0 | Maintenance Instructions TOPAX DX | BA-40100-0-V04
Chapter 5: Measuring values inputs
Ph which depends on the temperature of the measuring medium.
(see scientific publications or DIN 1961).
The correlation of Nernst voltage and temperature results into
the following table:
t (°C)
U (mV)
t (°C)
U (mV)
t (°C)
U (mV)
0
54,0
35
61.14
70
68,08
5
55,19
40
6,13
75
69,08
10
56,18
45
63,1
80
70,07
15
57,17
50
64,1
85
71,06
0
58,16
55
65,11
90
7,05
5
59,16
60
66,10
95
73,04
30
60,15
65
67,09
100
74,04
The Nernst voltage is measured by a pH-glass electrode and a
control-electrode integrated into a Ph-sensing electrode.
Calibration
Calibration of the electrode may be performed by ways of „-
point calibration“ with buffer solutions or by „1-point calibra-
tion” with final input of the resistivity value. The mandatory condi-
tions for using „1-point calibration“, is that the resistivity of the
sensing electrode must be priory measured in a lab.
The actual voltage of the electrode and the design value of buffer
solution are displayed on the TOPAX DX during calibration so that
the electrode can be actually graded while performing the cali-
bration process.
The reaction time for any brand new electrode is just a few sec-
onds and the electrode is considered as fully adjusted when the
physical reading becomes stable. Used second-hand electrodes
may have a longer reaction time.
Connect the electrode to your TOPAX DX by means of a BNC plug
connector or directly to input terminal clips.
2-point calibration
Calibration of the Ph-sensing electrode is performed by select-
ing the menu item “-points calibration” (see chapter 16, menu
1.).
The physical measuring value (mV) on the electrode is displayed
in the menu along with the design measuring value that should
be ideally assessed for the Ph.
Buffer-solution 1: zero-point calibration
Soak the pH-electrode in a buffer solution corresponding ap-
proximately 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
zero-point calibration a Lutz-Jesco buffer solution of pH 6.80 is
available. When soaking the Ph-electrode in this solution, a volt-
age of 1 mV should be displayed on the TOPAX DX. Keep in
mind that the physically measured value may slightly differ from
the design value.
When the physical value on the display becomes stable, you may
save the calibrated value by pressing „OK“.
BEWARE!!
Should the actual measured voltage strongly differ
from the design zero-point of the electrode, it means
there is a zero-point drift of the electrode. Zero-point
drift should not exceed the specifications of the DIN
Standards 19265. In the event of zero-point drift ex-
ceeding …… nV, TOPAX DX warns you about possi-
ble bad prove calibration.
Buffer-solution : resistivity calibration
Accurately clean the electrode with deionised or distilled water
before calibration of the resistivity.
WARNING!
Avoid rubbing off glass electrodes as you may provo-
ke accumulation of static electricity in the electrode
which in turn would result into faulty readings!!
To perform calibration of resistivity you need a buffer solution that
differs from the zero-point by at least Ph-points. Lutz-Jesco
buffer solutions with a Ph of 9.7 are available for this purpose.
Should you use a different buffer solution for calibration of the
resistivity, you first need to set TOPAX DX to the Ph of your other
solution. When soaking the Ph-electrode in this buffer solution
(Ph = 9.7), a voltage of -134 mV should be displayed on the
TOPAX DX. Keep in mind that the physically measured value may
slightly differ from the design value.
When the physical value on the display becomes stable, you may
save the calibrated value by pressing „OK“ and wait for the resis-
tivity of the electrode to be displayed.
In accordance with the DIN Standards 1965 the resistivity if an
electrode should range between 5 and 59 mV per pH-value.
Zero-point stability and accuracy of the measured resistivity are
consistently monitored.
BEWARE!
Resistivity values may not strongly differ from the va-
lues mentioned herein. If not, check the combination
electrode or the power supply cable and its quick
connector. Should zero-point and resistivity values
of the combination electrode exceed the admissible
tolerances, the system notifies the operator with a
warning message. In this case provide for immediate
replacement of the electrode.
Buffer solution
Voltage
3,06
9,15 mV
4,65
136,68 mV
6,80
11,63 mV
9,7
-13,0 mV
WARNING!
Store buffer solutions in a cool and dark place!
