Hanna Instruments HI 23 Series Instruction Manual page 24

The derivative function (rate action) compensates for rapid
changes in the system reducing undershoot and overshoot
of the EC or TDS value.
During PID control, the ON interval is dependent not only on
the error amplitude but even on the previous measurements.
Definitely PID control provides more accurate and stable
control than ON/OFF controllers and it is best suitable in
system with fast response, quickly reacting to changes due
to addition of low or high conductivity solution.
An example of how the response overshoot can be improved
with a proper rate action setting is depicted in the following
graphic.
PID TRANSFER FUNCTION
The transfer function of a PID control is as follows:
Kp + Ki/s + s Kd = Kp(1 + 1/(s Ti) +s Td)
with Ti = Kp/Ki, Td = Kd/Kp,
where the first term represents the proportional action, the
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