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Chapter 9:
Linear Scaling Examples
This chapter provides three linear scaling examples. The examples describe:
• A pressure sensor generating a 4 to 20mA signal
• A flow sensor generating a 0 to 5V signal
• A pulse encoder generating 900 pulses per inch of movement
Example 1: A 4-to-20mA Sensor
Situation
A pressure sensor that generates a 4 to 20mA signal is connected to the controller. The
specifications of the sensor state it generates 4mA at 0.0 pounds per square inch (PSI) and 20mA
at 50.0 PSI.
Setup
The sensor is connected to a loop input set up with a resistorscaling network producing 60mV at
20mA.
The
INPUT TYPE
for the loop is set to
LINEAR
. The sensor measures PSI in tenths, so the DISP
FORMAT
is set to -999.9 to 3000.0.
Table 9.1
—
Input Readings
PROCESS VARIABLE
DISPLAYED
SENSOR INPUT
READING, PERCENT OF FULL SCALE
(%FS)
50.0 PSI
20mA
100%FS
0.0 PSI
4mA
100% x (4mA/20mA) = 20%FS
The scaling values setup in the
SETUP LOOP INPUT
menu are shown in
Table 9.2
.
Table 9.2
—
Scaling Values
PARAMETER
PROMPT
VALUE
High Process Variable
HIGH PV
50.0 PSI
High Sensor Reading
HIGH RDG
100.0%FS
Low Process Variable
LO PV
0.0 PSI
Low Sensor Reading
LO RDG
20.0%FS
