Watlow Series D8 - Manual (Page 123)

Series D8 User’s Guide Chapter 4: Operation and Setup
Doc. 0600-3120-2000 Watlow Anafaze 105
Figure 4.13 Relationship Between the Process
Variable on the Master Loop and
the Set Point of the Ratio Loop
NOTE! Ratio control cannot be used on the same
control loop as cascade control.
How to Set Up Ratio Control
1. Adjust and tune the master loop for optimal performance
before implementing the ratio setup.
2. For the ratio loop, set the parameters in the Ratio menu.
3. Configure both the master loop and the ratio loop for in-
puts, outputs, and alarms
Ratio Control Example: Diluting KOH
A chemical process requires a formula of two parts water
(H
2
O) to one part potassium hydroxide (KOH) to produce di-
luted potassium hydroxide. The desired flow of H
2
O is 10 gal-
lons per second (gps), so the KOH should flow at 5 gps.
Separate pipes for each chemical feed a common pipe. The
flow rate of each feeder pipe is measured by a D8, with H
2
O
flow measured on loop 1 and KOH flow measured on loop 2.
The outputs of loops 1 and 2 adjust motorized valves.
High SP
Low SP
Input Range Input Range
Master PV * Control Ratio + SP Diff
erential
Ratio Loop Set P
oint
Master Loop Process Variable
SP Differential
SP = Set Point
PV = Process Variable
Low High