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11. Operation
11.1 Function
Various oxidation agents are used for the disinfection of
swimming-pool water and drinking water, for instance chlorine
(Cl
), chlorine dioxide (ClO
) and ozone (O
2
2
It is necessary to measure the concentration of the oxidation
agent and to regulate its dosing.
•
Concentration of oxidation agent is too low:
– Disinfection effect is too weak.
•
Concentration of oxidation agent is too high:
– Danger to health
– Unpleasant odour and taste
– Corrosion damage
– Increased operating costs.
The AQC-D12 measuring cell is used for measuring the
concentration of chlorine (Cl
), chlorine dioxide (ClO
2
(O
). It is equipped with a water sensor, an optional Pt100
3
temperature sensor and additional holders for pH electrodes and
redox electrodes.
Preassembled systems with Conex DIA, Conex DIS and DIP are
used when the values of decontamination agents, pH and
redox-potential have to be determined and controlled. The basic
element of the preassembled systems is the AQC-D12 measuring
cell.
11.1.1 Functional principle of measurement
•
Sample water is taken at a representative position and passed
to the measuring cell via an integral filter.
– The water flow rate can be adjusted on the measuring cell.
– A water sensor serves to trigger an alarm or to switch off the
control function, if there is a sample water deficiency. It is
also used for checking the cleaning wing.
– The agent to be determined (Cl
the gold or platinum electrode.
•
The agent to be determined generates an electric current in
the µA range, proportional to its concentration.
– The measuring cell is controlled by a potentiostat in the
measuring amplifier.
– A constant voltage is applied to the measuring electrode. An
exactly defined potential of the measuring electrode is
retained by means of the reference electrode. This results in
a linear response for the measuring cell as well as a stable
zero point for the measurement.
•
The Conex or DIP measuring amplifiers and controllers
– amplify the current
– calculate it using the calibration parameters
– display the concentration as a digital value
– control a gas dosing unit or a dosing pump (device
controlled).
11.1.2 Electrode cleaning
The measuring electrode and the counter-electrode are
continuously cleaned of deposits by a cleaning wing.
•
This ensures uniform sensitivity for the measuring cell over a
long period.
•
The cleaning wing is driven hydraulically.
18
).
3
) or ozone
2
, ClO
or O
) is measured at
2
2
3
11.1.3 Influence of temperature
The current generated on the electrodes depends on the
temperature of the sample water.
•
The measured value increases by approx. 4 % per 1 °C
increase.
•
Temperature variations can be compensated by the measuring
amplifier, if the temperature compensation function is
activated.
– The temperature can be measured with a Pt100 temperature
sensor optionally integrated into the measuring electrode.
– The temperature measuring signals are transferred to the
measuring amplifier and calculated using the electrode
signals.
11.1.4 Influence of the pH value on the chlorine measurement
Free, active chlorine is considered as the total of molecular
chlorine gas (Cl
), hypochlorous acid (HOCl) and hypochlorite
2
-
anions (OCl
). Dissolved chlorine gas in molecular form
practically does not exist at the application-dependent pH values
(pH 4.5 to 8.2), but is subject to hydrolysis in the presence of
water according to the equation
Cl
+ H
O
HOCl + HCl
2
2
The resulting hypochlorous acid is the actually effective
compound for disinfection of the water. The dissociation of acid to
anions is primarily according to an equilibrium dependent on the
pH value according to the equation
+
-
HOCl + H
O
H
O
+ OCl
2
3
% HOCl
100
80
60
40
HOCl
20
0
4
5
6
7
8
9
pH
Fig. 23 HOCl-OCl diagram
The measuring cell for chlorine primarily only determines the
portion of hypochlorous acid that is relevant for disinfection.
Since the dependence on the pH value is particularly large in the
actually relevant range between pH 7 and pH 8.2, this
dependence must be compensated for in the measuring amplifier
if the pH value varies considerably. The pH value must therefore
also be measured, and the pH compensation function must be
activated on the measuring amplifier.
-
% OCl
0
20
40
-
60
OCl
0 °C
80
10 °C
30 °C
100
10 11
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