HF scientific MicroTSCM Streaming Current Monitor - Manual (Page 20)

MICROTSCM (07/09) Page 16
REV 2.4
given change in coagulant dosing. The Integral Time tells the instrument how long it will
take to fully realize the effects of a change in coagulant dosing.
7.3 PI Control Procedure
The assumption is that the wiring between the MicroTSCM analyzer and the dosing pump
has been installed. The following procedure is recommended to determine the correct
Proportional - Integral (PI) values for any particular system. The MicroTSCM analyzer is
used to slow down the control loop to prevent overdosing of coagulant. A calculation
based on test data determines the Proportional Band or P. Band and is the final value that
will be entered into the analyzer. A hypothetical example of the procedure will be used to
demonstrate the calculation of these values. These values will need to be determined for
any particular system and can be plugged into the following equations to determine the
appropriate analyzer settings.
Steps:
1. Ensure the plant is operating at a steady SCU reading with a fairly constant flow
rate. Record this SCU value.
2. Adjust the dosing pump to give a 10% increasing dosing output.
3. Start timing when the change was made (a stopwatch is helpful).
4. Monitor the MicroTSCM and stop timing when the SCU value has changed and
leveled off.
5. Record this time period in seconds. This is the Integral Time setting.
6. Record the new SCU value when the change has fully leveled off.
Figure 9: Effect of Dosing Change
7.3.1 Proportional Band Calculation
For our hypothetical example, the cause of change was the 10% increase in pump output
i.e. 10% more coagulant was dosed into the water. Due to the requirement to mix the
coagulant with the carrier water and move it through the volume of the sample pot, we
observed a time lag. The total time from when the change was made to when the full
effect was noticed was 100 seconds. This will be entered later as the Integral Time (INT.
TIME). The SCU reading changed was from -2.5 SCU to -2.0 SCU; a change of 0.5 SCU.