ProMinent DULCOMETER diaLog DACb Assembly And Operating Instructions Manual page 68

Setting measured variables
HOCl
4 5 6 7 8
pH
Fig. 36: HOCl/OCL - equilibrium
68
Chlorine used in water disinfection comes in various forms, e.g. as
liquid sodium hypochlorite, as dissolved calcium hypochlorite, or as
chlorine gas. All of these forms can be measured with chlorine sen‐
sors. After adding chlorine to water, the chlorine splits into two frac‐
tions depending on the pH value:
1. Into hypochlorous acid (also known as subchlorous acid,
n
HOCl) – a strongly oxidising, efficient, anti-bacterial agent that
destroys most organisms very quickly.
2. Into the hypochlorite anion (OCl-) – with a weak anti-bacte‐
n
rial effect that takes a long time to kill off organisms.
The sensors for measuring free chlorine selectively measure the
very effective hypochlorous acid (HOCl), but not the hypochlorite
anion. If the pH value changes during the process, then the ratio of
the two chlorine fractions changes, and hence the sensitivity
(slope) of the chlorine sensor. If the pH value increases, the meas‐
ured HOCI concentration decreases. If there is an integrated con‐
trol, then the control tries to compensate for the change. If the pH
value now decreases again, it can lead to significant overmetering
of chlorine, even though no additional chlorine has been metered.
The use of a pH-compensated chlorine measurement can prevent
this.
As the graph shows, with pH values of > 8.5, less than 10% of the
HOCl is contained in the water and hence the disinfecting power is
lower. The chlorine value shown after compensation is a mathe‐
matically calculated chlorine value. The mathematically calculated
chlorine value does not change the effective disinfection effect in
OCl -
the water. Nevertheless, the above overmetering is avoided. The
recognised reference method DPD 1 (for free chlorine) is used as a
comparison method to calibrate the amperometric sensors. The
reference method is pH-independent (or buffers the pH value to
approx. 6.5) and therefore determines the free chlorine almost as
100% HOCl. The pH influence on the chlorine value measured by
the sensor can be compensated by the controller so that the con‐
centration value measured by the amperometric chlorine meas‐
uring system corresponds to this free chlorine value. The controller
can carry out this compensation automatically either using an inte‐
grated pH measurement or manually relative to a fixed pH value.
9 10 11
We recommend the automatic version. Nonetheless, it is also
essential to measure the sample water temperature, as it has a
significant influence on the pH measurement. If this influence were
not compensated, then the pH value would be measured incor‐
rectly and the chlorine value would then also be incorrectly com‐
pensated.
No calibration is possible with high pH values without pH compen‐
sation, as the difference between the measurement with the
chlorine sensor and the comparison DPD 1 reference method is
too great.
The working range of pH compensation: pH 4.00 ... 8.50, Tempera‐
ture: 5 ... 45 °C
Measurement of dissolved oxygen: You need to enter the following
correction variables in the event of exacting requirements relating
to measuring accuracy (see specification of sensor types): Air
pressure, Higher than NNl, Salinity or Conductivity. The tempera‐
ture correction variable is corrected in the sensor with sensor types
DO1, DO2 and DO3. If you switch the temperature to "OFF" for
these sensor types, then you need to update the values of the cor‐
rection variable at least prior to each calibration.