HART-capable transmitters are widely used in process automation. They provide a classic analog 4–20 mA signal and additionally make digital information available via the HART protocol. This is exactly where the major advantage lies: in addition to the process value, device status, diagnostic messages, measuring range, damping, sensor data and further parameters can be read out.
In practice, however, calibration often checks only the analog output signal. The transmitter is exposed to a pressure, temperature or process signal, the mA value is checked and the test is considered complete. Important digital parameters can remain undetected: an incorrectly set measuring range, excessive damping, a changed fault current direction or a diagnostic message that is not visible in the control system.
This article explains why, with HART transmitters, the analog signal and digital parameters should be assessed together, and how 4–20 mA, HART value, measuring range, damping, fault current, trim, as-found/as-left documentation and calibration report are meaningfully connected.
Table of contents
- Basics: What makes a HART transmitter special?
- Analog 4–20 mA signal and digital HART value
- Checking measuring range, unit and scaling
- Assessing damping, filters and response time
- Reading out device information and diagnostic messages
- Checking fault current direction and alarm behavior
- Clearly distinguishing calibration, adjustment and trim
- As-found and as-left: Why the condition before and after adjustment matters
- 4–20 mA signal testing with a loop calibrator
- Table: Which level is checked during HART calibration?
- Practical example: HART pressure transmitter with incorrect measuring range
- Table: Typical errors with HART transmitters
- Which measuring instruments / products are suitable?
- Conclusion: HART calibration is more than an mA check
- FAQ: Frequently asked questions about calibrating HART transmitters
Basics: What makes a HART transmitter special?
A HART transmitter combines an analog process signal with digital communication. In many plants, the analog signal remains the decisive signal for PLC, process control system or display. In addition, digital information can be read out via HART and, depending on the device, parameters can also be changed.
The advantage is that the transmitter not only delivers an mA value, but can also provide information about its internal condition. This includes, for example, the digital process value, the configured measuring span, the unit, device information, serial number, sensor status, diagnostic messages, damping, fault current behavior and, in some cases, maintenance or operating data.
In a simple analog check, it is often only verified whether the correct mA value is output for a specific input value. This is important, but not complete. A HART transmitter can appear to work correctly in analog mode and still be digitally parameterized incorrectly or show a diagnostic message.
A meaningful calibration therefore looks at several levels: the physical input value, the digital HART process value, the analog 4–20 mA output signal, the transmitter parameterization and the display or scaling in the control system.
Analog 4–20 mA signal and digital HART value
The digital HART value and the analog 4–20 mA signal should match, but they must be considered separately. The digital value shows which process value the transmitter calculates or measures internally. The analog signal shows which current value is transmitted to the control system.
Example: A pressure transmitter measures 0…10 bar. At 5 bar, the digital HART value should show approximately 5 bar. At the same time, with linear 4–20 mA scaling, the analog output signal should be approximately 12 mA. If the HART value is correct but the mA value does not match, the cause is more likely to be in the analog output, current loop, load, supply or output scaling.
If, on the other hand, the mA value matches the HART display but the PLC shows a different process value, the cause is often in the input card, control system scaling or visualization. This is exactly why it makes sense to compare HART value, mA signal and PLC value with one another.
This separation is particularly important during troubleshooting and commissioning. A single measured value says little about where in the measuring chain the error occurs. Only comparing the levels shows whether the sensor, transmitter, analog output, wiring or control system is affected.
Checking measuring range, unit and scaling
A very common error with HART transmitters is an incorrectly set measuring range. The transmitter may be physically suitable for a large range, but scaled for a specific working range in the plant. The lower range value, upper range value and associated unit are decisive here.
If a transmitter is set to 0…16 bar but the PLC calculates with 0…10 bar, systematically incorrect values occur. Conversely, a correctly working transmitter can appear incorrect in the control room if the unit or scaling in the control system does not match the device parameterization.
During calibration, the mA value should therefore not be the only thing checked. Checking the digital parameters is just as important. These include lower range value, upper range value, unit, transfer function, damping and, where applicable, sensor limits. These values should match the data sheet, measuring point sheet or plant specification.
