Function And Applications; Functional Principle Of Measurement - Grundfos AQC-D4 Installation And Operating Instructions Manual

5. Function and applications

The AQC-D4 preassembled system mainly consists of the
following components:
• potentiostatic measuring cell AQC-D4
• the measuring amplifier and controller Conex
• two peristaltic pumps
• sample water deficiency sensor 96680752 (12.4134-1).
Various oxidation agents are used for the disinfection of
swimming pool water and drinking water, for example
chlorine (Cl ).
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It is necessary to measure the concentration of the oxidation
agent and regulate its dosing because the disinfection effect is
too weak at too low concentrations.
Too high concentrations result in:
• danger to health
• unpleasant odour and taste
• corrosion damage
• increased operating costs.
For measuring the total chlorine concentration, one reagent and
one buffer are added to the sample water by the two pumps.
Only about 4.5 litres per hour are treated with the chemicals.
After a chemical reaction, the concentration is measured
electrochemically.
The basic element of the AQC-D4 preassembled system is the
potentiostatic measuring cell AQC-D4.
Electrode cleaning motor
The measuring electrode and the counter-electrode are
continuously cleaned of deposits by a gentle yet effective
cleaning motor.
• This guarantees a uniform sensitivity for the measuring cell for
a long time.
• The motor drives a magnetic stirring stone by a magnetic
clutch.
• The cleaning balls, which are constantly kept in motion by the
magnetic stirring stone, circulate between the measuring
electrode and the counter-electrode.
Influence of temperature
The current generated on the electrodes depends on the
temperature of the sample water.
• The measured value increases by approximately 5 % per 1 °C.
• Temperature variations are compensated in the measuring
amplifier.
– The temperature can be measured using a Pt100
temperature sensor.
– The temperature measuring signals are transferred to the
measuring amplifier and calculated using the electrode
signals.
Influence of pH value
The pH value is always constant (pH 4) in the AQC-D4
preassembled system due to the buffer dosing. Therefore there is
no influence of the pH value to be taken into account.
If the pH value is measured with a pH electrode, the
measurement takes place before the buffer dosing, so that the
correct pH value of the measuring water value is measured.

5.1 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.
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– A float switch serves to signal a sample water deficiency and
DIA
for example trigger an alarm.
• Part of the water is taken from the measuring cell and routed
through the three-electrode assembly.
– The agent to be determined (Cl
noble-metal electrode.
• The material to be determined generates an electric current:
– in the µA range
– proportional to the concentration of the parameter Cl
(total chlorine)
– The measuring cell is controlled by a potentiostat integrated
into the measuring amplifier.
– 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 measuring amplifier Conex
– amplifies the current
– calculates it using the calibration parameters
– displays the concentration as a numerical value
– controls a gas dosing unit or a dosing pump as final control
element.
If the concentrations of free and combined chlorine are measured
separately from time to time, a water sample can be taken at the
sample water tap. This tap is located before the measuring cell.
Note
) is converted at the
2
®
DIA:
After treatment with the reagents, only the total
chlorine concentration can be measured. Only use
untreated water (taken before passing the measuring
cell) to measure the concentrations of free and
combined chlorine.
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