If a pressure transmitter does not deliver a normal 4–20 mA signal, but permanently outputs approximately 3.6 mA, 21 mA or another conspicuous current value, this is an important indication for troubleshooting. Many users initially only see an incorrect pressure value in the PLC or on the display. In fact, however, the current value can already indicate whether the measured value is outside the range, whether there is a sensor fault, whether the wiring is faulty or whether the scaling in the control system is incorrect.
Especially with 4–20 mA pressure transmitters, it is important to distinguish between a real measured value, an overrange, an underrange and a fault current. A signal of 21 mA does not automatically mean that the sensor is defective. It may also be that the process pressure is above the configured measuring range. Conversely, a value around 3.6 mA may indicate a fault condition, but can also be related to underrange, parameterization or supply voltage.
This article explains what typical current values such as 3.6 mA, 4 mA, 20 mA or 21 mA can mean, how NAMUR fault currents should be classified and how sensor faults, cable breaks, short circuits, incorrect supply, overpressure, vacuum or PLC scaling errors can be systematically narrowed down.
Table of contents
- Basics: What does 4–20 mA mean for a pressure transmitter?
- Normal range: Correctly interpreting 4 mA to 20 mA
- Underrange: Why a signal below 4 mA can occur
- Overrange: Why a signal above 20 mA can occur
- Fault current: What 3.6 mA or 21 mA can mean
- NAMUR fault current and device settings
- Sensor fault, electronics fault or parameterization?
- Checking cable break, short circuit and loose terminals
- Checking supply voltage and load
- Incorrect measuring range or real overpressure?
- Checking PLC scaling and display correctly
- Testing with a multimeter or loop calibrator
- Table: Current value, possible cause and useful check
- Practical example: Pressure transmitter permanently shows 21 mA
- Which measuring instruments / products are suitable?
- Conclusion: Do not guess fault current, measure systematically
- FAQ: Frequently asked questions about 3.6 mA, 21 mA and fault current
Basics: What does 4–20 mA mean for a pressure transmitter?
A pressure transmitter with a 4–20 mA output converts the measured pressure into an analog current signal. The lower measuring range value is typically output as 4 mA, and the upper measuring range value as 20 mA. Between these values, the signal is proportional to the pressure.
Example: A pressure transmitter with a measuring range of 0 to 10 bar delivers approximately 4 mA at 0 bar, approximately 12 mA at 5 bar and approximately 20 mA at 10 bar. The PLC, display or process control system then converts this current value back into a pressure value.
The advantage of the 4–20 mA signal is that it can be transmitted robustly over longer cables and at the same time enables a certain degree of fault detection. A value of 0 mA, for example, would not be a normal measured value with an active 2-wire transmitter, but a clear indication of interruption, missing supply or a serious fault.
However, it is important to note: Not every value outside 4 to 20 mA is automatically a device defect. Many pressure transmitters can intentionally output below 4 mA or above 20 mA to signal underrange, overrange or fault conditions. It must therefore always be checked which setting and specification apply to the specific device.
Normal range: Correctly interpreting 4 mA to 20 mA
The normal measuring range of a 4–20 mA pressure transmitter lies between 4 mA and 20 mA. Within this range, the pressure is output proportionally. A deviation in the current value does not necessarily have to be a fault, but may simply represent a pressure change in the process.
For example, if a transmitter outputs 14.4 mA, this corresponds to a pressure of approximately 6.5 bar with a measuring range of 0 to 10 bar. The decisive factor is that the scaling in the PLC or display matches the actual measuring range of the transmitter.
Problems often occur when the pressure transmitter has a different measuring range than the one parameterized in the control system. In that case, the mA signal is electrically correct, but the displayed pressure value is wrong. A sensor with 0 to 16 bar must not accidentally be scaled as 0 to 10 bar in the PLC.
Before troubleshooting, it should therefore always be clear: What measuring range does the transmitter have? Which unit is used? Is the output signal 4–20 mA or possibly 0–10 V? And is the scaling entered correctly in the control system?
Underrange: Why a signal below 4 mA can occur
A signal below 4 mA can have various causes. It can be an intentional underrange, a parameterized fault current, a cable problem, insufficient supply voltage or an indication that the process pressure is below the configured measuring range.
With relative pressure transmitters with a measuring range of 0 to 10 bar, a real vacuum or an unfavorable zero point shift can cause the signal to fall below 4 mA. With absolute pressure or differential pressure measurements, it must be checked particularly carefully whether the measured value is actually below the scaled range.
Many devices output a defined current value in the event of faults, for example around 3.6 mA. This value is intentionally outside the normal measuring range and signals to the control system that no valid measured value is available.
A value below 4 mA should therefore not simply be interpreted as “pressure too low”. First, it must be checked whether it is a valid underrange, a fault current or a problem in the current loop.
