4-20 mA and HART: how field instruments send data

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 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.

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.

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.

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.

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.

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.