Calculation of RAW count for Pressure Transmitter in PLC Programming

Introduction

In industrial automation and process control systems, pressure transmitters play a critical role in monitoring and controlling processes such as fluid flow, tank levels, hydraulic systems, steam lines, pneumatic systems, and chemical processing. A pressure transmitter converts physical pressure into an electrical signal that can be interpreted by a PLC (Programmable Logic Controller), SCADA system, or DCS (Distributed Control System).

 

 

One of the most important concepts in PLC programming and instrumentation is Raw Count Calculation. PLCs do not directly understand engineering units such as bar, psi, MPa, or kg/cm². Instead, the PLC receives a numerical value called a raw count from the analog input module. This raw count must be mathematically converted into engineering units so operators and control systems can correctly interpret the process pressure.

Understanding raw count calculation is essential for:

PLC programmers

Instrumentation engineers

Maintenance engineers

Automation technicians

SCADA developers

Control panel designers

This document explains the concept of PLC raw counts for pressure transmitters in detail, including scaling formulas, analog signal conversion, practical examples, ladder logic concepts, troubleshooting methods, and industrial applications.

 

Understanding Pressure Transmitters

A pressure transmitter is an instrument that measures pressure and converts it into a proportional electrical signal.

Common output signals include:

Signal Type	Description
4–20 mA	Most widely used industrial analog signal
0–10 V	Voltage-based analog signal
1–5 V	Derived from 4–20 mA using resistor
Digital Communication	HART, Modbus, Profibus, Ethernet/IP

 

The most common signal used in industries is:

4–20 mA Analog Signal

In a 4–20 mA system:

4 mA represents the minimum pressure

20 mA represents the maximum pressure

Example:

Pressure	Output Current
0 bar	4 mA
10 bar	20 mA

This relationship is linear.

 

Why PLC Uses Raw Counts

A PLC analog input module cannot directly process current values like 4 mA or 20 mA. Instead, the module converts the analog signal into a digital value using an Analog-to-Digital Converter (ADC).

This digital value is called:

Raw Data

Raw Count

ADC Count

Digital Count

The PLC program then converts this raw value into actual engineering units.

 

Analog-to-Digital Conversion

The analog input module samples the incoming signal and converts it into binary data.

Example:

Current Signal	PLC Raw Count
4 mA	0
20 mA	32767

 

Some PLCs use:

PLC Brand	Typical Raw Range
Allen-Bradley	0–32767
Siemens	0–27648
Delta	0–4000
Mitsubishi	0–32000
Omron	0–4000

 

The raw count depends on the PLC manufacturer and analog module resolution.

 

Relationship Between Pressure and Raw Count

Pressure and raw count are directly proportional.

For a pressure transmitter:

Pressure	Current	Raw Count
0 bar	4 mA	0
10 bar	20 mA	32767

 

This creates a linear scaling relationship.

 

Linear Scaling Formula

The general scaling formula is:

Where:

Parameter	Meaning
Raw Count	Current PLC analog value
Raw Min	Minimum raw value
Raw Max	Maximum raw value
EU Min	Minimum engineering unit
EU Max	Maximum engineering unit

 

Example 1: Pressure Scaling Calculation

Given Data

Pressure transmitter range:

0 to 10 bar

Output: 4–20 mA

PLC raw range: 0–32767

Suppose PLC raw count = 16384

Find the pressure value.

 

Step-by-Step Calculation

Using the formula:

Simplifying:

Result:

Thus:

Raw count = 16384

Actual pressure ≈ 5 bar

 

Example 2: Siemens PLC Scaling

Siemens analog modules commonly use:

Signal	Raw Value
4 mA	0
20 mA	27648

 

Pressure range:

0–16 bar

Suppose raw count = 13824

Calculation:

Result:

 

Importance of Scaling in PLC

Scaling is necessary because operators cannot interpret raw counts directly.

