4-20 mA, voltage or digital: sensor outputs compared

A tender asks for a water level sensor with "4-20 mA output". Another asks for "RS-485 Modbus". A third, for an older station, quietly assumes 0-10 V. All three describe the same thing: how a sensor gets its reading to the data logger at the station. The choice decides noise problems, how much detail you can read back and how easily you can swap a sensor later. This article compares 4-20 mA, voltage and digital outputs, so that you can pick the one that suits your site.

What is a 4-20 mA signal?

A 4-20 mA signal is a current loop: the sensor varies the direct current flowing through a pair of wires so that 4 mA means the bottom of its measuring range and 20 mA means the top. The range is a long-standing industrial standard: IEC 60381-1 covers analogue direct current signals in industrial process measurement and control [1], and ANSI/ISA-50.00.01 is the American counterpart that names the 4-20 mA range [2].

Three ideas explain its popularity.

• Live zero. The scale starts at 4 mA, not 0 mA. A reading of 0 mA therefore cannot be a real measurement: it means a broken wire or a dead sensor.
• Current is the same everywhere in a loop. The loop is a single series circuit, so the logger sees the current the sensor set, whatever the resistance of the cable between them (within the limits of the supply voltage).
• Two wires do both jobs. In a loop-powered sensor the same pair carries power and signal.

The logger turns current back into a voltage with a small, accurate burden resistor, because analogue inputs measure voltage. A 250 Ω resistor turns 4-20 mA into 1-5 V (Ohm's law, V = I × R).

4-20 mA vs 0-10 V: which is better?

For field stations, 4-20 mA is usually the better choice, because the signal has to cross cable, junction boxes and sometimes different earth potentials. A voltage signal is simpler and cheaper, and it is fine over a short, clean cable inside one cabinet.

• Cable resistance. A voltage output feeding a high-impedance input loses very little to cable resistance, so length alone is not the main problem. The weak points are electrical noise picked up along the cable, and differences between the earth at the sensor and at the logger, both of which add directly to the voltage you read.
• Fault detection. With 4-20 mA a failed circuit is visible. With 0-10 V it can look like a legitimate low reading.

The price of the current loop is the supply voltage it needs, as the example shows.

Worked example: can the loop be driven? (illustrative)

A sensor needs at least 10 V across its terminals to work. The loop is fed from 24 V, the cable adds 40 Ω in total (both wires) and the burden resistor is 250 Ω. At full scale, 20 mA, the voltage drops are:

Burden:       0.020 A × 250 Ω = 5.0 V
Cable:        0.020 A × 40 Ω  = 0.8 V
Sensor minimum                 = 10.0 V
Total needed                   = 15.8 V   (supply 24 V, margin 8.2 V)

The loop works. On a 12 V supply the same loop would fail at the top of the range. The supply voltage, not the signal, sets the limit, so check the sensor's minimum terminal voltage.

The hidden cost: resolution

An analogue output ends at the logger's analogue-to-digital converter (ADC). Suppose a 12-bit converter covers 0-5 V, so 1-5 V (the 4-20 mA range on 250 Ω) uses about 3,277 of its 4,096 steps. A sensor ranged 0-10 m then reads in steps of 10 m ÷ 3,277 ≈ 3 mm. A 16-bit converter gives about 0.2 mm. A good radar level sensor (how radar sensing works) can resolve finer than that, so the converter becomes the limit. See accuracy, resolution and precision.

4-20 mA vs HART

HART is not a rival to 4-20 mA. It is an addition to it. According to the FieldComm Group, the body that maintains the protocol, HART provides two simultaneous channels on the same wires: the 4-20 mA signal carries the primary measured value, and a digital signal, encoded by frequency shift keying, carries device status, diagnostics and further values [3].

So a HART sensor works on a plain 4-20 mA input, and the extra information appears only if the logger or a hand-held configurator can read it. The digital channel is slow, so it suits configuration and diagnostics, not fast data.

4-20 mA vs Modbus and SDI-12

Digital outputs send the reading as numbers, so no converter limits it, and one cable can carry several values.

Modbus RTU over RS-485. The specification describes a master that talks to up to 247 slave devices on the same serial line, each with its own address from 1 to 247. In the common two-wire RS-485 arrangement the conductors form a balanced twisted pair, and the specification requires devices to support 9,600 and 19,200 bits per second [4]. Register maps differ between makers, so a new sensor needs its list of registers (see Modbus RTU explained).

SDI-12. This is a serial interface at 1,200 baud designed for battery-powered environmental sensors. The bus has three conductors: data, ground and a 12-volt line, and it is meant to connect one data recorder to several sensors on one cable. The specification states that a bus can carry at least ten sensors, each with 200 feet of cable [5]. Sensors can hold their own calibration, so a sensor can be replaced without re-programming the recorder [5]. More in SDI-12 and RS-485: the two sensor buses.

How do the outputs compare?

Which sensor output should you choose?

• Existing logger with analogue inputs, one sensor per cable: 4-20 mA. Use a good-quality converter and check the loop voltage.
• Many sensors at one station, or a need for diagnostics: a digital bus (RS-485 Modbus or SDI-12), checking that logger and sensor share the same protocol and register map.
• Short run inside one cabinet: 0-10 V is acceptable, with a shielded cable.
• Long outdoor runs: any output needs surge protection and earthing. See lightning and surge protection for remote stations.

Whichever you choose, record the output type, range (for example "0-10 m = 4-20 mA") and protocol settings in the tender and station record.

Key points

• 4-20 mA is a current loop with a live zero: 0 mA means a fault, and the loop works as long as the supply can drive it.
• 0-10 V is simple, but noise and earth differences add directly to the reading.
• HART adds digital information to the same 4-20 mA wires, without changing the analogue reading.
• Modbus RTU over RS-485 and SDI-12 send the value as numbers, so converter resolution is no longer a limit.
• Check loop voltage, converter resolution and register map before you buy.

Where ClimMaTech fits

Our devices have integrated data logging and GSM, so the station keeps its record without a separate logger. The product pages list the output options. Tarang, our radar water level sensor, lists 4-20 mA, RS-485, RS-232 and SDI-12 among its outputs, and PRAVAAH lists 4-20 mA, RS-485 and RS-232. Our RTDAS and hydromet telemetry page shows how such stations are put together.

References

1. International Electrotechnical Commission, IEC 60381-1:1982, Analogue signals for process control systems, Part 1: Direct current signals, catalogue entry (scope: analogue direct current signals used in industrial-process measurement and control systems). evs.ee
2. International Society of Automation, ANSI/ISA-50.00.01-1975 (R2012), Compatibility of Analog Signals for Electronic Industrial Process Instruments, ISA standards committee page. isa.org
3. FieldComm Group, HART Technology Explained, undated web page. fieldcommgroup.org
4. Modbus Organization, MODBUS over Serial Line Specification and Implementation Guide V1.02, 2006. modbus.org specifications (copy read: PDF)
5. SDI-12 Support Group, SDI-12: A Serial-Digital Interface Standard for Microprocessor-Based Sensors, Version 1.4, 20 February 2023. sdi-12.org