2.3.4 Multi-Point Calibration
After selecting the desired calibration set, click on [Calibrate] in the menu bar and choose [multi-point calibration] from
the drop-down menu.
Click on [Start] to start the calibration procedure.
The program will them prompt you to ‘’insert measuring tube in fluid 1’’. This is the fluid with the lowest concentration of
the calibration set. Follow the instructions and click on [OK] to start the measurement. Repeat these steps when
prompted for fluid 2 and fluid 3. Fluid 3 is the fluid with the highest ion concentration of the standard calibration set.
At the end of the procedure, the measurement results will change and should then be close to the known concentrations
of the last calibration fluid used.
After the calibration, one or more boxes may turn blue or black. This indicates that one or more of the sensors are working
poorly (blue) or are altogether defective (black). Additionally, blue or black boxes will appear when the multi-point
calibration procedure failed in one way or another. One possible reason for such a failure is that the temperature was not
kept stable during the whole procedure. If ever in doubt, simply repeat the procedure. If the black or blue boxes continue
to appear, please contact us.
It is advisable to perform a control measurement on a standard solution with known composition, like Evian bottled water,
after each multi-point calibration. Before the measurement, rinse the Sensor-Unit thoroughly with Evian. (Refer to Control
Measurements in Section 2.3.3)
For all calibration procedures and measurements, ionic concentrations should be measured from low to high. If a second
multi-point calibration has to be carried out immediately after the first one (that failed), first rinse the Sensor-Unit
thoroughly with (old) Fluid 1 and leave it in (old) Fluid 1 for at least 15 minutes. Restart the calibration procedure with
fresh Fluid 1.
A multi-point calibration should be carried out once every two weeks and preferably also at the end of a measuring
session. The need for a multi-point calibration is indicated in the (green) result boxes. Some of the result bars may
become grayed-out. If you place the mouse pointer over the result bar, the program will explain why it is grayed-out.
pH Multi-Point Calibration
Carry out the pH multi-point calibration from a high pH value to a low pH value.
A pH multi-point calibration should be carried out once every four weeks. Immediately following this calibration, the
Sensor-Unit must be kept for at least an hour in the Standard calibration fluid 2 used for the normal one point
calibration.
If both a pH calibration and a normal calibration are required at the same time, begin with the pH calibration. The
concentration of the HCO3-ion is calculated, based on the pH of the fluid and the measured concentration of dissolved
CO2 in the water. Thus, if the pH measurement is incorrect, the calculation of the HCO3- concentration will also be
incorrect!
The recommended buffer solutions for pH calibration are manufactured by Merck (http://www.merck.de)
pH 4 – ordering number: 109435
pH 7 - ordering number: 109439
pH 9 – ordering number: 109461
for most purposes using only pH 4 and pH 7 is sufficient.
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