Overrange: Why a signal above 20 mA can occur
A signal above 20 mA can mean that the pressure is above the configured measuring range end. Many pressure transmitters can slightly overdrive the measuring range and output values above 20 mA before a defined fault current or limitation is reached.
A value of 20.5 mA or 20.8 mA can therefore indicate a real overrange. The process pressure may then be slightly above the measuring range, or pressure peaks may occur that were not previously considered in the system.
A permanent signal around 21 mA can be a fault current depending on the device and setting. However, it can also mean that the measuring range has been significantly exceeded or that the transmitter is parameterized for an upper fault current direction.
Therefore, with a signal above 20 mA, it should always be checked whether the process pressure is actually too high, whether the measuring range of the transmitter was selected correctly and whether the PLC processes the value correctly. Premature sensor replacement will not solve the problem if the cause is real overpressure or incorrect sizing.
Fault current: What 3.6 mA or 21 mA can mean
A fault current is a defined output current with which a transmitter signals that no normal valid measured value is present. Typical fault current values are below the normal range, for example around 3.6 mA, or above the normal range, for example around 21 mA.
Which direction is used depends on the device and parameterization. Some applications prefer a lower fault current, others an upper fault current. It is important that the PLC, control system and alarm limits are configured accordingly.
A value of 3.6 mA can indicate a sensor fault, electronics fault, internal device fault, cable problem or a parameterized fault response. A value of 21 mA can also be a fault current, but can also be related to overrange, overpressure or an incorrect measuring range.
The displayed current value should therefore always be evaluated together with the process condition, device manual, parameterization and actual current measurement. The display in the PLC alone is often not sufficient for a reliable diagnosis.
NAMUR fault current and device settings
In many industrial applications, fault current ranges according to NAMUR recommendations are used. Valid measured values lie within the normal range, while fault conditions are detected by current values outside the measuring range. Common values are a lower fault current around 3.6 mA or an upper fault current around 21 mA.
The exact implementation can vary depending on manufacturer, device type and parameterization. It should therefore not be assumed across the board that every value of 3.6 mA always means the same fault. The decisive factor is how the specific pressure transmitter is configured.
With intelligent transmitters, it is often possible to define whether a fault is output as a lower or upper fault current. Alarm limits, damping, measuring range, diagnostic behavior and output limits can also be parameterizable.
In practice, this means: If a transmitter outputs 3.6 mA or 21 mA, the parameterization should be checked. Only then can it be reliably determined whether it is a diagnostic alarm, overrange, underrange or another device function.
Sensor fault, electronics fault or parameterization?
A fault current can be triggered by the sensor itself. Possible causes include a damaged measuring cell, overload, internal electronics fault, memory fault, temperature problem or internal diagnostic alarm. With smart devices, such conditions can sometimes additionally be read out via HART, display or diagnostic software.
However, a sensor fault is not the only possibility. Incorrect parameterization can also cause the output to run into an unexpected range. If measuring range, zero point, output behavior or fault current direction are set incorrectly, the signal appears faulty in the control system even though the device technically works.
Especially after replacing a device or after a parameter change, it should therefore be checked whether the new transmitter has the same measuring range, same pressure type, same unit and same fault current behavior as the previous device.
Overload due to excessive pressure can also lead to a fault condition. If the sensor has been mechanically overloaded, the zero point may be shifted, the signal may be permanently incorrect or a diagnostic fault may occur. In such cases, the device should be checked or calibrated.
Checking cable break, short circuit and loose terminals
A conspicuous current value can also be caused by the wiring. If the current loop is interrupted, the signal often drops to 0 mA or to an implausible value. A loose terminal can cause the signal to temporarily fail, jump or be interpreted as a fault.
A short circuit, damaged cable or faulty connection can also result in the transmitter not being supplied correctly or in the current value at the PLC not corresponding to the actual output current.
Particularly critical points include transitions at the connector, terminals in the field distributor, terminal connections in the control cabinet, damaged cable glands, crushed cables or moisture in the connection area. In harsh environments, vibration, temperature changes and corrosion can additionally cause contact problems.
During inspection, terminals, conductors, shielding, connectors, cable glands and the entire current loop should be checked. Work on electrical systems and in control cabinets may only be carried out by qualified personnel.
Checking supply voltage and load
A 2-wire pressure transmitter requires sufficient supply voltage to operate correctly and drive the 4–20 mA signal. If the supply is too low or collapses under load, the output signal may become unstable, limited or faulty.
The load plays an important role here. The load includes the PLC input, display instruments, isolation amplifiers, cable resistance and additional devices in the current loop. If the total load is too high, the voltage reserve may not be sufficient.
