Ammonium
The ammonium sensor should be calibrated every day the instrument is used. The ammonium sensor should
only be used in fresh water (salinity < 2 ppt) and to depths of 55 feet (17 meters) of water.
Ammonia is calculated from the ammonium, temperature and pH readings. pH greatly affects the ammonia
calculation. Therefore, for highest accuracy in the ammonia calculation, be sure to use a pH sensor in
conjunction with an ammonium sensor during measurements. If a pH sensor is not in use, the instrument will
assume the sample is neutral (pH 7) for the calculation.
CALIBRATION TIPS
1. If using an ammonium sensor with either a pH or ORP sensor on a 6051010 or Quatro cable, install the pH
or ORP sensor in port 1 and the ammonium sensor in port 2.
2. If using an ammonium sensor on a 6051010 or Quatro cable, calibrate the sensor in port 1 prior to calibrating
the sensor in port 2. The sensor in port 2 uses the reference of the sensor installed in port 1. Therefore, it is
important to verify that the sensor in port 1 is working properly before calibrating the sensor in port 2. See
ammonium Troubleshooting Tips for additional information on port configuration.
3. Exposure to the high ionic content of pH buffers can cause a significant, but temporary, drift in the
ammonium sensor. Therefore, if calibrating a pH sensor, either:
a. Remove the ammonium sensor from the cable bulkhead and plug the port. After pH calibration is
complete, reinstall the ammonium sensor and proceed with its calibration with no stabilization delay.
Or,
b. Calibrate pH first, immersing both sensors in the pH buffers. After calibrating pH, place the sensors
in 100 mg/L ammonium standard and monitor the reading. Usually, the reading starts low and may
take awhile to reach a stable value. When it does, proceed with the calibration. This may take
several hours.
4. The ammonium sensor can be calibrated with up to three calibration points. For highest accuracy, perform a
two point calibration with 1 and 100 mg/L standards within 10°C of your sample temperature
.
5. Rinse the sensors and cal cup with a small amount of ammonium solution (1 mg/L for the first point and 100
mg/L for the second point). Fill the cup so that the ammonium sensor tip and the temperature sensor are
submerged in solution. If using a Quatro cable or 6051030 cable, ensure that the conductivity sensor is also
submerged in the calibration solution. The salinity reading from the conductivity sensor is used in the
algorithm for the ammonium measurement.
6. After entering the calibration screen, change the calibration value if necessary.
7. If not using a conductivity sensor, enter the salinity value of the calibration standard. After calibration, change
the salinity correction value to the salinity value of the water you will be testing in the field in order to obtain
the most accurate ammonium measurement. You can change the salinity correction value in the Sensor
menu. As mentioned, the Ammonium sensor should only be used in fresh water salinity < 2 ppt). The salinity
of fresh water is typically 0 to 0.5 ppt.
8. Record the NH
4
millivolts for each calibration point. The acceptable mV outputs for each calibration solution
are shown below.
NH
4
1 mg/L = 0 mV +/- 20 mV (new sensor only)
NH
4
100 mg/L = 90 to 130 mV > 1 mg/L mV value
•
The mV span between 1 mg/L and 100 mg/L values should be
≈
90 to 130 mV. The slope should be
45 to 65 mV per decade.
9. Wait for the ammonium and temperature readings to stabilize in each calibration solution and then press
enter to accept each calibration point.
10. Rinse the sensor and cal cup between calibration points with a small amount of the next buffer.
11. After pressing enter to accept your last calibration point, press cal
to complete the calibration. Otherwise
you will continue calibrating up to 3 calibration points.
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