YSI 600DW-B Sonde - Manual (Page 223)

Principles of Operation Section 5
the sonde, the user should be aware that drift is almost certain to occur. The extent of the drift will vary
depending on the age of the probe, the flow rate at the site, and the quality of the water. For all monitoring
studies using ion selective electrodes, the user should acquire a few “grab samples” during the course of the
deployment for analysis in the laboratory by chemical means or with another nitrate sensor which has been
recently calibrated. Remember that the typical accuracy specification for the sensor (+/- 10 % of the
reading or 2 mg/L, whichever is larger) refers to sampling applications where only minimal time has
elapsed between calibration and field use.
CALIBRATION AND EFFECT OF TEMPERATURE
The nitrate sensor should be calibrated using solutions of known nitrate-nitrogen content according to the
procedures detailed in Sections 2.6.1 and 2.9.2. If a two point calibration protocol is used, the temperature
of the standards should be as close as possible to that of the environmental medium to be monitored. The
recommended calibration procedure is one involving three solutions. Two of the solutions should be at
ambient temperature while the third should be at least 10 degrees Celsius different from ambient
temperature. This protocol minimizes the effects of taking readings at temperatures that are significantly
different from ambient laboratory temperatures.
MEASUREMENT AND CALIBRATION PRECAUTIONS
(1) The temperature response of ion selective electrodes is not as predictable as that of pH sensors.
Therefore, be sure to carry out a 3-point calibration the first time you use the probe. This will provide a
default setting for the effect of temperature on your particular sensor. After this initial calibration, you can
use the less time-consuming 2 point and 1-point routines to update the 3-point calibration. However, we
strongly recommend a new 3-point calibration after each deployment of 30 days or longer.
(2) Ion selective electrodes may not stabilize as rapidly as pH sensors. Be sure to allow plenty of time for
the readings to come to their final values during all calibration routines.
(3) Ion selective electrodes generally drift more than pH sensors. To check for this drift, place the sonde in
one of your standards at the end of each deployment.
(4) Nitrate standards are good growth media for a variety of organisms. This growth can significantly
reduce the nitrogen content of your standards, an effect that is particularly important for the 1 mg/L
solution. It is best to use new standards for each deployment, but if you decide to save your solutions for
reuse, we recommend refrigerated storage to minimize the growth of these organisms.
(5) Remember that the nitrate sensor will take longer to stabilize after exposure to pH buffers. To
accelerate this process, soak the sensor in 100 mg/L standard for a few minutes after performing a pH
calibration. In addition, be particularly careful that readings are stable during nitrate calibration after
exposure to buffers.
CAUTION: The nitrate membrane module is for use only at depths less than 50 feet (15.2 meters). Use of
the probe at greater depths is likely to permanently damage the sensor.
5.09 AMMONIUM AND AMMONIA
The sonde ammonium probe employs a silver/silver chloride (Ag/AgCl) wire electrode in a custom filling
solution. Nonactin membrane separates the internal solution from the sample medium and this membrane
selectively interacts with ammonium ions. When the probe is immersed in water, a potential is established
across the membrane that depends on the relative amounts of ammonium in the sample and the internal
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-10