D8 Series User’s Guide
Chapter 4: Operation and Setup
83
High SP
Low SP
Input Range
Input Range
Master PV * Control Ratio + SP Dif
ferential
Ratio Loop Set
Point
Master Loop Process Variable
SP Differential
SP = Set Point
PV = Process Variable
Low
High
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 inputs, 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 diluted potassium hydroxide. The desired flow of H
2
O is 10 gallons 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.
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