Endress+Hauser Mycom S CPM 153 Operating Instructions Manual page 58

6 Start-up
58
X
–
W + E
Fig. 20: Schematic diagram of the CPM 153controller with K
X Actual value
W Set point
E Control difference
Y Set value
K Modulation gain (total gain)
R
T Integral action time (I component)
n
T Derivative action time (D component)
v
Range-dependent modulation gain
The majority of pH neutralisation processes are strongly non-linear (Example: titration
curve). If you specify a strong alkali in portions to a fixed volume of a weak acid, the pH
value changes. The change in pH value is, at the beginning, relatively small, larger in
the area of the so-called equivalence point and then ever smaller.
In the following diagram, such a titration curve is represented for a weak acid with a
strong alkali (y axis: pH value, x axis: volume units added to a strong alkali).
pH
12
11
10
9
8
7
6
5
5
Fig. 21: Schematic titration curve of a weak acid with a strong alkali.
For difficult neutralisations, the CPM 153 controller gives you the option of partial
compensating for the non-linearity by entering an inverse characteristic Y(X).
P
1
I
+
1/(T *S)
n
+
D
T *S
v
10 15 20 25 30
Mycom S CPM 153
K (X)
R
+
Y
*
(X) as the total gain
R
X [ml NaOH]
35
Endress+Hauser