105
CONTROL MODES
5. At this stage, stop dosing reagent.
6. Transfer the log file on a USB flash drive.
7. Connect to a PC and download the data from the USB flash drive and prepare the process
graphic.
8. On the chart draw a tangent to the maximum slope point until it intersects with the horizontal
line corresponding to the initial pH or mV value. Read the system time delay (Tx) on the time
axis.
9. The deviation, Reset Time and Rate Time can be calculated from the following:
Deviation = Tx * max. slope (pH or mV)
Reset time= Tx / 0.4 (minutes)
Rate time= Tx * 0.4 (minutes)
10. Set the above parameters and restart the system. If the response has too much overshoot or is
oscillating, the system can be fine-tuned by slightly increasing or decreasing the PID parameters
one at a time.
The graph, given as an example here, was obtained by dosing an alkaline solution to a weak acid
solution in a tank. For this, the initial settings have been:
Maximum slope = 3 pH/5 minutes = 0.6 pH/minute
Control period = Tx = approx. 7 minutes
Deviation = Tx * 0.6 = 4.2 pH
Reset time = Tx / 0.4 = 17.5 minutes
Rate time = Tx * 0.4 = 2.8 minutes
pH
Tx
3 pH
5 min.
To
Time
Figure 39:
Tunning PID Parameters, Dosing an Alkaline Solution to a Weak Acid
