18 | BA-40100-02-V17
TOPAX
®
DX
Multichannel controller
5 Measuring values inputs
This section describes the chemical and physical correlations which are
necessary to understand the behaviour of the measurement inputs.
The measurement inputs are processed for:
• Disinfection
• pH value
• Redox potential
• Temperature
• Total chlorine and display of combined chlorine
• Conductivity
A key element is the calibration of the sensors.
ATTENTION!
When set up the unit for the first time, care should be
taken to perform individual calibration of each output
immediately after connecting the sensors in the sys-
tem. The TOPAX DX monitors all calibration process-
es based on reasonable parameters (zero point and
transconductance). The readings are recorded. Non
calibrated and "poorly" calibrated measurements
inputs are shown in text form. "Poorly calibrated"
measurements are shown in the main display in red.
ATTENTION!
Note that time delays are possible due to the run-in
periods for the sensors.
The limits for "poorly calibrated" are:
Measurement
Limits
pH value transconductance
<40 mV/pH or
> 61 mV/pH
pH zero point
< -40mV or
> 40mV
slope rate Redox
< 0.85 mV / mV or
> 1.2 mV / mV
slope rate chlorine at
amperometric or potentiostatic meas-
uring cell
< 3 µA / mg/l or
> 100µA / mg/l
slope rate chlorine with encapsulated
electrode (0..2.00 mg/l)
< 3mA / mg/l or
> 13mA / mg/l
slope rate chlorine with encapsulated
electrode (0-5.00/10.00 mg/l)
< 1mA / mg/l or
> 5.2mg/l
The "poorly calibrated" display is only for reference and provides infor-
mation on the calibration quality and the state of the electrochemical
sensors.
Look for calibration faults and eliminate them. Replace the sensors as
soon as possible, if necessary. You can use the sensors unchanged until
replacement.
5.1 Measurement input Disinfection
The free chlorine measurement depends on the pH value of the water
sample. This is based on the reaction of the chloride ions at different
pH values. This relationship further determines the chlorine dissociation
curve.
free chlorine
Hyperchlorous acid
Fig. 19: 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 2 free chlorine values: The photometry value and
the effective chlorine content ("eff. chlorine") to kill germs at the current
pH value.
5.1.1 Technical data
The current signal for disinfection is sent to the measurement value
input by an amperometric measuring cell (2 electrode excess chlorine
measuring cells) a potentiostatic measuring cell or a diaphragm covered
measuring cell (measuring signal 4...20 mA).
Following measuring ranges are possible:
Sensor type
Measuring range
Amperometric electrode
(Excess chlorine detector with 2 electrodes
Type: CS 120)
0-1.00 mg/l
0-2.00 mg/l
0-5.00 mg/l
0-10.00 mg/l
potentialstatic electrode
(Type: PM)
0-1.00 mg/l or
0-2.00 mg/l
Encapsulated electrode
(Type: Cl 4.1 A 2)
20 mA type;
Set measuring range based on
type of detector
Accuracy
± 1 % after calibration
