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PROPORTIONAL CONTROL
In order to optimize the controlling
process and reduce the amount of
chemicals used, it is recommended
to set an appropriate proportional
dosage.
All models allow to set a proportional band ("Delta" = 0 to 2.0 pH
or 0 to 200 mV) as well as a time cycle (from 0 to 90 seconds). The
proportional dosage is obtained by personalizing the duty cycle and
frequency of the dosing relay activation/deactivation cycle.
The controller will enter proportional dosage at setpoint. Then it will
keep the dosing relay activated for a period proportional to the difference
between the measurement and the setpoint over the cycle. When the
measurement reaches the setpoint ± (depending on control direction)
the selected "Delta", the dosing relay is always on.
pH proportional control, example
Setpoint = 5.00 pH
Reading = 6.50 pH
Reading - Setpoint = 1.5 pH
Proportional settings: pH set to 2 and time cycle to 60 seconds.
The controller doses acidic solution to lower the pH value to the
desired limit.
Since it is 1.50/2.00 = 75% away from the ideal setting, it will
keep activated the dosing
terminals for 75% of the
time over the predetermined
60 seconds. The relay is
hence theoretically activated
for 45 seconds and off for
15 seconds.
The controller prevent over
dosage with fast responding
samples or highly concen-
trated chemicals, or under
dosage with slow reacting
or weak chemicals, stopping the dosage as soon as the difference
between the reading and the setpoint intersects the proportional line,
as shown in the graphs.
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