--- title: "4-20 mA and HART Explained: How Instruments Talk | Citia" description: "4-20 mA sends a reading as current, 4 mA low and 20 mA high; HART adds digital data on the same wires. Scaling formula, NE 43 fault levels and HART checks." url: https://citia-engineering.com/en/knowledge/4-20ma-hart language: en updated: 2026-11-13 type: article site: Citia Engineering Indonesia --- # 4-20 mA and HART: how field instruments send data A 4-20 mA loop sends a measurement as current; HART adds digital data on the same wires. How the loop works, how to convert mA to a value, and how to read faults. | Field | Value | | --- | --- | | Topic | Field instrumentation | | Author | Tim Engineering Citia | | Updated | 2026-11-13 | Key takeaways A 4-20 mA signal sends a measured value as current, with 4 mA for the lowest value and 20 mA for the highest; HART superimposes digital data on the same current so configuration and diagnostics can be read without extra wiring. A transmitter is the instrument that turns a physical quantity, such as pressure, level or temperature, into that standard signal for a PLC, DCS, SCADA or indicator. Current is used instead of voltage because it stays the same along the whole loop. Cable resistance and small voltage drops do not change the reading, which matters on long runs across a tank farm. The analogue direct-current signal range is standardised in IEC 60381-1. Most pressure, level, flow and temperature transmitters in Indonesian terminals and plants still deliver their main value this way, even when a digital bus runs beside them. - A 4-20 mA signal sends a measured value as current: 4 mA is the bottom of the range and 20 mA the top. - The value follows a straight line: value = LRV + (I - 4) / 16 x span, so 12 mA is always half of the range. - Under NAMUR NE 43, 3.8-20.5 mA carries measurement, while 3.6 mA or less and 21 mA or more signal a failure. - HART superimposes a digital FSK signal on the same loop, so configuration and diagnostics travel without extra wires. ## How a 4-20 mA loop works A 4-20 mA loop is a simple series circuit: a power supply, the transmitter and the input of the control system share one current. In the common two-wire, loop-powered design, the transmitter draws its own operating power from the loop and regulates the total current to match the measurement. Below its lowest output the electronics would starve, which is one reason the scale starts at 4 mA rather than 0 mA. The PLC or DCS input measures the current, usually as the voltage across a precision resistor. Because every element is in series, an open wire anywhere drops the current to zero, and a short can push it above the normal range. Both cases are easy to tell apart from a real reading. Four-wire transmitters, often analysers or flow meters, take power from a separate supply and drive the current output themselves. The input card must match: an active input supplies loop power, a passive input expects the transmitter or an external supply to do it. Mixing the two is a common commissioning mistake. Good wiring keeps the loop quiet. Use a twisted, shielded pair, ground the shield at one end only, usually in the control room, and keep signal cables away from motor and VSD power cables. In hazardous areas the loop also passes an isolator or intrinsic safety barrier, and its voltage drop must fit within the supply budget, together with the transmitter and the input resistor. - Transmitter: Sets the loop current from 4 to 20 mA and adds the HART signal. - Junction box and cable: Twisted, shielded pair; every point is in series. - Loop power supply: Usually 24 V DC, inside the input card or the panel. - PLC analog input: Measures current across a resistor; at least 250 ohm in the loop for HART. - HMI or SCADA: Shows the scaled value, alarms and trends. ## Converting current to a process value The current is a straight-line copy of the measurement: every 1.6 mA is one tenth of the range. Each transmitter is configured with a lower range value (LRV) at 4 mA and an upper range value (URV) at 20 mA. The 16 mA between them span the whole range. The control system reverses the line: value = LRV + (I - 4) / 16 x (URV - LRV). The same formula works for any range, including ranges that start below zero such as vacuum or sub-zero temperature. Most errors in scaling come from a range mismatch, not from the transmitter. If the transmitter is ranged 0 to 16 bar and the PLC is scaled 0 to 10 bar, every value is wrong by the same ratio. Check the range in both places, and check the unit, whenever a transmitter is replaced. Loop current across the measuring range | Label | Value | | --- | --- | | 0% | 4 mA | | 25% | 8 mA | | 50% | 12 mA | | 75% | 16 mA | | 100% | 20 mA | Source: Linear 4-20 mA signal range, IEC 60381-1 12 mA always means half of the configured range. ``` Pressure transmitter, range 0 to 10 bar (LRV = 0 bar, URV = 10 bar) value = LRV + (I - 4) / 16 x (URV - LRV) I = 12.0 mA -> 0 + (12.0 - 4) / 16 x 10 = 5.0 bar I = 7.2 mA -> 0 + ( 7.2 - 4) / 16 x 10 = 2.0 bar I = 20.0 mA -> 10.0 bar (top of range) I = 3.5 mA -> below 3.6 mA: failure signal (NAMUR NE 43), not a reading Reverse check: 2.5 bar should give 4 + 2.5 / 10 x 16 = 8.0 mA ``` Worked example for a 0-10 bar pressure transmitter. ## Why 4 mA, not 0 mA, and what 3.6 mA means Starting at 4 mA gives a "live zero": a healthy transmitter at the bottom of its range still sends current, so a reading of 0 mA can only mean a fault. NAMUR recommendation NE 43 standardises how transmitters report their own failures through the current. As summarised in vendor documentation such