20 | BA-40400-02-V14
Two-Channel Controller TOPAX DE
5.1 Measurement input Disinfection (amperometric
measurement cells)
To measure the disinfection, amperometric measuring cells are used.
These measuring cells are used to measure the concentration of the free
hypochlorous acid ions if chlorine is used and/or the chlorine dioxide
ions. The ion concentration at the work electrode results in an electric
current flow which is analysed in the measurement amplifier.
Amperometric measuring cells can be classified according to the fol-
lowing criteria:
• open measuring cells,
• encapsulated electrodes.
If chlorine is used for disinfection, the dissociation of chlorine in the
water depending on the ph value is to be observed.
free chlorine
Hyperchlorous acid
Fig.15: The dissociation curve shows the dependence on the pH value
For photometric measurements the pH value of the sample is buffered
to approx. pH 6.5. As a result the measurement has a higher effective
chlorine content than is actually in the process water. For high pH-values
significant differences will therefore occur between the expected and
actual disinfection if assessed by photometric analysis. The main display
will show therefore two free chlorine values: The photometry value and
the effective chlorine content ("eff. chlorine") to kill germs at the cur-
rent pH value. For adjusting the display of effective chlorine see chapter
“12.1.28 menu 5.17: Display” on page 36.
5.1.1 Technical data
Amperometric measurement of disinfection can be done using am-
perometric 2-electrode measurement cells, amperometric 3-electrode
measurement cells with potentiostatic control or encapsulated elec-
trodes (measurement signal 4...20 mA).
Following measuring ranges are possible:
Sensor type
Measuring range
Amperometric electrode
(Excess chlorine detector with 2 electrodes
type: CS120)
0…1.00 mg/l
0…2.00 mg/l
0…5.00 mg/l
0…10.00 mg/l
potentialstatic electrode
type: PE (for free chlorine or chlorine
dioxide)
0…1.00 mg/l or
0…2.00 mg/l
Encapsulated electrode
type: Cl 4.1 A 2 (for free chlorine)
type: CD 4 MA (for chlorine dioxide)
20 mA type;
Set measuring range based on
type of detector
Accuracy
± 1 % after calibration
5.1.2 Calibration
Calibration of amperometric, excess chlorine detector with 2
electrodes (CS120)
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 two-point calibration (see
section “12.1.5 Menu 1.2: Calibration” on page 30):
The physical measurement (µA) measured on the measuring cell is
shown on the display during calibration.
Reference value 1: Zero point calibration
To perform zero-point calibration, operate the detector in chlorine-free
water or just stop the water flow through the instrument. The value of the
physical quantity shown on the display (approx 5-10 µA) can be saved
as soon as it stops changing. The device automatically changes to the
next menu section.
Reference value 2: DPD
The sensor is operated with sample water. If the physical value on the
display does not change anymore, the chlorine concentration in the sam-
ple water is determined by means of the DPD method. 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 typical resistivity value is approx. 25–35 µA (depending on
water type) per mg/l of free chlorine. The plausibility of the transcon-
ductance measurement is monitored throughout the process.
1-point calibration is sufficient (reference value 2) for validation of the
chlorine content after optimization.
Calibration of amperometric 3-electrode measurement cell
with potentiostatic control (PE)
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 calibration (see
section “12.1.5 Menu 1.2: Calibration” on page 30):
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 typical resistivity value is approx. 35 µA (depending on water
type) per mg/l of free chlorine. The typical slope value is approx. 10 µA
per mg/l of free chlorine dioxide.
The plausibility of the transconductance measurement is monitored
throughout the process.