Without scaling:

PLC may show meaningless numbers

SCADA displays become confusing

Alarm settings become incorrect

PID control becomes unstable

After scaling:

Raw Count	Actual Pressure
0	0 bar
16384	5 bar
32767	10 bar

Pressure Transmitter Calibration Range

Every transmitter has:

LRV (Lower Range Value)

URV (Upper Range Value)

Example:

Parameter	Value
LRV	0 bar
URV	10 bar

The transmitter linearly maps pressure into current.

4–20 mA Signal Calculation

To calculate current from pressure:

Example:

Pressure = 5 bar

Range = 0–10 bar

Calculation:

Result:

Thus:

5 bar = 12 mA

 

PLC Scaling Methods

PLC scaling can be performed using:

Method	Description
Mathematical Formula	Manual programming
SCP Instruction	Allen-Bradley Scale with Parameters
SCALE Block	Siemens scaling block
Function Block	Dedicated analog scaling FB
SCADA Scaling	Scaling inside HMI/SCADA

 

Allen-Bradley PLC Scaling

In Allen-Bradley PLCs, scaling is commonly done using:

SCP instruction

Compute instruction

CPT instruction

Example:

Input raw range:

0–32767

Engineering range:

0–10 bar

Formula:

Siemens PLC Scaling

Siemens provides standard instructions:

SCALE_X

NORM_X

NORM_X

Normalizes raw value into 0–1.

SCALE_X

Converts normalized value into engineering units.

 

Ladder Logic Concept

Basic PLC scaling process:

1. Read analog input
2. Store raw count
3. Apply scaling formula
4. Store engineering value
5. Display on HMI

 

Example Ladder Logic Explanation

Suppose:

AI channel = IW64

Pressure range = 0–10 bar

Steps:

Read Analog Input

PLC reads:

IW64 = 16384

Apply Scaling

Program computes:

Pressure = (16384 × 10) / 32767

Output

Pressure = 5 bar

 

Importance in PID Control

Pressure scaling is critical for PID loops.

Incorrect scaling causes:

Oscillation

Unstable control

Wrong setpoint tracking

Valve hunting

Pump instability

Correct scaling ensures:

Accurate control

Stable process

Reliable operation

 

Real Industrial Applications

Pressure transmitter scaling is used in:

Industry	Application
Water Treatment	Pump pressure monitoring
Oil & Gas	Pipeline pressure
Food Industry	Tank pressure
Chemical Plants	Reactor pressure
HVAC	Air duct pressure
Power Plants	Steam pressure
Hydraulics	Hydraulic system monitoring

 

Differential Pressure Transmitter Scaling

Differential pressure transmitters measure pressure difference.

Applications:

Flow measurement

Filter monitoring

Tank level measurement

Example range:

-100 to +100 mbar

Scaling formula remains linear.

 

Negative Pressure Scaling

Some transmitters use negative ranges.

Example:

Pressure	Current
-1 bar	4 mA
+1 bar	20 mA

 

Scaling becomes:

Analog Input Resolution

Resolution determines accuracy.

Common resolutions:

Resolution	Counts
12-bit	4096
13-bit	8192
15-bit	32768
16-bit	65536

Higher resolution gives:

Better accuracy

Smoother readings

Improved PID control

 

Signal Noise Problems

Analog signals can contain noise due to:

Improper grounding

Electrical interference

VFD switching noise

Long cable runs

 

Effects:

Fluctuating pressure readings

Incorrect scaling

Unstable process control

 

Solutions:

Shielded cables

Proper grounding

Signal isolators

Analog filters

 

Open Circuit Detection

In 4–20 mA systems:

4 mA = minimum process value

Below 4 mA may indicate fault

Example:

Current	Condition
0 mA	Wire break
3 mA	Fault
4–20 mA	Normal
>20 mA	Over-range

PLCs can detect these faults using raw counts.

Over-Range and Under-Range

Industrial transmitters may exceed calibrated range.

