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Appendix: linear scaling examples, Example 1 – Watlow CAS User Manual

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Appendix: Linear Scaling Examples

CAS User’s Guide 75

Appendix: Linear Scaling

Examples

Example 1

Situation

A pressure sensor that generates a 4-20 milliamp signal is connected to
the CAS. The specifications of the sensor state that it generates 4
milliamps at 0.0 PSI and 20 mA at 50.0 PSI.

Setup

The sensor is connected to a channel input set up with a resistor scaling
network to produce 60 millivolts at 20 mA. (See the Inputs section of
Chapter 2: Installation for more information on scaling networks.)

The sensor measures PSI in tenths, so the appropriate display format is -
999.9 to +3000.0.

This table shows the input readings.

The scaling values are therefore:

PV

Displayed

Sensor Input

Reading (%FS)

50.0 psi

20

100%

0.0

4

100% x (4ma/20ma) = 20%

Parameter

Low Value

High Value

Process Value (PV)

0.0 psi

50.0 psi

Input Reading (RDG)

20.0

100.0