Pressure transmitters are among the most important sensors used in industrial facilities. They measure pressure in pipelines, vessels, hydraulic systems, or compressed air installations and typically transmit the measured values to a PLC or process control system via a 4–20 mA signal.
When incorrect readings occur, the PLC or process control system is often suspected first. In many cases, however, the root cause lies directly with the pressure transmitter, the wiring, or the current loop itself. To identify faults quickly, it is advisable to test the pressure transmitter independently of the control system.
Using a suitable loop calibrator such as the UPS4E, pressure transmitters can be tested, calibrated, and simulated directly in the field. This makes it possible to determine within minutes whether the problem originates from the sensor, the wiring, or the PLC.
In this article, you will learn which test equipment is required, how to test a pressure transmitter without a PLC, and how typical fault conditions can be diagnosed quickly.
Table of Contents
- What Is a Pressure Transmitter?
- Typical Pressure Transmitter Faults
- What Test Equipment Is Required?
- Testing a Pressure Transmitter Without a PLC – Step by Step
- Practical Maintenance Example
- Pressure Transmitter or PLC – Where Is the Fault?
- Multimeter or Loop Calibrator?
- FAQ
- Conclusion
What Is a Pressure Transmitter?
A pressure transmitter measures the pressure applied to a medium and converts it into an electrical output signal. In industrial automation, this signal is typically transmitted via a standardized 4–20 mA current loop.
The PLC or process control system then interprets the measured current value as a pressure reading. Typically, the lower range value corresponds to 4 mA and the upper range value corresponds to 20 mA.
For example, a pressure transmitter with a measuring range of 0 to 10 bar produces the following signal relationship:
| Pressure | Current Signal |
|---|---|
| 0 bar | 4 mA |
| 2.5 bar | 8 mA |
| 5.0 bar | 12 mA |
| 7.5 bar | 16 mA |
| 10.0 bar | 20 mA |
If the PLC displays an incorrect pressure value, this does not automatically mean that the pressure transmitter is faulty. Wiring problems, an incorrect power supply, or a misconfigured analog input can also be responsible.
For this reason, troubleshooting should always begin by verifying whether the pressure transmitter is actually producing the correct 4–20 mA signal. This is exactly where loop calibrators such as the UPS4E are useful, as they can measure, simulate, and document current signals.
Typical Pressure Transmitter Faults
If pressure readings displayed by the PLC or process control system appear implausible, the pressure transmitter is not necessarily the cause. In practice, several common fault conditions occur that can often be narrowed down with just a few measurements.
The faster the actual cause is identified, the shorter plant downtime and troubleshooting efforts will be.
No Signal Present
If the PLC continuously displays 0 mA or no reading at all, the cause is often a cable break, missing power supply, or an interruption in the current loop.
In this situation, the first step should be to verify that the loop is correctly powered and that the pressure transmitter is actually generating an output signal.
Reading Permanently Too High or Too Low
A constant measurement error often indicates a calibration shift, an incorrectly configured measuring range, or an improperly scaled PLC input.
By directly measuring the output current, it is possible to quickly determine whether the fault originates from the pressure transmitter or from signal processing.
Fluctuating or Jumping Readings
Unstable readings are often caused by loose connections, poor terminal contacts, EMC interference, or an inadequate power supply.
Damaged sensors or process disturbances can also lead to significant signal fluctuations.
3.6 mA or 22 mA Output Signals
Many modern pressure transmitters support NAMUR NE43 fault signaling. In this case, the transmitter intentionally indicates detected faults through defined current values.
| Signal | Possible Meaning |
|---|---|
| 3.6 mA | Underrange condition or device fault |
| 22 mA | Overrange condition or device fault |
| 0 mA | Cable break or power supply failure |
The exact meaning depends on the manufacturer and the device configuration.
Pressure Transmitter or PLC?
One of the most common questions in practice is: Is the pressure transmitter faulty, or is the PLC processing the signal incorrectly?
This is where the major advantage of a loop calibrator becomes evident. A device such as the UPS4E can both measure the actual output signal of the pressure transmitter and inject a defined 4–20 mA signal directly into the PLC.
This makes it possible to determine within minutes whether the root cause lies with the sensor, the wiring, or the controller’s analog input.
