Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 28)

Optimizing Your Process System with the WATLOW Series 988
General Software Features
3.8
Input Filter
Overview
In certain applications the process being
measured can be unstable, which makes it
difficult to control and also makes the con-
stantly changing display difficult to read. The
Series 988 input filter can solve these problems
by smoothing out just the display or the display
and the input signal.
You can set a time constant, in seconds, for a
TL W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
limit
limit
sensor
input 2
input 1
heater
output 1 temperature
sensor
Humidity Chamber
Display readings with input
filter off Display readings with input
filter on
time → time →
%RH
→
%RH
→
humidity
transmitter
Sample Application
A Series 988 controls the humidity in an environmental chamber. The
relative humidity (RH) sensor provides a 4-20mA signal over a 0-100% RH
range. The sensor is very sensitive to changes caused by air flow in the
chamber. The turbulence in the chamber has the 988 display jumping two
to three percent. To remove this display dithering the filter time constant
for input 1 is set to 2 seconds. This will smooth the display and
provide a more realistic reading.
low-pass filter that will affect the display only,
or you can configure the option to filter the
input signal itself. Use this feature with cau-
tion, because a large time constant could hide
system upsets.
Requirements
This feature is standard on all Series 988
controllers.