Hanna Instruments HI 9912 - Manual (Page 16)

16
0
SECONDS pH
2.0 0 90
PROPORTIONAL SETTINGS
0.5 1.5
D pH
Seconds 15 30 45 60
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
15 30 45 60
D pH
Seconds 15 30 45 60
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
15 30 45 60
D pH
Seconds 15 30 45 60
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
15 30 45 60
e.g. pH proportional control
Setpoint = 7.4 pH
Measured value = 8.90 pH
Delta = 8.90 - 7.40 = 1.50 pH
Proportional settings = pH set to 2 and time cycle to 60 seconds
The controller will be dosing
acids to reduce pH to the
desired limit. Since it is 1.50/
2.00 = 75% away from the
ideal setting, it will keep the
dosing relay activated for 75%
of the time over the predeter-
mined 60 seconds. The
terminals are hence theoreti-
cally activated for 45 seconds
and off for 15 seconds.
In order to avoid over dosage
of fast responding samples or
highly concentrated chemicals
or under dosage with slow
reacting or weak chemicals,
the controller provides even a
more accurate control.
As the graphs show, it does
that by stopping the dosage
as soon as the current pulse
curve intersects the dosage
curve. This means shortening
the dosage period if the chemi-
cals have reacted quickly or lengthening it if the measured pH
continues to drift from the ideal setpoint as can be seen from the
graphs.
to the difference between the measurement less the setpoint over the
cycle.
NOTE:
• If the setting is left at 0 pH or 0 mV, the controller will operate
as an ON/OFF control with no proportional dosage. In this case
the controller will operate with a 0.1 pH or 7 mV hysteresis.
• Do not set the time cycle to 0 Do not set the time cycle to 0 Do not set the time cycle to 0 Do not set the time cycle to 0 Do not set the time cycle to 0. This causes the relay to chatter
and can be detrimental to the controlling system and pump.