Common Faults at a Glance
| Symptom | Possible Cause |
|---|---|
| 0 mA | Cable break or missing power supply |
| 3.6 mA | Sensor fault or underrange condition |
| 22 mA | Overrange condition or device fault |
| Reading fluctuates | Wiring or EMC issues |
| Reading permanently incorrect | Calibration or scaling error |
| Signal correct, PLC displays incorrect value | Check analog input or PLC configuration |
Most faults can be clearly identified simply by measuring the actual current signal. Therefore, troubleshooting should always begin at the pressure transmitter rather than at the control system.
What Test Equipment Is Required?
Only a few pieces of test equipment are required to test a pressure transmitter independently of a PLC or process control system. The goal is to apply a defined pressure to the transmitter while simultaneously monitoring the resulting 4–20 mA output signal.
With the correct test setup, sensor faults, wiring problems, and PLC issues can be distinguished from one another within just a few minutes.
1. Pressure Source
A pressure source is required to generate the desired test pressure. Depending on the pressure range, either pneumatic or hydraulic hand pumps are typically used.
| Pressure Range | Typical Pressure Source |
|---|---|
| Vacuum to approximately 35 bar | Pneumatic hand pump |
| Up to several hundred bar | Hydraulic hand pump |
| Laboratory and calibration applications | Automatic pressure controller |
2. Reference Pressure Instrument
To verify the actual applied pressure, a reference pressure instrument is required. This serves as the comparison value during testing.
Depending on the required accuracy, digital precision pressure gauges, pressure calibrators, or automatic pressure controllers may be used.
3. Loop Calibrator
The most important tool for troubleshooting is the measurement of the output signal. This is performed using a loop calibrator.
A device such as the UPS4E Loop Calibrator can not only measure 4–20 mA signals but can also generate and simulate defined current values. This makes it possible to verify whether the PLC correctly processes the injected signal.
Key functions include:
- Measurement from 0 to 24 mA
- Simulation of 4–20 mA signals
- Integrated 24 V loop power supply
- Voltage measurement up to ±30 V DC
- HART support via integrated 250 Ω resistor
- Step and ramp functions
- Data logger for service applications
Detailed technical information can be found in the UPS4E Datasheet.
Typical Test Setup
During troubleshooting, the pressure transmitter is first disconnected from the PLC. A test pressure is then applied while the output current is measured directly.
If the pressure and current signal match, the pressure transmitter is functioning correctly. If deviations occur, the cause is likely related to the sensor or its calibration.
In the next step, the loop calibrator can be used to inject a defined 4–20 mA signal directly into the PLC input. This allows the technician to determine conclusively whether the controller is operating correctly.
Testing a Pressure Transmitter Without a PLC – Step by Step
Testing a pressure transmitter without a PLC is much easier than many users expect. The objective is to measure the transmitter’s output signal directly and compare it with the actual applied pressure.
This approach makes it possible to quickly determine whether the fault lies within the pressure transmitter, the wiring, or the PLC.
Step 1: Disconnect the Pressure Transmitter from the Control System
The pressure transmitter should first be electrically disconnected from the PLC or process control system. This prevents other components from influencing the measurement.
At the same time, it ensures that only the behavior of the pressure transmitter itself is being evaluated.
Step 2: Set Up the Test Equipment
The required test equipment is now connected:
- Pressure source
- Reference pressure instrument
- Pressure transmitter
- Loop calibrator
If the transmitter does not have its own power supply, the integrated 24 V loop power supply of the UPS4E Loop Calibrator can be used.
Step 3: Check the Zero Point
With no pressure applied, the pressure transmitter should output the lower range value.
For a measuring range of 0 to 10 bar, the expected output is:
| Pressure | Expected Output Current |
|---|---|
| 0 bar | 4.000 mA |
Many faults can already be identified at this stage. If the signal deviates significantly, a zero shift is often present.
Step 4: Check the Full-Scale Value
The upper range value is then applied.
| Pressure | Expected Output Current |
|---|---|
| 10 bar | 20.000 mA |
If the full-scale value is incorrect, this often indicates a calibration issue or span error.
Step 5: Verify Intermediate Points
For meaningful testing, several intermediate points should also be checked.
| Pressure | Target Value |
|---|---|
| 25% | 8.000 mA |
| 50% | 12.000 mA |
| 75% | 16.000 mA |
This makes it possible to determine whether the pressure transmitter operates linearly across the entire measuring range.
