D8 Series User’s Guide
Chapter 4: Operation and Setup
70
%
0
0
1
%
0
20%
Input Signal
Pr
ocess
V
aria
ble
8
28
Ran
ge
for set points and alarms
Figure 4.6
– Input Scaling
The range for set points and alarms is bound by the process variables that correspond to the 0
percent and 100 percent input signals. Bear in mind that the range for set points and alarms is
not
bound by the low and high process variable ranges that you enter in the scaling parameters.
Input Scaling Example: 4 to 20mA Sensor
Situation
Suppose the controller has a 0 to 20mA process input that is connected to a pressure sensor. The
pressure sensor has a range of 4 to 20mA, representing 0.0 to 50.0 pounds per square inch (PSI).
Setup
Set the scaling parameters in the
Input
menu as follows:
• For the
Input
type parameter, choose
process
.
• For the
Disp forma
t parameter, choose
-999.9to 3000.0
, because the sensor measures PSI
in tenths.
• For the
Input signal low
and
Input signal high
parameters, use the minimum and maximum
range of the sensor. In this case, the sensor range is 4 to 20mA. The range must be
expressed in percent of full scale. To determine the percentages, divide the minimum and
maximum sensor range (4mA and 20mA) by the maximum signal that the controller can
accept (20mA):
–
Input signal low
= 4mA/20mA = 0.2 = 20%
–
Input signal high
= 20mA/20mA = 1.0 = 100%
• For the
Input range low
and
Input range high
parameters, enter the process values that
correspond to the low and high signals. In this case, a 20 percent (4mA) signal represents
0.0 PSI. A 100 percent (20mA) signal represents 50.0 PSI.