Special care is required with replacement devices. A new transmitter can fit mechanically and electrically, but may not yet be parameterized correctly. Without a HART check, an incorrect measuring range often remains unnoticed at first until process values or limit values appear implausible.
Assessing damping, filters and response time
Damping influences how quickly a transmitter responds to changes in the process value. Higher damping stabilizes a fluctuating signal, but also delays the response to real process changes. Too little damping, on the other hand, can lead to an unstable signal that interferes with control or display.
This point is often overlooked during calibration. The device is checked at static points and the values appear correct. In the real process, however, the configured damping can cause fast pressure, temperature or level changes to become visible too late.
The appropriate damping depends on the application. Stronger damping can be useful for a steady level indication. For a test bench, fast pressure control or safety-related monitoring, excessive damping can be problematic.
With HART transmitters, it should therefore be checked which damping value is set and whether it matches the measuring task. If a plant reacts unusually slowly, the cause may not only be in the PLC or mechanics, but also in unfavorable transmitter parameterization.
Reading out device information and diagnostic messages
A major advantage of HART is the ability to read out diagnostic information. The transmitter can provide indications of sensor faults, electronics faults, limit violations, configuration problems, loop errors or maintenance requirements. This information is not always visible during a pure mA measurement.
During a recurring inspection, not only a measured value should therefore be documented. Device status, diagnostic messages, device designation, serial number, tag number, firmware version and important configuration parameters can also be important for traceability.
Comparing the measuring point identification in the transmitter with the plant documentation is particularly helpful. If tag number, measuring range or unit do not match the measuring point, this may indicate a replacement device, incorrect parameterization or an undocumented change.
Diagnostic messages should not be ignored simply because the analog signal still looks plausible. A device may still deliver a usable 4–20 mA signal and nevertheless already output internal warnings that are relevant for maintenance or operation.
Checking fault current direction and alarm behavior
HART transmitters with an analog output can output a defined current value in the event of a fault. Depending on the parameterization, this fault current lies above or below the normal 4–20 mA range. In many plants, this is used to signal that the measured value is no longer valid.
It is important that the fault current behavior matches the plant logic. If a transmitter goes to a low fault current in the event of a fault but the PLC does not recognize this condition as a fault, a dangerously incorrect process value can arise. Conversely, an incorrect alarm direction can trigger unnecessary faults.
During testing, it should therefore be checked which fault current direction is configured and how the control system responds to it. This concerns not only the transmitter, but also the input card, alarm limits, visualization and, where applicable, safety logic.
Fault current tests must be carried out with care because they can trigger real alarms or switching actions in running plants. Before such tests, it must be clear whether the measuring point is in operation, which functions depend on it and whether simulation or test mode is required.
Clearly distinguishing calibration, adjustment and trim
With HART transmitters, the term calibration is often used even though different activities may be meant. Calibration first means determining and documenting the deviation between setpoint and actual value. Adjustment means setting the device in order to reduce the deviation. A trim is an internal device correction that can affect different areas depending on manufacturer and device.
A sensor trim typically influences the internal measured value acquisition of the transmitter. An output trim, on the other hand, influences the analog 4–20 mA output. This distinction is important because an incorrect trim can make the measuring chain worse rather than better.
If the digital HART value is correct but the mA signal deviates, an output trim may be useful. If the digital process value itself is incorrect, the input or sensor path must be considered. If only the PLC display is incorrect, the cause may not be in the transmitter at all.
A trim should only be performed if the cause of the deviation is clear and a suitable reference is used. After every adjustment or trim, it should be checked and documented again how the transmitter performs afterwards.
As-found and as-left: Why the condition before and after adjustment matters
In professional calibration, the as-found value is particularly important. It shows how the transmitter was found before any possible adjustment. This condition is decisive for assessing whether the measuring point operated within the permissible deviation during the previous operating period.
The as-left value describes the condition after adjustment, trim or renewed setting. It shows the condition in which the device is returned to operation. Both values belong together if the calibration is to be traceable and quality-relevant.
Especially with HART transmitters, as-found and as-left should not only represent the mA signal. Relevant digital parameters should also be documented. These include measuring range, unit, damping, tag number, fault current behavior and any existing diagnostic messages.
