What Is a HART Temperature Transmitter and How Does It Work?
Most process plants still run on 4-20mA loops, and replacing that wiring wholesale isn’t realistic. A HART temperature transmitter works with that existing wiring instead of against it: it keeps the 4-20mA analog signal for the control system and adds a digital layer on the same two wires for configuration and diagnostics.
This article covers how HART temperature transmitters work, what the digital layer actually gives you beyond the analog signal, and what to check when comparing devices.
What Is a HART Temperature Transmitter?
A HART temperature transmitter takes input from a temperature sensor, typically an RTD, thermocouple, resistance or mV signal, and outputs both a 4-20mA current and a HART digital signal superimposed on the same loop.
The HART signal doesn’t replace the analog value. It rides on top of it, so existing 4-20mA infrastructure keeps working exactly as before, and a HART-capable handheld or host system can additionally read and write digital data over the same two wires.
How Does a HART Temperature Transmitter Work?
1. Sensor Signal Acquisition
The transmitter reads the connected sensor. Common inputs are RTD types (Pt100, Pt1000 and others), resistance signals, thermocouples, and mV signals, depending on the specific model.
2. Signal Processing
The raw sensor signal is linearized against the sensor’s known characteristic curve and converted into a temperature value. On models with dual sensor inputs, this includes handling two independent measurement channels, or a primary/backup sensor pair, according to the device’s supported wiring combinations.
3. Output: 4-20mA Plus HART
The temperature value drives the 4-20mA loop current as usual. In parallel, the HART modem in the transmitter modulates a digital signal onto the same current loop, carrying configuration parameters, measurement data and device status that a HART communicator or host system can read without interrupting the analog signal.
What Does HART Communication Actually Add?
HART (Highway Addressable Remote Transducer) is a widely used field communication protocol built specifically to coexist with 4-20mA. Two things it adds in practice:
Configuration without opening the device. A HART-capable transmitter can be configured over the loop using a HART modem and a PC, a handheld communicator, or dedicated configuration software, instead of adjusting settings at the device terminals. As one example, the NCS-TT106H series supports Microcyber’s own HartMPT configuration software as well as third-party HART tools such as SDC625, for setting device tag and address, range and damping, sensor type and wiring, current calibration, and live variable monitoring.
Built-in fault detection. Some HART temperature transmitters include a self-diagnostic function that watches for specific fault conditions. On the NCS-TT105H, for example, the transmitter detects sensor open circuit, sensor short circuit, or an AD conversion error, and automatically drives the loop current to a pre-configured alarm level (set via a dial switch, either above roughly 21.75 mA or below roughly 3.7 mA depending on configuration) so the fault is visible to the control system without needing a separate diagnostic channel.
This kind of loop-level fault signaling and remote parameter access is the practical value of HART over a plain analog transmitter — not a vague “more information,” but specific, checkable functions that reduce the number of times someone has to walk out to the field device.
HART Temperature Transmitter vs. Traditional 4-20mA Transmitter
| Recurso | Traditional 4-20mA Transmitter | Transmissor de Temperatura HART |
|---|---|---|
| Analog current output | Sim | Sim |
| Digital communication on the same loop | Não | Sim |
| Configuration method | Local adjustment at the device | Local adjustment, or via HART modem/handheld/host software |
| Fault signaling | Alarm current only, if supported | Alarm current, plus HART-readable fault status on models with self-diagnostics |
| Compatibility with existing 4-20mA wiring | N/A | Yes, no rewiring needed |
Where HART Temperature Transmitters Are Used
Oil and gas: refining processes, pipeline systems, storage and processing equipment, where existing 4-20mA loops are already in place and full fieldbus rewiring isn’t practical.
Chemical processing: reaction monitoring, heat exchanger systems, process equipment where loop-level diagnostics reduce unplanned downtime.
Power generation: equipment and process temperature monitoring supporting plant control operations.
Manufacturing: process visibility and stable production conditions where a mix of legacy analog and newer smart instrumentation coexists on the same plant floor.
Como selecionar um transmissor de temperatura HART
1. Input signal compatibility. Confirm the transmitter accepts your sensor type and wiring: RTD (2-, 3- or 4-wire), thermocouple, resistance, or mV, and check whether dual-channel input is available if you need a backup sensor or a second measurement point.
2. Communication needs. Decide whether the application needs 4-20mA only, 4-20mA with HART, or a fully digital fieldbus protocol (FOUNDATION Fieldbus H1, PROFIBUS PA) — HART is the middle ground when you want digital diagnostics but can’t or don’t want to rewire for a fieldbus network.
3. Installation format. Head-mount, DIN-rail and other module formats are available depending on whether the transmitter needs to sit on the sensor head or in a cabinet.
4. Environment and hazardous area rating. Match operating temperature range, IP rating and explosion-protection marking to the installation, and verify the certificate for the specific marking rather than assuming it from the product family name.
Example: Microcyber HART Temperature Transmitters
NCS-TT105H is a dual-channel, high-accuracy HART transmitter. It accepts RTD (Pt100, Pt200, Pt500, Pt1000, Cu50, Cu100), resistance (0-500Ω, 0-4000Ω), mV (-100 to 100mV) and thermocouple (B, E, J, K, N, R, S, T) input. Current accuracy is 0.03% with a temperature drift of 0.003%/°C, response time is 0.8-1.3s depending on sensor type and wiring, and the enclosure is rated IP67 for operation from -40°C to 85°C (extendable to -55°C to 85°C as an option). It includes the self-diagnostic alarm function described above.
NCS-TT106H is a single-channel HART 7 transmitter built as a compact module. It accepts the same range of RTD, resistance, mV and thermocouple inputs, with current accuracy of 0.03%, a 0.5s response time, and an Ex ia IIC T4 Ga explosion-protection rating for use in hazardous areas. It supports configuration through Microcyber’s HartMPT software or third-party HART configuration tools.
Perguntas Frequentes
What is a HART temperature transmitter? A temperature measurement device that outputs a standard 4-20mA signal while also supporting HART digital communication for configuration and diagnostics on the same two wires.
What is the difference between HART and 4-20mA? 4-20mA is the analog current signal used for the primary process value. HART adds a digital signal superimposed on that same current loop, without changing the analog value the control system sees.
Can a HART temperature transmitter work with RTD sensors? Yes. Most HART temperature transmitters, including Microcyber’s NCS-TT105H and NCS-TT106H, accept RTD inputs such as Pt100 and Pt1000 alongside thermocouple, resistance and mV signals.
Does HART configuration require special tools? Yes, typically a HART modem plus a PC running configuration software (either the manufacturer’s own tool or a generic HART configurator), or a dedicated HART handheld communicator.
Why use a HART temperature transmitter instead of a plain analog one? The main practical benefit is being able to read diagnostic and configuration data over the existing loop instead of walking out to the device, while keeping the 4-20mA signal the control system already relies on.
