44
CORN I NG ASCENT FBR PD SYSTEM
7.9.2 MCV pH Control
The base pump minimum and maximum Controller Values (CV), and CO
2
MFC sides of the PID split range are configurable in the pH
Control faceplate (Figure 7-31).
Figure 7-31.
MCV pH control.
The first pH control input column is for base pump configuration, and the second input column is for CO
2
MFC. In each column, the PID
Controller Value Min. and Max. can be entered in percentage. This is the PID output range which the CO
2
flow rate and the base pump
are controlled. The next two fields are Device Output Min. and Max. to enter CO
2
in SCCM, and the base pump output in mL/min. pH
Deadband limits can be set for CO
2
in the Upper field, and for Base in the Lower field. The P, I, and D Gains outputs can be entered, and pH
deadband can either be Enabled or Disabled.
NOTE:
The vast majority of applications will be best served by leaving the D-Gain at zero for both the Base and CO
2
sides of the pH
controller configuration.
The pH control loop uses the PID loop to control the base pump and CO
2
flow rate in sparging gas mixture. The PID loop operates using a
configurable Controller Value split range control with a deadband around the set point. The control loop is indirect acting with respect to the CO
2
flow rate and direct acting with respect to base flow rate. The Split Range Control graph in Figure 7-32A illustrates how the control loop works.
PID CV is 50% when the loop is at its minimum output. This is within the default split range Controller Value scaling region, wherein both
the CO
2
MFC flow rate and base pump flow rate are set to their minimum values. By default, the split range Controller Value scaling limits
are 49.9% and 53%.
Base Pump and CO
2
MFC Setpoints
By changing the base pump PID Controller Value limits and CO
2
MFC, the user can change the Controller Value range so that the base
pump and CO
2
MFC are limited to ensure stable control.
When the PID output is in Region I, the base pump flow rate is set to its minimum. In this region the controller is indirect acting with
respect to the CO
2
, that is, as the controller output goes down, the CO
2
output goes up.
When the PID output is in Region II, the CO
2
MFC flow rate is set to the calculated minimum CO
2
MFC flow rate. In this region the
controller is direct acting with respect to the base pump, that is, as the controller output goes up, the base pump output goes up.
Figure 7-30.
Example of MCV DO control
configuration table when the system is connected
to four gases (air, oxygen, nitrogen and CO
2
).
While using this configuration, the user would be
able to maintain MCV DO level as low as 24% (5%
oxygen concentration in purging gas mixture). The
system will automatically calculate the amount of
nitrogen needed to maintain MCV DO level below
atmospheric saturation.
