Hanna Instruments HI 23 Series Instruction Manual page 26

Note
SIMPLE TUNING PROCEDURE
26
The user can vary five different parameters, i.e. the setpoint
(S1 or S2), the deviation (D1 or D2), the reset time, the rate
time and the proportional control mode period T
to 30 minutes).
User can disable the derivative and/or integrative action
(for P or PI controllers) by setting Td = 0 and/or Ti = MAX
(Ti) respectively through the setup procedure.
The following procedure uses a graphical technique of ana-
lyzing a process response curve to a step input.
1. Starting from a solution with an EC or TDS value quite
different from the dosed liquid, 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 (T
0
more delay, the EC or TDS will reach a maximum rate of
change (slope). Note the time that this maximum slope
occurs and the EC or TDS value at which it occurs. Note
the maximum slope in EC or TDS 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
EC or TDS value. Read the system time delay Tx on the time
axis.
) the EC or TDS starts to vary. After
(from 1
c