wtw 330i - Instrument Calibration

Operation pH 330i/340i
18
4.3 Calibrating
Why calibrate? pH electrodes age. This changes the asymmetry (zero point) and slope
of the pH electrode. As a result, an inexact measured value is dis-
played. Calibration determines the current values of the asymmetry
and slope of the electrode and stores them in the measuring instru-
ment.
Thus, you should calibrate at regular intervals.
When to calibrate?
l After connecting another electrode
l When the sensor symbol flashes:
– after the calibration interval has expired
– after a voltage interruption, e. g. after changing the batteries
Calibration interval The calibration interval (Int 3) is set to 7 days in the factory. You can
select the interval in the range of 1 ... 999 days (see section
4.6 C
ONFIGURATION).
You can select between 3 calibration procedures:
AutoCal TEC is specially adapted to the WTW technical buffer solutions as a fully au-
tomatic two-point or three-point calibration. The buffer solutions are
automatically recognized by the measuring instrument. Depending on
the instrument setting (see section 4.6 C
ONFIGURATION), the instrument
displays the relevant buffer nominal value or the current electrode volt-
age in mV. The calibration can be terminated after the first buffer solu-
tion. This corresponds to a single-point calibration. For this, the
instrument uses the Nernst slope (-59.2 mV/pH at 25 °C) and deter-
mines the asymmetry of the electrode.
AutoCal DIN is specially adapted to the permanently programmed buffer solutions in
accordance with DIN19266 as a fully automatic two-point or three-
point calibration. The buffer solutions are automatically recognized by
the measuring instrument. Depending on the instrument setting (see
section 4.6 C
ONFIGURATION), the instrument displays the relevant buffer
nominal value or the current electrode voltage in mV. The calibration
can be terminated after the first buffer solution. This corresponds to a
single-point calibration. For this, the instrument uses the Nernst
slope (-59.2 mV/pH at 25 °C) and determines the asymmetry of the
electrode.
ConCal This function is a conventional two-point calibration using two buffer
solutions (pH 7.0 ± 0.5 and any other buffer solution) or a single-point
calibration using any buffer solution and is used as a high-speed
method.