If the inspection shows that a transmitter was metrologically correct but parameterized incorrectly, this change must also be documented. An unnoticed parameter change can later be just as critical as a measurement deviation.
4–20 mA signal testing with a loop calibrator
Since HART transmitters continue to output an analog 4–20 mA signal, testing the current loop remains a central part of calibration. Digital communication does not replace the analog signal; it complements it. In many plants, the mA value is still used for control, display or archiving.
The UPS4E current loop calibrator / loop calibrator is suitable for testing the current loop. It can measure and simulate mA signals and helps assess transmitter output, wiring, input card and PLC scaling separately.
A practical approach is to first check the process value or a simulated input value at the transmitter. Then the real mA signal is measured. Afterwards, a defined mA value can be simulated directly at the PLC input in order to check input card and scaling independently of the transmitter.
This creates a clear separation of the measuring chain. If HART value and mA output match but the PLC displays incorrectly, the cause is more likely in the control system. If HART value and mA output do not match, the transmitter output or its parameterization must be examined more closely.
Table: Which level is checked during HART calibration?
| Test level | What is checked? | Why is this important? |
|---|---|---|
| Physical input | Pressure, temperature, level or other input signal | Shows whether the transmitter responds correctly to the actual process value |
| Digital HART value | Internal process value of the transmitter | Helps identify whether internal measured value acquisition is plausible |
| Analog 4–20 mA signal | Current value at the transmitter output | Decisive for PLC, display, controller or process control system |
| HART parameters | Measuring range, unit, damping, tag, fault current and device information | Prevents errors due to incorrect parameterization |
| Control system / PLC | Scaling, display, limit values and alarm behavior | Shows whether the transmitted value is interpreted correctly in the system |
Practical example: HART pressure transmitter with incorrect measuring range
In a plant, a HART pressure transmitter is used at a measuring point with 0…10 bar. During a recurring inspection, a pressure of 5 bar is applied. The transmitter shows a plausible pressure value via HART. However, the mA signal is not at the expected 12 mA.
Further inspection shows that the transmitter is internally scaled to 0…16 bar. The sensor is therefore measuring correctly, but the analog output is parameterized for a different measuring range. The PLC continues to calculate with 0…10 bar and therefore displays incorrect process values.
Without a HART check, this error would have been harder to detect. A pure mA measurement would only have shown that the output does not match the expectation. Only looking at measuring range, unit and HART process value reveals that the cause lies in the parameterization.
After checking against the plant documentation, the measuring range is set correctly. As-left values are then documented: digital HART value, mA output at several test points, PLC display and relevant parameters. This means that not only the signal has been checked, but the complete measuring chain has been assessed traceably.
Table: Typical errors with HART transmitters
| Error | Possible consequence | Better approach |
|---|---|---|
| Only 4–20 mA checked | Incorrect HART parameters or diagnostic messages remain undetected | Check analog signal and digital parameters together |
| Measuring range set incorrectly in the transmitter | PLC or control system displays incorrect process values | Compare LRV, URV, unit and scaling with measuring point sheet |
| Damping set too high | Process changes are displayed with delay | Assess damping according to the measuring task |
| Fault current direction does not match plant logic | Fault conditions are detected incorrectly or not at all | Check alarm behavior of transmitter and PLC together |
| Incorrect trim performed | Measuring chain is worsened instead of improved | Clearly separate sensor trim, output trim and PLC scaling |
| As-found values not documented | Retrospective assessment of the measuring point is hardly possible | Fully document condition before and after adjustment |
Which measuring instruments / products are suitable?
Process calibrators / electrical calibrators are particularly relevant for calibrating and testing HART transmitters. They support the measurement and simulation of electrical process signals and, depending on the model, are also suitable for more advanced communication and documentation tasks.
A suitable instrument for comprehensive calibration tasks is the DPI 620 Genii pressure calibrator / process calibrator. It is especially suitable for users who want to test process instruments, measure or simulate electrical signals, integrate pressure modules and, depending on the version, use HART communication.
