Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 40)

Enhanced Software Features
4.7
Optimizing Your Process System with the WATLOW Series 988
TL TL W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
Ratio
Overview
This feature allows the 988 to control one
process as a ratio of another process. This is
especially useful in applications that mix two
materials, whether steam, paint or food ingredi-
ents.
Input 2 of the 988 measures the part of the
process that is either uncontrolled or controlled
by another device. The part of the process
controlled by the 988 will be maintained at a
input 2
output 1 input 1
mixed paint
uncontrolled
flow of
unmixed paint
flow
transmitter
flow
transmitter
motorized
valve
controlled
flow of pigment
Mixing Tank
level equal to the quantity measured at input 2
multiplied by the ratio term set by the user.
Input 1 monitors the controlled part of the
process.
Requirements
Ratio control requires enhanced software. Two
analog inputs are required to monitor the
process, and at least one output adjusts the
controlled part of the process.
Sample Application
Blue pigment must be added to paint at a ratio of one part per 100 to
create a mixed paint of the desired color. The uncolored paint flows into
the mixer in an uncontrolled stream that is set manually and sensed by
input 2. A motorized valve controls the flow of pigment, which is moni-
tored by the flow sensor to input 1. The flow rate of the uncolored paint
determines the set point for the motorized valve that controls the pigment
flow. If an operator needs to change the rate of flow for the uncolored
paint, the set point will shift accordingly to maintain the correct ratio in
the mixing tank.
The application engineer set up this feature in software by choosing ratio
as the control parameter in the Global menu. The set point
value displayed was then a ratio value. He entered 0.01 to maintain an
input 1:input 2 ratio of 1:100.