Example:

Signal	Condition
22 mA	Over-range
3.5 mA	Under-range

PLC programs should include alarm logic.

Practical Example: Tank Pressure Monitoring

System Description

Pressure transmitter: 0–6 bar

PLC raw range: 0–27648

HMI displays pressure

Suppose raw count = 18432

Calculation:

Result:

Importance of Engineering Units

Engineering units provide meaningful interpretation.

Examples:

Raw Count	Engineering Unit
12000	Meaningless
4.5 bar	Useful

Operators always require engineering units.

Common PLC Scaling Errors

Incorrect Raw Range

Using wrong PLC raw values causes inaccurate readings.

Wrong Engineering Range

If transmitter range differs from program range:

Display becomes incorrect

Alarms fail

PID loops malfunction

 

Integer Division Errors

Using integer math can reduce accuracy.

Solution:

Use floating-point calculations

 

Floating Point Scaling

Better accuracy is achieved using REAL data type.

Example:

REAL Pressure

Instead of:

INT Pressure

Benefits:

Decimal precision

Accurate analog scaling

Better control stability

 

HMI Display Considerations

HMIs display scaled engineering values.

Common display features:

Numeric display

Bar graph

Trend graph

Alarm indication

Historical data logging

 

SCADA Integration

SCADA systems use scaled values for:

Process monitoring

Trending

Alarm handling

Data logging

Remote monitoring

Incorrect scaling affects the entire control system.

 

Pressure Transmitter Accuracy

Accuracy affects PLC readings.

Typical transmitter accuracy:

Type	Accuracy
Standard	±0.5%
Industrial	±0.25%
High Precision	±0.1%

Calibration and Verification

Pressure transmitters should be calibrated periodically.

Calibration steps:

1. Apply known pressure
2. Measure output current
3. Compare with expected value
4. Adjust transmitter if neede

Example Calibration Table

Pressure	Expected Current
0 bar	4 mA
2.5 bar	8 mA
5 bar	12 mA
7.5 bar	16 mA
10 bar	20 mA

 

Importance in Industry 4.0

Modern smart factories use pressure scaling for:

Predictive maintenance

Cloud monitoring

IIoT systems

Energy optimization

AI-based diagnostics

Accurate raw count conversion is essential for reliable analytics.

 

Best Practices for PLC Raw Count Calculation

Use Proper Scaling

Always verify:

Raw input range

Transmitter range

PLC module specifications

Use Floating Point Math

Improves precision.

Add Filtering

Removes signal noise.

Include Fault Detection

Detect:

Wire break

Over-range

Sensor failure

Verify Calibration

Regular maintenance improves reliability.

 

Troubleshooting Analog Scaling Problems

Problem	Possible Cause
Wrong pressure display	Incorrect scaling
Fluctuating reading	Electrical noise
Always zero	Wiring fault
Reading too high	Wrong transmitter range
Unstable value	Grounding issue

Future of Smart Pressure Measurement

Modern transmitters now support:

Wireless communication

Self-diagnostics

Remote calibration

Digital protocols

Cloud integration

Despite advanced technologies, PLC raw count scaling remains a fundamental concept in automation engineering.

 

PLC raw count calculation for pressure transmitters is one of the most important topics in industrial automation and instrumentation. A pressure transmitter converts physical pressure into a standard analog signal such as 4–20 mA, and the PLC analog module converts this signal into digital raw counts.

Since PLCs process raw numerical data rather than engineering units, scaling becomes essential to convert these counts into meaningful pressure values such as bar, psi, or MPa. Proper scaling ensures accurate monitoring, stable PID control, reliable alarm systems, and efficient industrial operation.

Understanding raw count calculation helps engineers design accurate automation systems, troubleshoot analog problems, optimize control loops, and improve plant reliability. Whether in water treatment, oil and gas, manufacturing, chemical processing, or smart factories, correct pressure scaling forms the foundation of precise industrial control systems.