Jesco TOPAX DX, Руководство по эксплуатации

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20 | BA-40100-02-V17
TOPAX
®
DX Multichannel controller
5.2 pH value measurement input
5.2.1 Technical data
Power output Values
Input Voltage signal from a pH single-rod measuring cell
Input resistance 10
9
ohms
Measuring accuracy 1 % (after calibration)
The ph single-rod measuring cell supplies a voltage which is propor-
tional to the pH value. This voltage is defined by the Nernst voltage. The
Nernst voltage is the change in voltage per pH unit. It depends on the
temperature of the medium to be measured (see corresponding techni-
cal literature or German Standard DIN 19261).
The following table shows the temperature dependence of the Nernst
voltage:
t (°C) U (mV) t (°C) U (mV) t (°C) U (mV)
0 54.20 35 61.14 70 68.08
5 55.19 40 62.13 75 69.08
10 56.18 45 63.12 80 70.07
15 57.17 50 64.12 85 71.06
20 58.16 55 65.11 90 72.05
25 59.16 60 66.10 95 73.04
30 60.15 65 67.09 100 74.04
The Nernst voltage is measured between the pH glass electrode and a
reference electrode. These two electrodes are physically integrated in a
pH single-rod measuring cell.
5.2.2 Calibration
Calibration of the single-rod measuring cell may be performed via a
"2-point calibration" with 2 buffer solutions or a "single point calibra-
tion" with subsequent input of the single-rod measuring cell transcon-
ductance. The mandatory conditions for using 1-point calibration , is
that the resistivity of the sensing electrode must be previously measured
in a lab.
The actual voltage of the electrode and the design value of buffer solu-
tion are displayed on the TOPAX DX during calibration so that the elec-
trode can be actually graded while performing the calibration process.
This allows the rating of the single-rod measuring cell to be determined
during calibration, assuming fresh buffer solutions.
The response time for a new single-rod measuring chain is a few sec-
onds and is set when the physical reading becomes stable. For older
single-rod measuring cells the response time may be longer.
The single-rod measuring cell is directly connected to the input termi-
nals of the circuit board of the input module 5-fold (see section “4.3
Structure of the device” on page 8).
5.2.3 2-point calibration
The physical measuring value (mV) on the electrode is displayed in the
menu (see chapter “19.1.4 Menu 1.2: Calibration” on page 31) along
with the design measuring value that should be ideally assessed for
the Ph.
Buffer 1: Zero point calibration
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 zero-point cali-
bration a Lutz-Jesco buffer solution of pH 6.80 is available. When im-
mersing the pH single-rod measuring cell in this solution, a voltage of
12 mV can be displayed on the TOPAX DX theoretically. The physical
value actually measured is, however, always different from the thero-
retical 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 DX warns you about possi-
ble 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 DX 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 DX. The physical value actually meas-
ured however always differs from the theoretical one. Confirm presetting
by pressing "OK".
The transconductance value of the single-rod measuring cell is then dis-
played. In accordance with DIN 19265 the transconductance of the sin-
gle-rod measuring cell should lie between 52 and 59 mV per pH-value.
Zero-point stability and accuracy of the measured resistivity are con-
sistently 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.
buffer solution voltage value
3.06 229.15 mV
4.65 136.68 mV
6.80 11.63 mV
9.27 -132.02 mV
Tab.: Theroretical voltage value of different buffer solutions at 20°C