Hanna Instruments HI 9910 - Manual (Page 27)

27
e.g. pH proportional control
Setpoint = pH 5.00
Measured value = 6.50
Delta = 6.5 - 5.0 = 1.5 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 terminals
activated for 75% of the time
over the predetermined 60
seconds. The relay is hence
theoretically activated for 45
seconds and off for 15 sec-
onds.
In order to avoid over dosage with 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 chemicals have
reacted quickly or lengthen-
ing it if the measured pH
continues to drift from the
ideal setpoint as can be seen
from the graphs.
SECONDS
0 90
45
0
pH
2.0
0.5
1
1.5
PROPORTIONAL SETTINGS
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