Hach 6120118 - Manual (Page 48)

48
General pH Information
B.2 PID Controller Basics
A pH control loop operates as follows: The pH meter measures the value of the pH in the
effluent, and, if the pH is different from the setpoint, the controller actuates the reagent
pump (or valve) that adds reagent to a mixing tank. The added reagent adjusts the pH
value of the process.
The physical layout of the loop, the sizing of the pump (valve), type of mixing tank, and
location of the pH electrodes all have a major impact on the ultimate performance of the
loop, after the controller is tuned for optimal performance. The largest single performance
factor is the delay time around the loop. This includes the response time of the
electrode/meter, time required to deliver the reagent to the process water, time required
for the reagent to mix with and react with the process water, and the time required to
deliver the completely mixed water to the electrode. If the delay times are too long or the
mixing is not complete, the control will be poor regardless of how well the controller is
tuned.
The Process pH Meter uses a PID (proportional, integral (reset), derivative (rate) control)
control algorithm. Each of the instrument settings along with their effects on the control
loop, are described below.
Mode
Manual: The manual output is specified in percent of full-scale PID output
(4–20 mA) and is commonly used for testing the output device.
Auto: Allows the process to be controlled automatically using information specified in the
Phase, Setpoint, Proportional Band, Integral, and Derivative menus as follows:
Phase
Direct: The control output action will cause the process value to increase.
Reverse: The control output action will cause the process value to decrease.
Setpoint
The setpoint is defined as the desired process value in pH
Proportional Band
The proportional band is the range in pH from the setpoint value where the controller
provides proportional control. For example, the desired setpoint for the process is pH 7.0
and the process requires that a reagent must be added to the process water to bring it up
to pH 7.0. If the proportional band is set to pH 1.0, the controller will provide proportional
output control over the range of pH 6.0 to 8.0. When the process is at pH 6.0, the
controller will provide a 100% control output level (assuming that Phase is set to Direct).
When the process is at pH 7.0, the proportional control will provide a 0% control output
level. When the process is at pH 6.5 the proportional control will provide a 50% output.
The output action is equal to the difference between the setpoint and the process value,
divided by the proportional band value.