The question arises early in the specification process: does an analog transmitter solve the problem, or is HART justified? The answer depends less on the protocol and more on what the plant will do with the available information.
One piece of information that changes the framework of the comparison: HART does not replace the analog signal. It operates on top of the 4-20mA loop, transmitting digital data via FSK (Frequency Shift Keying) without interfering with the process current. The two signals coexist on the same pair of wires.
How does the 4-20mA signal work?
The analog current loop represents the process variable through electrical current variation: 4mA is equivalent to 0% of the configured range, 20mA is equivalent to 100%. Outside this range (below 3,6mA or above 21mA), the system interprets it as a fault in the instrument or in the loop.
It is a straightforward protocol, without additional layers, compatible with virtually any PLC or DCS available on the market. The cost per point is low and installation does not require special configuration in the control system.
The limitation is structural: you receive a variable. If you need to check the sensor's status, adjust the span, or identify the cause of a deviation, you need physical access to the instrument.
What HART adds to the analog loop
HART keeps the 4-20mA signal intact. The digital protocol travels as a low-amplitude AC signal superimposed on the direct current, without compromising it.
With this, the HART TTHP1-A temperature transmitter In addition to the primary variable, it provides a set of information accessible via digital communication:
- Device status (open sensor, saturation, hardware failure)
- Ambient temperature of internal electronics
- Sensor resistance, in the case of RTDs.
- Tag identification, calibration date, and configuration history.
- Secondary variables, according to the model.
To access this data, you need compatible infrastructure: HART-enabled input cards in the DCS, an asset management system (AMS Suite, FieldCare, PACTware), or at least a portable HART communicator.
Remote diagnosis: the main argument for HART.
In plants with 50 or more temperature instruments, the maintenance cost per point starts to weigh heavily on OPEX. Each field visit incurs costs for technical hours, safety procedures, PPE, and, depending on the classified area, process downtime.
With HART, the initial screening of any anomaly happens from the control room. The technician accesses the instrument status, checks the sensor resistance, and identifies if there is signal saturation. If the problem is confirmed remotely, the field visit is surgical: you already know what you will find and what you need to bring.
This reduction in unqualified field visits is the most concrete argument for the HART temperature transmitter in medium and large-scale installations.
Configuration without opening a field.
Changing the range of a conventional 4-20mA transmitter means opening the transducer in the field, adjusting via physical trim, or connecting a communicator locally. In hard-to-reach areas, harsh environments, or continuous processes where interrupting the measurement is costly, this procedure is expensive.
With HART, configuration adjustments (range, sensor type, damping, engineering unit) are made remotely via asset management or a HART communicator connected at any point in the loop, without physical access to the instrument.
For instrumentalists who manage hundreds of points, this represents a real change in their work routine.
Multivariate and traceability
Some HART transmitter models publish more than one variable via digital protocol. A transmitter installed on a thermocouple can simultaneously provide the process temperature and the cold junction temperature. For the DCS to access these secondary variables, the input cards need to support this reading.
Traceability is the second key value for industries with regulatory requirements. HART stores the tag identification, the date of the last adjustment, and the configuration history in the instrument's memory. In ISO 9001 audits, in pharmaceutical processes with GMP requirements, or in plants with calibration plans traceable to INMETRO/RBC, this record on the device itself simplifies compliance documentation.
Maintaining traceability via manual spreadsheets is possible, but it creates an error surface that the digital protocol eliminates by default.
When is 4-20mA still the right choice?
HART where it won't be used is wasted CAPEX. Before specifying, three questions need to be answered:
Does the control infrastructure support HART? If the DCS or PLC input cards do not read HART, the transmitter only functions as analog. You pay for the protocol and do not use any of the additional features.
Does an asset management system exist, or will it exist in the future? Remote diagnostics and configuration without opening the field depend on an interface to reach the engineer. Without AMS, HART is limited to a portable communicator for occasional interventions.
What is the volume of instruments and the maintenance cost per point? In plants with fewer than 30 to 40 temperature points and a maintenance team that already performs periodic inspections at a controlled cost, the 4-20mA system gets the job done without any technical compromises.
O TR213 from Alutal This is the answer for these contexts: a compact 4-20mA transmitter, compatible with type B and DIN B heads, designed for direct installation on thermocouples and RTDs. If the process measures, transmits, and controls via an analog loop without the need for digital communication, this is the solution.
How Alutal caters to both scenarios
Alutal manufactures temperature transmitters for both protocols, with calibration traceability and technical documentation aligned with IEC 60751 and ISO 9001 standards.
For applications where an analog loop is sufficient, the TR213 offers 4-20mA output with simplified installation and compatibility with virtually any control system.
For plants with HART infrastructure or with asset management under implementation, the TTHP1-A It implements HART 7, with backward compatibility to previous revisions and DD available for the main asset management systems on the market.
The choice between the two protocols depends on the installation context. If you need technical support to determine which transmitter best suits your plant, contact Alutal's experts.
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