Hanna Instruments HI 9931 - Manual (Page 18)

18
PROPORTIONAL CONTROL PROPORTIONAL CONTROL PROPORTIONAL CONTROL PROPORTIONAL CONTROL PROPORTIONAL CONTROL
In order to optimize the controlling process and reduce the amount of
chemicals and fertilizers used, it is recommended to use a proportional
dosage appropriate for the system.
HI 9931 and HI 9934 allow for a proportional band (delta) of 0 to
1.6 mS/cm and 0 to 400 ppm for conductivity (EC) and TDS,
respectively. Each controller also provides for a time cycle from 0 to 90
seconds. The proportional dosage is obtained by personalizing a
curent pulse whose height equals the EC or TDS proportional delta
and the length corresponds to the selected time cycle.
The controller will enter proportional dosage at the setpoint minus the
preselected delta. It will then keep the dosing relay activated for a
period proportional to the difference between the actual measurement
less the setpoint over the cycle.
If the setting is left at 0 mS/cm or 0 ppm, the controller will operate
as an ON/OFF control with no proportional dosage. The hysteresis of
such an ON/OFF control will be 0.1 mS/cm or 15 ppm, respectively.
0
SECONDS mS
1.6 0 90
45
PROPORTIONAL SETTINGS
0.8
PROPORTIONAL SETTINGS
0
SECONDS ppm
400 0 90
45 200
e.g. HI 9931 proportional control
Setpoint = 2.2 mS/cm (EC)
Measured value = 1.6 mS/cm
Delta = 2.2 - 1.6 = 0.6 mS/cm
Proportional settings = EC set to 0.8 and time cycle to 60 seconds
The controller will be dosing fertilizers to increase the EC to the desired
limit. Since it is 0.6/0.8 = 75%
away from the ideal setting, it will
keep the dosing relay activated for
75% of the time over the preselected
60 seconds. The terminals are hence
theoretically activated for 45 seconds
0
SECONDS mS
1.6 0 90
45
PROPORTIONAL SETTINGS
0.8
D mS
Seconds 15 30 45 60
0.20
0.40
0.60
0.80
15 30 45 60