as the Vaisala PR-33 user guide, the range from 3.8 to 20.5 mA carries measurement information. A current of 3.6 mA or less, or 21 mA or more, signals a diagnostic failure. The small bands between 3.6 and 3.8 mA and between 20.5 and 21 mA show a process that has left the configured range. Set the alarm limits in the PLC to match these levels, and decide for each loop whether the transmitter should fail low or fail high. A level transmitter that fails high on a tank can trip a high-high alarm; one that fails low can hide an overfill. - 0 mA: Open loop or no power - 3.6 mA or less: Transmitter failure, low (NE 43) - 3.8 to 4 mA: Below range, still a measurement - 4 to 20 mA: Configured measuring range - 20 to 20.5 mA: Above range, still a measurement - 21 mA or more: Transmitter failure, high (NE 43) ## What HART adds to the loop HART lets you talk to a transmitter digitally over the same two wires that carry its 4-20 mA signal. The Siemens application guide "Working with HART networks" (AG012712, 2020) describes the method. The main process value stays on the analogue current. Everything else travels as Bell 202 frequency shift keying superimposed on it: a burst at 1200 Hz is a binary 1 and a burst at 2200 Hz is a binary 0. The bursts average to zero, so the slow 4-20 mA signal is generally not affected. Through HART you can read the digital process value, secondary variables such as sensor temperature, the tag name, range settings and device diagnostics. HART is a master/slave protocol with up to two masters, typically the control system and a handheld communicator. The guide notes that HART was first developed in the early 1980s and that HART 7 devices use addresses 0 to 63. - Process value: One analogue value | Analogue value plus digital copy - Extra variables: None | Secondary values such as sensor temperature - Configuration: At the device, by local buttons or display | Remotely from the communicator or the asset system - Diagnostics: Failure current only (NE 43) | Detailed device status and alerts - Wiring: Two wires | Same two wires; at least 250 ohm loop resistance ## Calibrating and checking a transmitter with HART Calibration with HART means checking two things separately: that the sensor reads the true process value, and that the current output matches that reading. A HART communicator connects in parallel across the loop, at the transmitter terminals or at the marshalling cabinet. It needs enough loop resistance to see the signal; the Siemens guide sets a minimum of 250 ohm, which most HART input cards already provide. With a laptop modem on a short bench loop, add the resistor yourself. The analogue output can then be forced to fixed currents, for example 4, 12 and 20 mA, while someone reads the value on the HMI. This loop test proves the whole path from transmitter to screen without moving the process. Record every as-found and as-left value in the calibration sheet. Those records serve as evidence in audits and as the starting point when the same loop misbehaves again. - Isolate and inform: Put the loop in bypass or manual and tell the control room before touching it. - Connect and identify: Connect the communicator, confirm the tag, range and unit match the loop drawing. - Check the sensor: Apply known reference values with a calibrator and compare the digital reading. - Check the output: Run a loop test at fixed currents and compare with a reference meter and the HMI. - Trim only if needed: Trim sensor or output when the error exceeds the allowed tolerance, then recheck. - Record and restore: Save as-found and as-left values, remove the bypass and confirm with operations. ## HART vs Modbus: different jobs HART and Modbus solve different problems: HART adds digital data to an analogue loop, while Modbus is a digital network on its own cable. HART keeps the 4-20 mA value as the primary measurement and adds configuration and diagnostics on top. Its digital channel is slow; the Siemens guide puts the rate at about two messages per second. That is fine for setup and health checks, not for fast control. Multidrop HART exists, but the analogue value is then lost and the update rate drops further. Modbus RTU puts many devices on one RS-485 pair and reads each by address, with no analogue signal at all. Tank gauges, flow computers and power meters often use it because they deliver many values per device. On a terminal you will find both: transmitters on 4-20 mA with HART into PLC inputs, and multi-value devices on Modbus into gateways. - Medium: Superimposed on a 4-20 mA loop | Own RS-485 pair - Main value: Analogue current | Digital registers - Devices per cable: Usually one (point to point) | Many, by address - Typical use: Configuring and diagnosing transmitters | Reading gauges, meters and drives ## In the field: transmitter work at Citia sites On Citia projects, most 4-20 mA problems are found during loop checks, before the plant relies on the signal. At ITJ LPG Priok, areas PMB III and IV , Citia installed pressure and temperature transmitters whose data the PLC processes and the HMI displays. The work covered installation, calibration, wiring, configuration, testing and commissioning. Wiring and device communication problems were solved by checking the wiring thoroughly and adjusting the PLC and transmitter configuration. During the Cikampek tank-monitoring upgrade , loop checks, I/O tests and functional tests verified every analogue and digital signal, alarm and interlock. The Field Instrumentation service delivers the same records as standard: datasheets, I/O lists, loop diagrams, calibration certificates