YSI 6000UPG - Manual (Page 113)

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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 ammonium 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 ammonium sensor should be calibrated using solutions of known total ammonium-nitrogen
content according to the procedures detailed in sections 3.2 and 4.2. 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 which are significantly different 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. Be sure to check for this drift
by placing the sonde in one of your standards at the end of each deployment.
(4) Nitrogen standards are good growth media for a variety of organisms. This growth can
significantly reduce the nitrogen content of your standards, an effect which 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 ammonium sensor will take longer to stabilize after exposure to buffers in
a sonde pH calibration. 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 ammonium calibration after exposure to buffers.
CAUTION: The ammonium 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.