A typical symptom is that the transmitter still works plausibly at low current values, but no longer reaches 20 mA cleanly at higher values or enters a fault condition. An apparent 21 mA fault can also be related to supply, load or loop configuration.
The supply should therefore be checked not only at the power supply, but also directly at the transmitter under operating conditions. In addition, the total load should be compared with the manufacturer’s specifications for the pressure transmitter.
Incorrect measuring range or real overpressure?
An output signal above 20 mA is often an indication that the pressure is above the scaled measuring range. This can be a real process condition or can result from an incorrectly selected measuring range.
If a pressure transmitter with 0 to 6 bar is used in a system where 7 bar or short pressure peaks regularly occur, the output will inevitably run into overrange. The transmitter is then not necessarily defective, but may be incorrectly specified for the application.
An incorrect pressure type can also lead to misinterpretation. Relative pressure, absolute pressure and differential pressure must not be confused. An absolute pressure transmitter behaves differently at ambient pressure than a relative pressure transmitter.
For verification, the actual process pressure should be compared with a suitable reference instrument. If the reference pressure is also above the measuring range, the problem is not the current output, but the process condition or device selection.
Checking PLC scaling and display correctly
Sometimes the current value is correct, but the display in the PLC or visualization is wrong. This often happens after sensor replacement, a change in measuring range or copying programs and parameters between similar systems.
The PLC must know which pressure value corresponds to 4 mA and which corresponds to 20 mA. If a transmitter with 0 to 25 bar is installed, but the PLC is still scaled to 0 to 10 bar, incorrect pressure displays will result even though the current value is correct.
Fault current limits must also be set correctly. If the PLC, for example, detects values below 3.8 mA or above 20.5 mA as faults, this must match the output behavior of the transmitter. Otherwise, a valid overrange can be treated as a fault or a fault current as a normal measured value.
With conspicuous values, it should therefore always be checked: Which raw values arrive at the input card? How are 4 mA and 20 mA scaled? Which unit is used? And which alarm limits are parameterized for underrange, overrange and fault current?
Testing with a multimeter or loop calibrator
To interpret a fault current correctly, the current value should be measured as directly as possible in the current loop. A digital multimeter can be used for this if it is suitable for the measuring task and correctly connected in series. In practice, however, a current loop calibrator is often much more convenient.
A loop calibrator can measure and simulate mA signals. This makes it possible to check whether the transmitter is actually outputting 3.6 mA or 21 mA, whether the current value arrives correctly at the PLC and whether the input card scales correctly with a simulated signal.
The simulation of defined values is particularly helpful. If a calibrator provides 4 mA, 12 mA and 20 mA, the PLC must correctly display the measuring range start, midpoint and measuring range end. Scaling errors can be detected quickly in this way.
For all measurements in current loops, the system must be checked safely and professionally. Opening a current loop can affect process signals. In running systems, it should therefore be clarified in advance whether measurement during operation is permissible and what effects it may have on control system or process.
Table: Current value, possible cause and useful check
| Measured current value | Possible meaning | Useful check |
|---|---|---|
| 0 mA | No supply, cable break, open loop or incorrect connection | Check supply, wiring, terminals and loop |
| approx. 3.6 mA | Lower fault current, diagnostic alarm or underrange | Check device parameterization, process pressure and diagnostics |
| below 4 mA, but above fault limit | Underrange, zero point shift or pressure below measuring range | Check reference pressure, zero point and measuring range |
| 4 mA | Measuring range start or valid zero point | Compare with process pressure and PLC scaling |
| 12 mA | Midpoint of measuring range with linear scaling | Check PLC display and measuring range |
| 20 mA | Measuring range end | Check whether process pressure is at the upper range |
| above 20 mA | Overrange, real overpressure or upper fault current | Check reference pressure, measuring range and fault current direction |
| approx. 21 mA | Upper fault current or significant overrange | Evaluate device setting, process pressure and diagnostics |
| Strongly fluctuating value | Loose terminal, EMC, process pulsation, supply or sensor problem | Compare directly at the transmitter and at the PLC |
Practical example: Pressure transmitter permanently shows 21 mA
In a production system, the PLC reports an excessively high pressure value. The pressure transmitter is parameterized as a 4–20 mA device with a measuring range of 0 to 10 bar. In the visualization, a value above the measuring range is displayed. The maintenance technician measures the loop current and finds: The transmitter permanently outputs approximately 21 mA.
At first, it is suspected that the pressure transmitter is defective. Before the device is replaced, the actual process pressure is checked with a reference pressure gauge. This shows that the pressure actually rises above 10 bar during certain operating states.
The transmitter therefore does not output a fault arbitrarily, but signals an overrange or upper alarm direction. The cause is not primarily in the output signal, but in the process condition and in the design of the measuring point.
