How to Select a 4–20 mA Pressure Transmitter for Industrial Control Systems
Learn how to select a 4–20 mA pressure transmitter by range, loop power, wiring, accuracy, process connection and PLC input compatibility.
A pressure transmitter must do more than measure pressure accurately. It also needs to deliver a signal that the connected control system can interpret reliably. In many industrial applications, a 4–20 mA pressure transmitter is used to send pressure measurements to a programmable logic controller (PLC), distributed control system (DCS) or industrial monitoring system.
The 4–20 mA signal is widely used because current-loop systems can provide reliable measurements over industrial wiring distances when properly designed. However, choosing the wrong pressure range, power supply, wiring configuration or PLC input can result in inaccurate readings, installation problems or equipment damage.
This guide explains how to select a transmitter that suits your application, what to check in the datasheet and how to avoid common compatibility mistakes.
Why Are 4–20 mA Signals Widely Used?
Industrial control systems often need to transmit sensor measurements from equipment in the field to a controller located in a control cabinet. A current signal is useful because the measurement is represented by the current flowing through the circuit rather than solely by a voltage measured at the receiving end.
A correctly designed 4–20 mA loop can tolerate voltage drops along the wiring, provided the transmitter has sufficient operating voltage and the loop remains within its electrical limits.
The signal also has a useful feature: 4 mA represents the lower end of the configured measurement range, rather than zero current. This is commonly called a live zero. A current near zero may indicate a broken loop or loss of power, although fault interpretation depends on the transmitter configuration and receiving system.
For example, a transmitter configured for 0–10 bar may use 4 mA to represent 0 bar and 20 mA to represent 10 bar. The PLC converts the measured current into an engineering value for display, control or alarm functions.
How a 4–20 mA Signal Represents Pressure
A standard linear 4–20 mA pressure transmitter maps its configured pressure range to a current between 4 mA and 20 mA.
The relationship is:
Pressure = Lower range value + [(Measured current − 4 mA) ÷ 16 mA] × Pressure span
The pressure span is the upper range value minus the lower range value.
For example, consider a transmitter configured for 0–10 bar.
| Current signal |
Represented pressure |
| 4 mA |
0 bar |
| 8 mA |
2.5 bar |
| 12 mA |
5 bar |
| 16 mA |
7.5 bar |
| 20 mA |
10 bar |
At 12 mA, the signal is halfway between 4 mA and 20 mA, so it represents 50% of the configured span.
This calculation assumes a linear transfer function and the stated range. Some transmitters can be configured for different lower and upper range values, so the actual scaling must be confirmed.
Understand the 4 mA live zero
The 4 mA starting point allows the control system to distinguish a valid zero-scale measurement from a signal of zero current under normal operating conditions. However, a current below 4 mA does not automatically identify a particular fault. Some devices use configured underrange and overrange current levels for diagnostics.
Check the transmitter’s fault signalling specification and the PLC’s input configuration before relying on these signals for alarms.
Check Measurement Range and Engineering Units
The first selection step is to define the pressure the transmitter needs to measure.
Start with the normal operating pressure, then identify the minimum, maximum and any pressure spikes that may occur during startup, shutdown or abnormal conditions.
A transmitter that operates close to the top of its range may leave little room for pressure surges. A range that is unnecessarily large, on the other hand, can reduce the useful resolution of the measurement in some systems.
Choose the correct pressure reference
Confirm whether the application requires:
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Gauge pressure: Pressure relative to local atmospheric pressure.
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Absolute pressure: Pressure relative to a perfect vacuum.
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Differential pressure: The difference between pressures at two ports.
These measurement types are not interchangeable. For example, a gauge pressure transmitter is often appropriate for monitoring pressure in a compressed-air line, while an absolute pressure transmitter may be more suitable for a vacuum process requiring an absolute reference.
Confirm units and configured range
Check whether the process specification uses bar, kPa, MPa, psi or another unit. The transmitter’s range, PLC scaling and operator display must use compatible values.
Also confirm whether the transmitter is factory-configured for the required range or needs configuration before commissioning.
Confirm Supply Voltage and Loop Requirements
Before purchasing a transmitter, verify its electrical supply requirements and the limits of the complete current loop.
Two-wire loop-powered transmitters
A common industrial arrangement uses a two-wire, loop-powered transmitter. The same pair of wires supplies power to the transmitter and carries the 4–20 mA signal.
In this configuration, the transmitter, DC power supply and receiving input form part of the same electrical loop. The available voltage must be sufficient for the transmitter to operate while overcoming voltage drops across the wiring and other loop components.
Other transmitter configurations
Not every 4–20 mA transmitter is a two-wire loop-powered device. Some models use three-wire or four-wire arrangements, separate power connections or additional signal connections.
Do not assume that a wiring method suitable for one transmitter will work for another.
Check the loop voltage budget
Review the manufacturer's minimum and maximum supply voltage requirements. Then consider the voltage needed by the receiving input, the resistance of the wiring and any other components in the circuit.
For a simple series loop, the available supply voltage must cover the required transmitter voltage, the voltage drop across the receiver and the voltage drop across the wiring and other series resistance.
If the available voltage is insufficient, the transmitter may not maintain the expected output across its full operating range.
Wiring and PLC Input Compatibility
A transmitter can have the correct pressure range and output but still be unsuitable if it is incompatible with the PLC analog input.
Confirm the PLC input type
Check whether the PLC has a current input that supports the transmitter’s output range. Verify the input channel configuration, current limits, input resistance, isolation arrangements and whether the channel is active or passive, where relevant.