Step 6: Evaluate the Results
The measured values are now compared with the theoretical target values.
| Result | Assessment |
|---|---|
| All values correct | Pressure transmitter operating properly |
| Constant deviation | Possible zero shift |
| Incorrect full-scale value | Possible span error |
| Incorrect intermediate values | Possible linearity issue |
| No signal present | Check power supply or wiring |
Step 7: Test the PLC Input
If the pressure transmitter is operating correctly, the next step is to inject a defined current signal directly into the PLC analog input.
Using the UPS4E, signals such as 4 mA, 12 mA, or 20 mA can be simulated.
If the PLC then displays an incorrect pressure value, the cause is not the pressure transmitter but rather the PLC configuration, wiring, or analog input channel.
This straightforward procedure typically allows the fault source to be identified within just a few minutes.
Practical Maintenance Example
A typical real-world scenario: The control room reports highly unstable pressure readings in a process plant. The displayed pressure fluctuates between 45 and 60 bar within seconds, even though operating personnel confirm that the actual process pressure remains stable.
The first assumption is often that the pressure transmitter has failed. However, before replacing a sensor, the root cause should be clearly identified.
Initial Situation
| Parameter | Value |
|---|---|
| Measured Variable | Pressure |
| Measuring Range | 0 … 50 bar |
| Output Signal | 4 … 20 mA |
| Reported Issue | Fluctuating pressure readings |
Testing the Pressure Transmitter
The pressure transmitter is first disconnected from the PLC and connected to a pressure source and a loop calibrator.
Several test points are then applied and the output current values are measured.
| Pressure | Target Value | Measured Value |
|---|---|---|
| 0 bar | 4.000 mA | 4.001 mA |
| 25 bar | 12.000 mA | 11.999 mA |
| 50 bar | 20.000 mA | 20.001 mA |
The results show that the pressure transmitter is operating within the specified tolerances. A sensor fault can therefore be ruled out.
Testing the PLC
In the next step, the pressure transmitter is completely disconnected. Instead, a defined current signal is injected directly into the PLC input using the UPS4E Loop Calibrator.
| Simulated Current | Expected Display | Actual Display |
|---|---|---|
| 4 mA | 0 bar | 0 bar |
| 12 mA | 25 bar | 31 bar |
| 20 mA | 50 bar | 62 bar |
Within just a few minutes it becomes clear that the PLC is interpreting the current values incorrectly. The pressure transmitter is functioning correctly, while the actual problem lies in the analog input configuration.
Root Cause
Further investigation reveals that the PLC analog input had been configured for a measuring range of 0 to 60 bar, while the installed pressure transmitter was designed for a range of 0 to 50 bar.
As a result of the incorrect scaling, all pressure values were calculated incorrectly and displayed inaccurately within the control system.
Result
After correcting the PLC configuration, the displayed values once again matched the actual process pressure. An unnecessary replacement of the pressure transmitter was avoided.
This example demonstrates why a systematic troubleshooting approach is so important. By combining measurement and simulation, loop calibrators can quickly pinpoint faults and help prevent unnecessary downtime, spare parts costs, and maintenance effort.
Pressure Transmitter or PLC – Where Is the Fault?
When incorrect pressure readings occur, the same question often arises: Is the pressure transmitter faulty, or is the PLC processing the signal incorrectly?
Without a structured testing procedure, sensors are frequently replaced unnecessarily or valuable troubleshooting time is wasted. With a few targeted tests, however, the root cause can usually be identified very quickly.
Method 1: Measure the Pressure Transmitter Output Signal
The first step is to measure the actual output signal directly at the pressure transmitter.
If the measured current corresponds to the applied pressure, the transmitter is fundamentally operating correctly. In that case, the fault is likely located elsewhere.
| Test | Assessment |
|---|---|
| 4 mA at 0% of range | Zero point correct |
| 20 mA at 100% of range | Full-scale value correct |
| Intermediate values plausible | Transmitter operating correctly |
Method 2: Simulate a Current Signal Directly into the PLC
In the second step, the pressure transmitter is disconnected from the analog input. A loop calibrator is then used to inject a defined signal directly into the PLC.
A device such as the UPS4E can generate precise values such as 4 mA, 12 mA, or 20 mA.
