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CONTROL MODES
On/Off control of a batch pH process using a pump as external dosing device
A dosing solution can be an acid or a base, depending on the desired results; and control mode can
be set High or Low.
With On/Off control type enabled in Setup, the algorithm uses configured “Set point” and “hysteresis”
parameters. See MENU section for further details.
With High mode control, the hysteresis is below the Set point. With Low mode control, the hysteresis
is above the Set point.
• When in High control mode the controlled process value is too high. The dosing pump will run
(adding an acid to bring down the pH) until the process value decreases to the Set point minus
hysteresis value. Above the Set point, the relay is activated. The dosing pump turns off and remains
off until process value reaches Set point value.
• When in Low control mode, the controlled process value is too low. The dosing pump will start
running (adding a base to bring up the pH) until it reaches the Set point plus hysteresis. The pump
remains off until the process value decreases to a value equal to Set point.
High control mode
Relay status
Relay status
On
Off
Low control mode
On/Off Control
Set point
Measured value
Set point
Measured value
Hysteresis
Hysteresis
On
Off
Figure 21:
On/Off Control, High /Low Control Mode
Following graphs exemplify how the input parameters work.
Here’s an example of hystersis-free control output.
SP
Control mode High
Control mode Low
Process value
Control output
Control output
On
Off
On
Off
Figure 22:
General On/Off Control
