22 | BA-40400-02-V14
Two-Channel Controller TOPAX DE
bration a buffer solution of pH 6.80 is available. When soaking the Ph-
electrode in this solution, a voltage of 12 mV should be displayed on
the TOPAX. The physical value actually measured is, however, always
different from the theroretical one.
When the physical value on the display becomes stable you can save the
reference value with "OK".
IMPORTANT!
Should the actual measured voltage strongly dif-
fer from the design zero-point of the electrode, it
means there is a zero-point drift of the electrode.
This should not exceed the specifications of the DIN
Standards 19265. In the event of zero-point drift ex-
ceeding ± 40 mV,
TOPAX
warns you about
possible bad prove calibration.
Buffer 2: Transconductance calibration
Clean the single-rod measuring cell before calibrating the transconduct-
ance (flush out with distilled water).
ATTENTION!
Avoid rubbing off glass electrodes as this will pro-
duce a static charge on the electrode. This would
result in faulty readings.
For transconductance calibration a buffer solution must be used which
differs at least 2 pH units from the zero point. For transconductance cali-
bration the manufacturer offers a buffer solution of pH 9.27. Should you
use a different buffer solution for calibration of the resistivity, you first
need to set TOPAX to the pH of your other solution. When soaking the
Ph-electrode in this buffer solution (Ph = 9.27), a voltage of -134 mV
should be displayed on the TOPAX. The physical value actually measured
however always differs from the theoretical one. Confirm presetting by
pressing "OK".
The transconductance value of the single-rod measuring cell is then
displayed. In accordance with DIN 19265 the transconductance of the
single-rod measuring cell should lie between 52 and 59 mV per pH-
value.
Zero-point stability and accuracy of the measured resistivity are consist-
ently monitored.
IMPORTANT!
If the slope value differs considerably from these
values, check the combination electrode or the
connecting cable and the plug connectors. If the
"zero-point" and "transconductance" values of the
single-rod measuring cell exceed the tolerances, the
system notifies the operator with a warning mes-
sage. The measuring cell should then be replaced as
soon as possible.
Theroretical voltage value of different buffer solutions at 20°C:
buffer solution
voltage value
3.06
229.15 mV
4.65
136.68 mV
6.80
11.63 mV
9.27
-132.02 mV
IMPORTANT!
Store buffer solutions in a cool and dark place. Con-
sider their useful life. Make sure that the buffer solu-
tion is not contaminated. For this reason single-rod
measuring cells should not be moved directly from
one buffer solution to another.
1-point calibration
Single-point calibration may also be used for pH calibration with a sin-
gle-rod measuring cell (see section “12.1.5 Menu 1.2: Calibration” on
page 30).
Submerge the pH combination electrode in a buffer solution which is
equivalent or close to the zero point of the electrode. The ideal electrode
zero-point (0 mV) is at a pH of 7.00. However, the real zero-point shows
minimum variations as against this minimum value. For the zero point
calibration a buffer solution of pH 6.80 (TOPAX default setting) is avail-
able from the manufacturer.
When soaking the Ph-electrode in this solution, a voltage of 12 mV
should be displayed on the TOPAX. The measured physical value how-
ever always differs from the theoretical one.
When the physical value on the display becomes stable you can save the
reference value with "OK".
Now enter the transconductance of the single-rod measuring cell.
IMPORTANT!
If the actual measured voltage differs significantly
from the theoretical zero-point of the measuring
cell, there is a zero-point drift on the measuring cell.
Zero-point drift should not exceed the specifications
of the DIN 19265 (±40 mV).
5.2.3 Offset compensation
In accordance with DIN 19643, the pH value should be checked via an
electro-metric pH value measurement.
External influences can cause the pH value of the electro-metric meas-
urement, measured by hand with the photometer, to deviate by a con-
stant value. The menu "offset alignment" offers the posibility to eliminate
the difference.
ATTENTION!
The offset value is set to "0" after each new calibra-
tion.