If the PLC displays the expected pressure values, the analog input is functioning correctly. If the displayed values differ, the fault usually lies within the PLC configuration or the analog input itself.
Method 3: Check the Wiring
Even if both the pressure transmitter and PLC are functioning correctly, wiring issues can still cause incorrect readings.
Typical causes include:
- Loose terminals
- Corroded contacts
- Damaged cables
- Reversed connections
- Voltage drops over long cable runs
- EMC interference
For this reason, the entire current loop should always be considered during troubleshooting.
Systematic Fault Analysis
| Test Result | Most Likely Cause |
|---|---|
| Transmitter outputs incorrect signal | Check sensor or calibration |
| Transmitter correct, PLC incorrect | Check PLC input or scaling |
| Signal fluctuates significantly | Check wiring or power supply |
| No signal present | Cable break or power failure |
| Simulation works, transmitter does not | Pressure transmitter likely faulty |
| Simulation also fails | Check PLC or wiring |
Why Signal Simulation Is So Valuable
Measuring a current signal only shows what the pressure transmitter is currently outputting. By additionally simulating a defined signal, the entire signal chain can be tested.
This makes it possible to determine conclusively whether the fault lies with the sensor, the wiring, or the PLC. This combination of measurement and simulation is precisely what makes a loop calibrator one of the most valuable tools for service, maintenance, and commissioning applications.
Multimeter or Loop Calibrator?
When troubleshooting pressure transmitters, a multimeter is often the first tool used. For simple voltage or current measurements, this is usually sufficient. However, when pressure transmitters, PLC inputs, or complete 4–20 mA current loops need to be tested, a multimeter quickly reaches its limits.
A loop calibrator has been specifically designed for these applications and provides additional functions that make troubleshooting significantly easier.
| Function | Multimeter | UPS4E |
|---|---|---|
| Measure 4–20 mA signals | ✓ | ✓ |
| Simulate 4–20 mA signals | ✗ | ✓ |
| 24 V loop power supply | ✗ | ✓ |
| Test PLC inputs | ✗ | ✓ |
| HART support | ✗ | ✓ |
| Data logger | ✗ | ✓ |
A multimeter is an excellent tool for basic electrical testing. However, for targeted troubleshooting of pressure transmitters and current loops, a loop calibrator such as the UPS4E offers significantly greater capabilities.
In particular, the ability to simulate defined 4–20 mA signals often saves considerable time during troubleshooting and allows a clear distinction to be made between sensor, wiring, and PLC-related faults.
FAQ – Testing a Pressure Transmitter Without a PLC
Can I test a pressure transmitter without a PLC?
Yes. Using a pressure source and a loop calibrator, a pressure transmitter can be tested completely independently of a PLC or process control system.
How can I tell if a pressure transmitter is faulty?
If the transmitted 4–20 mA signal does not correspond to the applied pressure or fluctuates significantly, the sensor or electronics may be defective. A direct measurement of the output signal allows this to be verified quickly.
What does an output signal of 3.6 mA mean?
Many pressure transmitters use NAMUR NE43 fault signaling. A signal of approximately 3.6 mA often indicates a device fault or a measured value below the permissible measuring range.
What does an output signal of 22 mA mean?
A signal of approximately 22 mA typically indicates an overrange condition or a fault detected by the transmitter.
Can I test a pressure transmitter using a multimeter?
Yes. The output signal can generally be measured with a multimeter. However, for complete troubleshooting and signal simulation, a loop calibrator is a much more suitable solution.
How can I verify whether the PLC is processing the signal correctly?
The pressure transmitter is disconnected from the analog input and a defined current signal is injected directly into the PLC. If the displayed values do not match the simulated signal, the fault most likely lies within the PLC or its configuration.
How can I test a PLC analog input?
A loop calibrator can generate defined values such as 4 mA, 12 mA, or 20 mA. The PLC should then display the corresponding process values.
What equipment is required for testing?
For most applications, a pressure source, a reference pressure instrument, and a loop calibrator are sufficient.
Which loop calibrator is suitable for this task?
Suitable loop calibrators should be capable of measuring and simulating current signals while also providing loop power. One example is the UPS4E Loop Calibrator.
Where can I find technical information about the UPS4E?
A complete overview of technical specifications and functions can be found in the UPS4E Datasheet.