23
The low setpoint relay may be used to control a low conduc-
tivity dosing pump.
P.I.D. CONTROL MODE (HI 23x2z and HI 24x2z models only)
PID control is designed to eliminate the cycling associated
with ON/OFF control in a rapid and steady way by means
of the combination of the proportional, integral and de-
rivative control methods.
With the proportional function, the duration of the activated
control is proportional to the error value (Duty Cycle Con-
trol Mode): as the measurement approaches setpoint, the
ON period diminishes.
The following graph describes the EC/TDS process control-
ler behavior. Similar graph may apply to the controller.
ON
OFF
EC
Setpoint +
Hysteresis
Setpoint
During proportional control the process controller calculates
the relay activation time at certain moments t
0
, t
0
+T
c
, t
0
+2T
c
etc. The ON interval (the shaded areas) is then dependent
on the error amplitude.
With the integral function (default), the controller will reach a
more stable output around the setpoint providing a more
accurate control than with the ON/OFF or proportional
action only.
de-energized when the EC value is above the setpoint plus
the hysteresis.
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