Watlow PM LEGACY™ Limit Controller • 88 • Chapter 8 Features
Filter Time Constant
Filtering smooths an input signal by applying a first-order filter time constant to the signal. Filtering
the displayed value makes it easier to monitor. Filtering the signal may improve the performance of
PID control in a noisy or very dynamic system.
Adjust the filter time interval with Filter Time fi L (Setup Page, Analog Input Menu). Example: With a
filter value of 0.5 seconds, if the process input value instantly changes from 0 to 100 and remained at 100,
the display will indicate 100 after five time constants of the filter value or 2.5 seconds.
Sensor Selection
You need to configure the controller to match the input device, which is normally a thermocouple,
RTD or process transmitter.
Select the sensor type with Sensor Type SEn (Setup Page, Analog Input Menu).
Scale High and Scale Low
When an analog input is selected as process voltage or process current input, you must choose the
value of voltage or current to be the low and high ends. For example, when using a 4 to 20 mA input,
the scale low value would be 4.00mA and the scale high value would be 20.00mA. Commonly used
scale ranges are: 0 to 20mA, 4 to 20mA, 0 to 5V, 1 to 5V and 0 to 10V.
You can create a scale range representing other units for special applications. You can reverse scales
from high values to low values for analog input signals that have a reversed action. For example, if 50
psi causes a 4 mA signal and 10 psi causes a 20mA signal.
Scale low and high low values do not have to match the bounds of the measurement range. These
along with range low and high provide for process scaling and can include values not measurable by
the controller. Regardless of scaling values, the measured value will be constrained by the electrical
measurements of the hardware. Select the low and high values with Scale Low s.Lo and Scale High
s.hi. Select the displayed range with Range Low r.Lo and Range High r.hi (Setup Page, Analog Input
Menu).
Filtered Input Signal
Time
Unfiltered Input Signal
Time
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