Knick Protos 3400, User Manual

Page: 114 / 212

114
• Calibration: Detecting deviations without readjustment
• Adjustment: Detecting deviations with readjustment
Calibration / Adjustment
Note: HOLD mode active
Current outputs and relay contacts behave as configured
Caution:
Without adjustment every pH meter delivers an imprecise or wrong output
value! Every pH electrode has its individual zero point and its individual slope.
Both values are altered by aging and wear.
To determine the correct pH value, the pH meter must be adjusted to the elec-
trode. The analyzer corrects the voltage delivered by the electrode with regard
to electrode zero and slope and displays it as the pH value.
Be sure to perform an adjustment after having replaced the electrode!
Procedure
First, a calibration is performed to detect the deviations of the electrode (zero,
slope). To do so, the electrode is immersed in buffer solutions whose pH value
is exactly known. The measuring module measures the electrode voltages and
the buffer solution temperature and automatically calculates the electrode zero
and slope. These data are stored in a calibration record. By “Adjustment” the
determined calibration data can be used for correction (see following page).
Parameters Determined by Calibration
of an electrode is the voltage change per pH unit. For an ideal
pH electrode, it lies at -59.2 mV/pH.
Slope
of the process solution must be detected since pH measure-
ment is temperature-dependent. Many electrodes have an
integrated temperature probe.
Temperature
is the pH value at which the pH electrode outputs the voltage
0 mV. It is different for each electrode and changes with age
and wear.
Zero
Calibration and adjustment