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13. Operation
13.1 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 for various reasons:
•
The disinfection effect is too weak at too low concentrations.
•
Too high concentrations have consequences:
– danger to health
– unpleasant odour and taste
– corrosion damage
– increased operating costs.
The AQC-D3 potentiostatic measuring cell is used for
measuring the concentration of chlorine (Cl
(ClO
) or ozone (O
) and is equipped with additional holders for
2
3
pH single-rod electrodes and redox single-rod electrodes, and
also for a water sensor and an integrated Pt100 temperature
sensor.
The preassembled system 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-D3
potentiostatic measuring cell.
13.1.1 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 (optional), for example, serves to trigger an
alarm or to switch off the control function if there is a sample
water deficiency.
– The agent to be determined (Cl
the noble-metal electrode.
•
The material to be determined generates an electric current:
– in the µA range
– proportional to the concentration of the parameters Cl
ClO
and O
2
3.
– The measuring cell is controlled by a potentiostat integrated
into the measuring amplifier
– A constant voltage is applied to the measuring electrode.
An exactly defined potential of the working electrode is
retained by means of the third electrode (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 as the device
controlled.
13.1.2 Electrode cleaning
The measuring electrode and the counter-electrode are
continuously cleaned of deposits by a cleaning wing which has a
gentle, yet effective action.
•
This ensures uniform sensitivity for the measuring cell over a
long period.
•
The cleaning wing is driven hydraulically.
).
3
), chlorine dioxide
2
, ClO
or O
) is measured at
2
2
3
,
2
13.1.3 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 for by the
measuring amplifier if the temperature compensation function
is activated.
– The temperature can be measured using a Pt100
temperature sensor integrated into the measuring electrode.
– The temperature measuring signals are transferred to the
measuring amplifier and calculated using the electrode
signals.
13.1.4 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 8.5), 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. 22 HOCl-OCl diagram
The potentiostatic 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 8.5, 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.
13.2 Operation
13.2.1 Switching on
See section 12.7 Switching on.
13.2.2 Operating state
Observe the installation and operating
instructions of the measuring amplifier and the
Caution
device controlled (for instance a gas dosing unit
or dosing pump)!
•
The operation of the measuring cell in the operating state can
be taken from the installation and operating instructions of the
relevant measuring amplifier.
The measuring cell may be damaged if the
Caution
reference electrode B1 is not connected, and the
electricity supply is switched on!
-
% OCl
0
20
40
-
60
OCl
0 °C
80
10 °C
30 °C
100
10 11
19
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