19
18
PROPORTIONAL CONTROL MODE
The user can vary three different parameters, i.e. the setpoint
(S1 or S2), the deviation (D1 or D2) and the proportional
control mode period T
c
from 1 to 30 minutes. Duration of the
activated control is directly proportional to the error value
(Duty Cycle Control Mode): as the measurement approaches
setpoint, the ON period diminishes.
The following graph describes the pH process controller be-
havior. Similar graph may apply to the mV controller.
During proportional control the process controller calculates
the relay activation time at certain moments t
0
, t
0
+T
c
, t
0
+2T
c
etc. The ON interval (the shaded areas) is then proportional
to the error amplitude.
For example with S1 representing High Setpoint
Setpoint (S1) = 7.00 pH
Deviation (D1) = 1.00 pH
T
c
= 1 minute
If measurement > 8.00 pH, then ON all the time.
If measurement = 7.60 pH, then ON for 36 seconds
OFF for 24 seconds.
If measurement = 7.10 pH, then ON for 6 seconds
OFF for 54 seconds.
The number of strokes per minute of the pump can be
changed only by means of the pump’s command.
Referring to the following diagram (low setpoint) the relay is:
• ALWAYS ON if pH < setpoint-deviation;
• ON proportionally to the error
if setpoint - deviation < pH < setpoint
ON
OFF
14
Se
Hysteresis
Setpoint
t
0
t
0
+T
c
t
0
+2T
c
t
0
+3T
c
An upper boundary is imposed for acid/base dosage time
when relays are energized continuously, i.e. when relay works
in ON/OFF mode or in proportional mode but in the latter
case only if the relay is always ON. This parameter can be set
through setup procedure. When the maximum boundary is
reached, an alarm is generated; device stays in alarm condi-
tion until relay is de-energized.
ON/OFF CONTROL MODE
Either for mode 1 or 2 (base or acid dosage) the user has to
define the following values through setup:
• relay setpoint (pH/mV value);
• relay hysteresis (pH/mV value).
Connect your device to the COM
and NO (Normally Open) or NC
(Normally Closed) terminals.
The ON relay state occurs when relay is energized (NO and
COM connected, NC and COM disconnected).
The OFF relay state occurs when relay is de-energized (NO
and COM disconnected, NC and COM connected).
The following graphs show relay states along with pH mea-
sured value (similar graph can be derived for mV control).
As shown below, a high setpoint relay is activated when the
measured pH exceeds the setpoint and is deactivated when it
is below the setpoint value minus hysteresis.
Such a behavior is suitable to control an acid dosing pump.
A low setpoint relay as can be seen from the following graphs
is energized when the pH value is below the setpoint and is
de-energized when the pH value is above the sum of setpoint
and the hysteresis. The low setpoint relay may be used to
control an alkaline dosing pump.
ON
OFF
Setpoint
14
Setpoint –
Hysteresis
