11. Proportional dosage (TWM): Time with modulation) with or
without overdose alarm
The pH pump setup menu has two sub-menus that are
inaccessible to the user: The "Dosing Period" and "Prop. Band"
are set to 10 minutes and 0.80 pH units respectively by default,
so as not to complicate the settings to be made by the user.
The operating principle for proportional dosing with overdose
alarm is as follows:
As soon as the threshold defined by the set value is exceeded
(maximum threshold if acid is injected - minimum threshold if
base is injected), the dosing process starts up, taking into
account the progressive steps for the injection time / pause time
ratio
These steps are as follows:
• 25%= 2.5 minutes injection and 7.5 minutes pause
• 50%= 5 minutes injection and 5 minutes pause
• 75%= 7.5 minutes injection and 2.5 minutes pause
• 90%= 9 minutes injection and 1 minute pause
If the first step (25%) is not sufficient to stop the pH drift, the
instrument activates the second step (50%) when the pH
deviates from the 0.8 setpoint. If the drift of the pH still
continues exceeding the setpoint of 2 x 0.8 = 1.6, the 3rd stage
is activated by the instrument.
Once the pH drift is arrested and the pH is converging towards
the setpoint, the instrument will decrease the step as the pH
approaches the setpoint to the same limits as before.
In parallel, the overdose alarm counts the time from the moment the setpoint threshold is crossed and
triggers the first pump activation rate step (25%). As soon as the *continuous dosing time exceeds the time
setting for this alarm, dosing is stopped, irrespective of whether the pH value has been brought back to the
setpoint level or not.
* Dosing time = duration of injection phases + duration of pause phases, the proportion of which varies
according to the activated stage.
The purpose of this alarm is to avoid the risk of overdosing, potentially due to a problem with the pH probe,
a problem with the water circulation, or a risk of spreading product due to a blockage or leak in the pH
corrector injection circuit.
The overdose alarm generates two alarm levels:
• Pre-alarm: is triggered when the continuous dosing time has reached 80% of the alarm time setting
(e.g. set time = 150 minutes; the pre-alarm is triggered after 0.8 x 150 = 120 minutes of
continuous dosing). The pre-alarm results in a warning message on the display of the device but
does not stop the dosing
• Alarm: When 100% of the alarm time setting is reached, the device stops dosing (150 minutes in
our example).
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