Calibration process

Before calibrating the sensor is important to read the chapters "ISE sensors frequent AQ" and "Before calibrating sensors" of this technical guide. The calibration process is very sensitive and it must be carried out carefully and following the recommendations of this technical guide. Calibrating is needed for both the normal (Single and Double) sensor probes and the PRO sensor probes; the process is just the same. The complete calibration process must be done by the final user, and Libelium is not responsible for the final result of the process. Next steps describe a typical calibration process, but other methods can be followed.

Figure: Image of the material necessary for the ion sensors calibration process

[Things that can affect to the calibration process]{.underline}

  • Ambient noise: electromagnetic fields can affect directly to the voltage measurement. The voltage values generated by the ISE Sensors are low, and be easily affected by interferences.

  • Temperature changes: the ISE Sensors are very sensitive to temperature changes. Deviations by more than about 2 °C from the original calibration temperature, cause deviations in the voltage value measurement.

  • Contamination of the calibration solutions: During the calibration process, the calibration solutions can be contaminated. Before calibrating be sure that the sensor and the beaker is clean.

  • Clean the material with deionized water: soaking the electrodes in de-ionized water for a few seconds before each measurement.

  • Use of ISA Buffers: ISAB (Ionic Strength Adjustment Buffer) is added to all standards and samples to increase the ionic strength to the same high level, and hence generate a uniform difference between activity and concentration. In this case the potential reading can then be directly related to concentration. The choice of ISAB is critical and varies depending on the ion being analyzed and the reference electrode being used.

Calibration process for ISE Sensors

In this example we are going to calibrate the Calcium Ion (Ca2+) Sensor Probe in the socket1 of the Smart Water Ions Sensor Board.

  • Turn on the Waspmote with the Smart Water Ions Sensor Board and the Calcium Ion (Ca2+) Sensor Probe connected.

  • Upload the example "Socket1 reading for Smart Water Ions" to the Waspmote board and make sure of receiving the data in the serial monitor.

  • Pour one of the solutions (about 50 ml should be enough) in a clean beaker. In this case we are going to use 10 mg/L solution of Calcium.

  • Immerse the probe in pure de-ionized water for about 30 seconds to ensure that the membranes were always in the same state at the beginning of each measurement.

  • Introduce the Calcium Ion (Ca2+) Sensor Probe in the solution and wait for a stable output. Make sure that the sensor is completely immersed in the solution and that it is not close to the beaker wall, which may affect the field between the electrodes and disturb the measurement. Once the output is steady, annotate the value in volts obtained.

Figure: Calcium Ion (Ca2+) Sensor Probe in 10 mg/L solution

After getting the sensor from the first solution, carefully rinse it and repeat the process explained in step 3 with the next solution.

  • Introduce the values noted and the concentration of the calibration solutions in your code, as shown in the next images.

Figure: In this define, you should write the value of the calibration solution used
Figure: In this define, you should write the voltage value obtained

The function setCalibrationPoints() is used to configure the calibration parameters. The pH Sensor Probe (for Smart Water Ions), has its own calibration function called setpHCalibrationPoints().

  • During the process, the temperature of the solution must be measured and annotated. The Soil/Water Temperature (Pt-1000) Sensor can be used in combination with the corresponding example, "Temperature Sensor Reading".

  • Upload the code again with the new calibration values obtained from the calibration process. At least, it is recommended the use of three points, but if the sensors is working in the linear range, good results can be obtained with only two points.