For pure testing of the 4–20 mA current loop, the UPS4E current loop calibrator / loop calibrator should also be considered. It is particularly helpful when mA signals need to be measured or simulated in order to separate transmitter, wiring, analog input and PLC scaling from one another.
When selecting measuring instruments, it should be considered whether only an analog signal needs to be checked or whether HART communication, pressure generation, pressure measurement, temperature or electrical simulation, documentation and Ex use are also required. For a complete HART transmitter test, the combination of process calibrator, HART communicator and loop calibrator is often useful.
Conclusion: HART calibration is more than an mA check
A HART transmitter should not be assessed only via its analog 4–20 mA signal. The mA signal remains important, but the digital HART level provides additional information that can be decisive for commissioning, maintenance, troubleshooting and documentation.
Measuring range, unit, damping, fault current direction, device status, diagnostic messages and as-found/as-left values are part of a clean inspection. Especially with replacement devices, parameter changes or implausible process values, HART communication can provide the decisive clue.
With a suitable process calibrator such as the DPI 620 Genii, a direct 4–20 mA test instrument such as the UPS4E and clean documentation, the result is a calibration that not only checks individual measuring points, but assesses the complete measuring chain.
FAQ: Frequently asked questions about calibrating HART transmitters
What is a HART transmitter?
A HART transmitter is a measuring transducer that provides an analog 4–20 mA signal and additionally makes digital information available via HART communication. This allows process value, parameters and diagnostic information to be read out.
Why is a pure 4–20 mA check not always sufficient?
A pure mA check only shows whether the analog output matches the expected value. It does not reliably show whether measuring range, unit, damping, fault current behavior or diagnostic messages are correct. This information is located on the digital HART level.
What is the difference between HART value and mA value?
The HART value is the digital process value of the transmitter. The mA value is the analog output signal. Both should match the parameterization, but they can reveal different error causes.
Which parameters should be checked on a HART transmitter?
Important parameters include lower range value, upper range value, unit, damping, tag number, device information, fault current direction, output behavior and diagnostic messages. Depending on the application, further device-specific parameters may be relevant.
What do LRV and URV mean?
LRV stands for lower range value. URV stands for upper range value. These values determine which process value corresponds to 4 mA and which process value corresponds to 20 mA.
Why is damping important?
Damping influences how quickly the transmitter responds to process changes. Excessive damping can delay important changes, while insufficient damping can cause an unstable signal.
What does fault current direction mean?
Fault current direction defines whether the transmitter outputs a current above or below the normal 4–20 mA range in the event of a fault. This setting must match the alarm and safety logic of the plant.
What is an output trim?
An output trim corrects the analog output of the transmitter. It is relevant when the internal digital measured value is plausible but the output mA value deviates. It should only be performed with a suitable reference and a clearly identified error cause.
What is a sensor trim?
A sensor trim influences the internal measured value acquisition of the transmitter. It therefore affects the relationship between physical input value and digital process value. A sensor trim should be performed particularly carefully and only with a suitable reference.
What does as-found mean?
As-found describes the condition in which a device was found before adjustment or change. These values are important for assessing whether the measuring point operated within the permissible deviation during previous operation.
What does as-left mean?
As-left describes the condition after adjustment, trim or parameter change. These values show the condition in which the transmitter was returned to operation.
How does the UPS4E help with HART transmitters?
The UPS4E helps test the 4–20 mA current loop. It can measure or simulate mA signals and thus separately check transmitter output, wiring, analog input and PLC scaling. It does not replace HART communication itself.
When do I need a process calibrator with HART communication?
A process calibrator with HART communication is useful when, in addition to the mA signal, digital parameters, device status, diagnostic messages or measuring ranges need to be read out and documented.
Can a transmitter measure correctly and still display incorrectly?
Yes. The transmitter can measure correctly internally but be scaled incorrectly, output an incorrect mA signal or be interpreted incorrectly in the control system. HART value, mA signal and PLC display should therefore be compared with one another.
What is the most important practical tip?
The most important practical tip is: Always check HART transmitters on several levels. The physical input value, digital HART value, 4–20 mA signal, parameterization and PLC scaling must match.