After checking the system, it is determined that a pump generates short pressure peaks when a valve is closed. The measuring point is reassessed, the limit values are adjusted and it is checked whether a transmitter with a higher measuring range or additional damping is useful. The sensor did not have to be replaced immediately.
Which measuring instruments / products are suitable?
A current loop calibrator is particularly helpful for troubleshooting 4–20 mA pressure transmitters. A suitable device is the Druck UPS4E current loop calibrator. It is suitable for measuring and simulating mA signals and therefore supports systematic testing of transmitter, cable, PLC input and scaling.
With a loop calibrator, it can be checked directly whether a pressure transmitter actually outputs a fault current such as 3.6 mA or 21 mA. In addition, a defined signal can be injected, for example 4 mA, 12 mA or 20 mA. This makes it visible whether the PLC scales correctly and whether the fault comes from the transmitter or from the evaluation.
For additional checks, professional digital multimeters are useful, for example for checking supply voltage, wiring, continuity and terminal connections. When working in control cabinets, measuring instrument, test leads and measurement category must match the application.
If the actual process pressure also needs to be checked, suitable digital pressure gauges or reference pressure measuring instruments can be used. This makes it possible to clarify whether a 21 mA signal is caused by real overpressure, a pressure peak or an incorrectly selected measuring range.
Conclusion: Do not guess fault current, measure systematically
A pressure transmitter that outputs 3.6 mA or 21 mA provides an important diagnostic indication. The value may indicate a fault current, underrange, overrange, sensor fault, wiring problem, supply problem or incorrect scaling.
The decisive factor is not to look at the current value in isolation. Process pressure, measuring range, parameterization, fault current direction, supply voltage, load, wiring and PLC scaling must be evaluated together.
With a measurement directly in the current loop and a loop calibrator, many faults can be quickly narrowed down. This avoids unnecessarily replacing a functioning pressure transmitter or incorrectly treating a real process fault as a sensor problem.
FAQ: Frequently asked questions about 3.6 mA, 21 mA and fault current
What does 3.6 mA mean on a pressure transmitter?
A value around 3.6 mA can be a lower fault current. Depending on the device, it may indicate a diagnostic alarm, sensor fault, underrange or corresponding parameterization.
What does 21 mA mean on a pressure sensor?
A value around 21 mA can be an upper fault current or indicate that the measuring range has been exceeded. Process pressure, measuring range and device setting must be checked.
Is 21 mA always a defective sensor?
No. 21 mA can also be caused by real overpressure, overrange, incorrect measuring range or a parameterized fault current direction.
Why does a pressure transmitter output less than 4 mA?
This can be caused by underrange, fault current, zero point shift, too low process pressure, incorrect parameterization or a problem in the current loop.
What is a NAMUR fault current?
A NAMUR fault current is a defined current value outside the normal 4–20 mA range with which a device can signal a fault condition or diagnostic alarm.
Can a cable break cause 3.6 mA?
A complete cable break often leads to 0 mA or an open input. Depending on the loop configuration and input card, however, other implausible values can also occur. The wiring must therefore be checked.
How do I check whether the fault comes from the sensor or from the PLC?
The current should be compared directly at the transmitter and at the PLC input. In addition, a loop calibrator can simulate a defined signal to check the PLC scaling.
Can incorrect scaling simulate a fault current?
Yes. If the PLC is scaled incorrectly or incorrect alarm limits are used, a correct current value can be displayed as an incorrect pressure value or fault.
What does a signal above 20 mA mean?
A signal above 20 mA can indicate overrange, real overpressure, pressure peaks or an upper fault current. The exact meaning depends on the device and parameterization.
Why must the supply voltage be checked?
If the supply is too low or the load is too high, the transmitter cannot drive the loop current correctly. This can lead to unstable or limited output values.
What role does the load play?
The load is the total resistance in the current loop. If it is too high, the voltage reserve may not be sufficient, especially at higher current values close to 20 mA.
How does a loop calibrator help with 3.6 mA or 21 mA?
A loop calibrator can measure the actual current and simulate defined mA values. This makes it possible to check whether transmitter, cable, input card and scaling are working correctly.
Can real overpressure cause 21 mA?
Yes. If the process pressure exceeds the measuring range, the transmitter can output above 20 mA or run into an upper fault current.
When should the pressure transmitter be calibrated?
Calibration is useful if the transmitter deviates at known test points, behaves conspicuously after overload, the zero point drifts or the fault remains after checking wiring, supply and scaling.
When should the pressure transmitter be replaced?
Replacement is useful if an internal sensor or electronics fault has been confirmed, the device has been mechanically overloaded, the measuring cell has been damaged or the device is unsuitable for the measuring range and process conditions.