The terms active and passive describe how power is supplied to the loop or input arrangement. Their exact meaning and wiring implications depend on the equipment. Confirm the PLC and transmitter documentation rather than relying on terminology alone.
Match the PLC scaling
The PLC program must convert the measured current into the transmitter’s configured pressure range.
For a linear 0–10 bar transmitter:
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4 mA should map to 0 bar.
-
12 mA should map to 5 bar.
-
20 mA should map to 10 bar.
The raw PLC input counts depend on the particular analog module, its resolution and configuration. Use the module’s documentation to establish the correct scaling values.
Follow the specific wiring diagram
Do not use a generic wiring diagram as a substitute for the manufacturer's terminal diagram. Terminal numbers, polarity, grounding, shielding and power arrangements vary by model.
Before installation or wiring changes, isolate power where required, follow the equipment manufacturer's instructions and comply with applicable electrical requirements. Have a suitably qualified person perform the work where required.
Accuracy, Response Time and Environmental Conditions
Electrical compatibility is only one part of transmitter selection. The device must also perform reliably under the actual operating conditions.
Accuracy and stability
Check the manufacturer's stated accuracy and how it is specified. Depending on the datasheet, accuracy may be expressed as a percentage of full scale, span or another defined reference.
Also review repeatability, long-term stability, temperature effects and any compensation limits relevant to the application. A transmitter with a large measurement range may not provide the precision required for a low-pressure process.
Response time
Some processes need a fast response to changing pressure, while others benefit from stable readings with less fluctuation. Check the transmitter's response time and whether filtering or damping can be configured.
Excessive damping can delay important changes. Too little filtering may allow pressure pulsations or noise to affect the displayed measurement.
Process connection and wetted materials
Confirm the required thread type, size, sealing arrangement and mounting orientation. Also verify that the materials in contact with the process medium are compatible with the liquid or gas, including any corrosive or cleaning agents.
Do not assume that two mechanically similar connections are interchangeable.
Temperature and environmental rating
Review process and ambient temperature limits, ingress protection, vibration resistance and any required hazardous-area or industry approvals. The enclosure rating alone does not establish suitability for every installation condition.
Fault Detection and Common Installation Mistakes
A properly selected transmitter can still produce unreliable readings if the loop is incorrectly installed or configured.
Common problems include:
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Incorrect range scaling: The PLC displays the wrong pressure because its engineering range does not match the transmitter.
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Insufficient loop voltage: The supply cannot maintain the required voltage across the transmitter and loop resistance.
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Wrong input configuration: The PLC channel is set for voltage rather than current, or is otherwise configured incorrectly.
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Incorrect polarity or terminal connections: The circuit does not operate as intended.
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Poor shielding or grounding practices: Electrical interference affects the measurement.
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Unverified fault thresholds: The PLC treats an underrange or overrange signal as a normal process value.
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Wrong pressure reference: The transmitter measures gauge pressure when the application requires absolute or differential pressure.
For troubleshooting, compare the transmitter's indicated or measured output with the PLC input value and the configured scaling. Use the manufacturer's diagnostic procedures and appropriate test equipment.
Before testing or changing wiring, follow the required isolation procedure and ensure that the work is safe for the electrical and process conditions.
4–20 mA Pressure Transmitter Datasheet Checklist
Use this checklist when comparing products or requesting technical confirmation.
| Specification |
What to verify |
| Measurement type |
Gauge, absolute or differential |
| Pressure range |
Normal operating range, maximum pressure and expected transients |
| Output |
4–20 mA and any configured fault signalling |
| Wiring |
Two-wire, three-wire or four-wire configuration |
| Supply |
Required voltage and permitted operating range |
| Loop capacity |
Available voltage after receiver and wiring voltage drops |
| PLC input |
Current-input compatibility, channel configuration and input limits |
| Accuracy |
Stated accuracy, reference basis, repeatability and temperature effects |
| Response |
Response time and available damping or filtering |
| Process connection |
Thread, size, sealing and mounting requirements |
| Wetted materials |
Compatibility with the process medium |
| Environment |
Temperature, ingress protection and required approvals |
| Documentation |
Manufacturer datasheet, terminal diagram and configuration instructions |
FAQs
What does 4–20 mA mean?
It is an analog current signal in which 4 mA commonly represents the lower configured measurement value and 20 mA represents the upper value. The receiving system converts the current into an engineering measurement such as pressure.
Is a 4–20 mA pressure transmitter always two-wire?
No. Two-wire loop-powered models are common, but three-wire and four-wire configurations are also available. Confirm the specific model's power and wiring requirements before connecting it.
How do I match a pressure transmitter to a PLC?
Check that the PLC analog input supports the transmitter's current output, that the power and loop arrangements are compatible, and that the configured pressure range matches the PLC scaling. Review both manufacturers' documentation.
Can a 4–20 mA transmitter operate over a long cable?
It can be suitable for long industrial cable runs, provided the complete loop remains within its electrical limits. Cable resistance, supply voltage, receiver resistance, electrical interference and installation requirements must all be considered.
Conclusion
Choosing a 4–20 mA pressure transmitter involves more than matching a pressure range. The measurement reference, configured span, power supply, wiring arrangement, PLC input, accuracy, process connection and environment all affect compatibility.
Start with the measurement task, then check the complete electrical loop and control-system requirements. Before purchasing, verify the output and wiring details against the individual manufacturer's datasheet.
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