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1. Starting from a solution with a pH or mV value different
from the dosed liquid (at least a 3 pH or 150 mV differ-
ence) turn on the dosing device at its maximum capacity
without the controller in the loop (open loop process).
Note the starting time.
2. After some delay the pH or mV starts to vary. After more
delay, the pH or mV will reach a maximum rate of change
(slope). Note the time that this maximum slope occurs and
the pH or mV value at which it occurs. Note the maximum
slope in pH or mV per minute. Turn the system power off.
3. On the chart draw a tangent to the maximum slope point
until intersection with the horizontal line corresponding
to the initial pH or mV value. Read the system time delay
Tx on the time axis.
4. The deviation, Ti and Td can be calculated from the fol-
lowing:
• Deviation = Tx * max. slope (pH or mV)
• Ti = Tx / 0.4 (minutes)
• Td = Tx * 0.4 (minutes).
5. Set the above parameters and restart the system with the
controller in the loop. If the response has too much over-
shoot or is oscillating, then the system can be fine-tuned
slightly increasing or decreasing the PID parameters one
at a time.
Example:
the chart recording in the figure
aside was obtained continuously
dosing an alkaline solution to a
weak acid solution in a tank. The
initial settings will be:
Max. slope = 3 pH/5 mins = 0.6
pH/min
Time delay = Tx = approx. 7 mins
Deviation
= Tx * 0.6 = 4.2 pH
Ti
= Tx / 0.4 = 17.5 mins
Td
= Tx * 0.4 = 2.8 mins