and loop-check sheets. Communicator cannot find the device? Check the loop resistance: If a HART communicator finds nothing on a loop that reads a normal current, the usual cause is missing loop resistance. Connecting directly across a power supply or a low-impedance input card shorts the HART signal. Connect across a resistor of at least 250 ohm, as the Siemens HART guide requires, and never across the supply terminals alone. ## Terms to know - Transmitter: An instrument that converts a physical quantity such as pressure or level into a standard signal for a control system. - Current loop: The series circuit of power supply, transmitter and input in which one current carries the measurement. - LRV and URV: Lower and upper range values: the process values that correspond to 4 mA and 20 mA. - Live zero: The practice of starting the scale at 4 mA so that 0 mA always indicates a fault. - Loop-powered (two-wire): A transmitter that takes its operating power from the same two wires that carry the signal. - HART: Highway Addressable Remote Transducer: digital FSK communication superimposed on a 4-20 mA loop. - HART communicator: A handheld or laptop tool used to configure, test and diagnose HART devices. - NAMUR NE 43: A NAMUR recommendation that sets current levels for measurement and for transmitter failure signals. - Loop test: Forcing a transmitter output to fixed currents to check the path to the control system. ## Frequently asked questions Q: Why does a transmitter signal start at 4 mA instead of 0 mA? A: The signal starts at 4 mA so that a broken wire, which gives 0 mA, can never be mistaken for a real zero reading. The 4 mA also leaves enough current to power a two-wire transmitter at the bottom of its range. Q: What does a current below 3.8 mA mean? A: A current below 3.8 mA means the signal has left the NAMUR NE 43 measuring band. Between 3.6 and 3.8 mA the process is below range; at 3.6 mA or less the transmitter is reporting a failure, and near 0 mA the loop is open or unpowered. Q: What is the difference between HART and Modbus? A: HART adds digital data on top of a 4-20 mA loop, while Modbus is a separate digital network on RS-485 or Ethernet. HART is mainly for configuring and diagnosing one transmitter; Modbus reads many values from many devices on one cable. Q: How do I calibrate a transmitter with a HART communicator? A: Calibrate a transmitter with HART by checking the sensor against a reference, then checking the current output with a loop test. Connect across at least 250 ohm, put the loop in bypass, record as-found values, trim only when out of tolerance, and record as-left values. Q: Does HART disturb the 4-20 mA reading? A: HART generally does not disturb the 4-20 mA reading because its 1200 Hz and 2200 Hz bursts average to zero. Most analogue input cards filter the signal out. The Siemens guide notes that rare incompatibilities exist, so check the card when a reading jitters only during HART traffic. Q: What does a transmitter do? A: A transmitter converts a measured physical quantity, such as pressure, level, flow or temperature, into a standard signal like 4-20 mA. The PLC, DCS or indicator reads that signal and turns it back into an engineering value. ## Sources and further reading 1. IEC 60381-1:1982 Analogue signals for process control systems, Part 1: Direct current signals, IEC, 1982 (https://webstore.iec.ch/en/publication/1948) 2. NAMUR Recommendations and Worksheets (NE 043, signal level for failure information of digital transmitters), NAMUR (https://www.namur.net/en/recommendations-and-worksheets/current-nena.html) 3. Default mA output value, PR-33 / PR-33-AC user guide (NAMUR NE 43 signal ranges), Vaisala (https://docs.vaisala.com/r/M212411EN-B/en-US/GUID-E40476FD-D92F-4BE2-ABBD-CF64B0D2D65A) 4. SITRANS: Working with HART networks, application guide AG012712, Siemens, 2020-09 (https://cache.industry.siemens.com/dl/files/810/109783810/att_1038663/v1/AG012712.pdf) 5. Pertamina ITJ LPG Priok: transmitter replacement in areas PMB III and IV, Citia (https://citia-engineering.com/en/projects/pertamina-itj-lpg-priok-transmitter-replacement-in-areas-pmb-iii-and-iv) 6. Field Instrumentation, Citia (https://citia-engineering.com/en/products/field-instrumentation) 7. Cover photo by Retired electrician, CC0, Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Screw-type_terminal_block,_10mm_spacing_01.jpg) Need transmitters installed or checked?: Send siTia your instrument list or loop problem, and a Citia engineer will discuss calibration, loop checks or replacement. ## Contact - Company: PT Citia Engineering Indonesia - siTia - Virtual Assistance: [WhatsApp +62 813-2206-507](https://wa.me/628132206507). siTia answers questions about our products, projects and enquiries on WhatsApp. Replies are automated, and a person from the Citia team follows up. - Email: services@citiaengineering.com - Phone / WhatsApp (siTia): +62 813-2206-507 - Location: Bandung, Indonesia [Contact Citia](https://citia-engineering.com/en/contact.md) ## Structured data (JSON-LD) ```json { "@context": "https://schema.org", "@graph": [ { "@type": "Organization", "@id": "https://citia-engineering.com/#organization", "name": "Citia Engineering Indonesia", "legalName": "PT Citia Engineering Indonesia", "alternateName": "Citia", "url": "https://citia-engineering.com", "logo": { "@type": "ImageObject", "url": "https://citia-engineering.com/media/citia-logo.png", "width": 950, "height": 328 }, "description": "Citia is an engineering and system integration company in Bandung, Indonesia. 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