BA-40400-02-V14 | 21
Operating instructions
ATTENTION!
When operating in a heating water system, electro-
chemical processes at the measuring electrode may
cause a shift of the zero point by means of electro-
chemical processes. In this case, a two point calibra-
tion is required. The zero point must be adjusted with
chlorine-free heating water. Thereafter, the slope of
the measuring cell can be adjusted by using the
DPD method.
NOTICE!
When measuring the free chlorine with a potentio-
stat it is possible to perform a compensation of the
pH-value and the temperature. This is done by con-
necting a pH-electrode and a temperature sensor to
TOPAX
(see chapter “12.1.17 Menu 5.1: Inputs” on
Calibration of encapsulated electrode
Before starting the calibration, the upper value of the measuring range
of the sensor must be defined in the TOPAX (see chapter “12.1.17 Menu
5.1: Inputs” on page 35).
Then the input must be calibrated using a one-point or two-point cali-
bration (see section “12.1.5 Menu 1.2: Calibration” on page 30):
ATTENTION!
When measuring free chlorine by means of a fully
encapsulated electrode, make sure to connect both
terminal clips 51 B and 52 B. These clips are respec-
tively used to feed the 20 mA signal of the electrode
into the controller and to supply 24 VDC operating
power to the sensing electronics of the electrode.
5.1.3 Zero-point calibration (in case of 2-point calibration)
Zero point calibration of the sensing electrode is normally not a must-
requirement, as tuning of the 4 … 20 mA output signal of the electrode
is done on input 4 … 20 mA (4 mA corresponds to 0,00 mg/l).
However, a zero-point calibration may be still required due to the toler-
ances of the sensing electronics of the electrode. For this reason the
zero-point must be set upon first setup of the instrument.
Calibration method: DPD
The sensor is operated with sample water. A nearly stable physical value
should be displayed. In order to avoid signal variations and consequent
reading errors, water must be taken at the sensing element and the
actual signal must be saved directly in the TOPAX upon taking of the
water sample. The chlorine content in the sample water is determined
by means of the DPD method. The measurement must be set in the
controller and saved by pressing OK.
After storage the transconductance value of the chlorine sensor is
shown.
The plausibility of the transconductance measurement is monitored
throughout the process.
IMPORTANT!
The signal from the excess-chlorine sensing detector
is converted by the electrode into an impressed cur-
rent pulse of 4...20 mA. Whenever your
TOPAX
fea-
tures a lower value than 3.5 mA, an alert message
is displayed „total chlorine sensor failure", the alarm
relay is switched.
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 Adjustment
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 during calibration so that the electrode
can be actually graded while performing the calibration process. This al-
lows 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 “4.4
Technical components” on page 8).
2-point calibration
The physical measuring value (mV) on the electrode is displayed in the
menu (see chapter “12.1.5 Menu 1.2: Calibration” on page 30) 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-
