Chlorine, Bromine, Chlorine Dioxide, Chlorite, Dissolved Oxygen And Ozone - ProMinent DULCOMETER Assembly And Operating Instructions Manual

Setting measured variables
Chlorine, bromine, chlorine
8.1.5
dioxide, chlorite, dissolved
oxygen and ozone
Measured variable chlorine, bromine,
chlorine dioxide, chlorite, dissolved
oxygen and ozone:
The measured variables chlorine , bro‐
mine, chlorine dioxide, chlorite, dissolved
oxygen and ozone are always measured
using a mA signal because the measuring
transducer is located in the sensor.
The temperature compensation takes
place automatically inside the sensor
(exception: CDP, chlorine dioxide sensor).
For further information see the operating
instructions of the sensor used.
Measurement of chlorine with pH compen‐
sation
Chlorine for disinfecting water is available
in different forms, for example as liquid
sodium-calcium hypochlorite, as dissolved
calcium hypochlorite or as chlorine gas.
All these forms can be measured with
DULCOTEST chlorine sensors. After
chlorine has been added to water, the
chlorine splits into two parts depending on
the pH value:
1. into hypochlorous acid (also known
n
as HOCI) – a strongly oxidising disin‐
fectant that destroys most organisms
in a very short time.
2. into the hypochlorite anion (OCI) –
n
with a weak disinfectant effect that
takes a very long time to destroy
organisms.
The sensors for measuring free chlorine
selectively measure the very effective
hypochlorous acid (HOCI), but not the
hypochlorite anion. If the pH value in the
process changes, then the ratio of the two
56
chlorine parts changes and thus also the
sensitivity (slope) of the chlorine sensor.
The HOCI concentration measured is
lower as the pH value increases. If a con‐
trol is integrated, the control tries to com‐
pensate for this. If the pH value becomes
lower again, this could result in a signifi‐
cant overdosing of chlorine, although
there was no further metering. The use of
a pH-compensated chlorine measurement
can prevent this.
HOCl
4 5 6
Fig. 28: HOCl/OCL equilibrium
As the diagram shows, less than 10% of
the HOCI is contained in the water with
pH values of > 8.5 and the disinfectant
effect is therefore lower. The chlorine
value displayed after compensation is a
mathematically calculated chlorine value.
The mathematically calculated chlorine
value does not alter the effective disinfec‐
tant effect present in the water. However,
the aforementioned overdosing is
avoided. The recognised DPD 1 reference
method (for free chlorine) is used as a
comparative method to calibrate the
amperometric sensors. The reference
method is pH-independent (or buffers the
pH value to approximately 6.5) and there‐
OCl -
7 8 9 10 11
pH
-