XNX / 4-20mA Gas Transmitter Symbol
Definition: The XNX / 4-20mA Gas Transmitter symbol represents a loop-powered industrial gas detection transmitter that outputs a 4–20 mA analogue current signal proportional to gas concentration, used in process-safety schematics and P&ID drawings to identify a field-mounted gas-sensing device with two-wire 4–20 mA process loop output and discrete alarm relay contacts; it is documented in accordance with ISA-5.1 instrumentation symbol standards.
Also known as: 4-20mA gas transmitter, loop transmitter, gas detector transmitter, gas analyser transmitter, XNX gas monitor, process gas transmitter, AT (analogue transmitter).
What the XNX / 4-20mA Gas Transmitter symbol means
The XNX / 4-20mA Gas Transmitter symbol in a P&ID or electrical schematic represents a field-mounted industrial gas detection instrument. The transmitter contains an electrochemical, catalytic bead (pellistor), infrared (IR), or photoionisation (PID) sensor that measures the concentration of a target gas (flammable, toxic, or oxygen). The analogue output — a 4–20 mA current proportional to 0–100% full-scale range — is transmitted over the two-wire loop back to a control system, safety PLC, or dedicated gas controller.
The XNX Universal Transmitter (manufactured by Honeywell Analytics) is a widely used example of this class of device, hence the naming convention. In P&ID and electrical drawings, the symbol is annotated with the instrument tag (e.g., AT-101 for Analyser Transmitter), the measured variable (flammable gas % LEL, toxic gas ppm, or O2 % vol.), and the loop number. Alarm relay outputs (Alarm A and Alarm B) represent discrete contacts that trip at pre-set concentration thresholds.
How to identify the XNX / 4-20mA Gas Transmitter symbol
The XNX / 4-20mA Gas Transmitter symbol is typically drawn as a circle (ISA-5.1 instrument bubble) divided horizontally, with the upper half labelled with the measured variable code (e.g., 'AT' for Analyser Transmitter or 'GT' for Gas Transmitter) and the lower half labelled with the loop number. In electrical schematics it may appear as a rectangular block with four terminal lines: positive supply (+ 24 V) and loop return (- Loop) on the input side (left), and Alarm A and Alarm B relay contact outputs on the output side (right). The 4–20 mA current loop is indicated by a two-wire line with a current-loop annotation.
Function and usage boundary
The XNX / 4-20mA Gas Transmitter converts a gas concentration measurement into a standardised 4–20 mA analogue current signal. 4 mA represents 0% of the measurement range (or 'live zero') and 20 mA represents 100% full scale (e.g., 100% LEL for flammable gas, or 25 ppm for toxic gas). The live zero (4 mA at zero concentration) allows the receiving system to distinguish a valid zero-concentration reading from a broken wire (0 mA) — a critical safety feature in hazardous-area gas detection systems. The transmitter also provides two discrete alarm relay outputs (Alarm A and Alarm B) that de-energise at programmable concentration thresholds, driving audible/visual alarms or safety shutdown systems. The device is powered from the 24 V DC loop supply (typically 14–30 V).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60079 series (Explosive Atmospheres) governs the use of gas transmitters in hazardous areas; the transmitter must be certified to Ex ia, Ex d, or Ex e categories. IEC 61508 / IEC 61511 covers functional safety requirements for gas detection systems used in safety instrumented functions (SIFs). The 4–20 mA signal standard is defined in IEC 60381-1. |
|---|---|
| ANSI/IEEE 315 | ISA-5.1 (ANSI/ISA-5.1-2009) defines the P&ID instrumentation symbols for transmitters and analysers. The instrument designation follows ISA tag coding: first letter = measured variable (A = analyser, G = gauging), subsequent letters = function (T = transmitter). ANSI/UL 2075 covers gas and vapour detectors for safety applications. |
| Key difference | ISA-5.1 (North American P&ID standard) uses a circle bubble with letter codes for the transmitter symbol, while IEC/ISO P&ID conventions (ISO 10628-2) use similar circle symbols with slightly different letter codes. The 4–20 mA current loop standard (IEC 60381-1) is identical worldwide. Both conventions show alarm relay outputs as discrete contact lines from the instrument symbol. |
The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.
Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| pos | + (24V) |
| neg | - (Loop) |
| a | Alarm A |
| b | Alarm B |
Where the XNX / 4-20mA Gas Transmitter symbol is used
- Oil and gas facilities: offshore platforms, refineries, gas processing plants — detecting hydrocarbon gas leaks (methane, propane) and H2S
- Chemical and petrochemical plants: continuous toxic gas monitoring (chlorine, ammonia, CO) at process equipment and storage areas
- Wastewater treatment plants: H2S, methane, and oxygen-deficiency monitoring in enclosed spaces and pump stations
- Mining operations: methane (CH4) monitoring in tunnels and confined spaces per mining safety regulations
- Commercial and industrial buildings: CO and CO2 monitoring in car parks, boiler rooms, and plant rooms with gas-triggered ventilation control
- Power generation: transformer rooms (SF6 gas leak), battery rooms (H2 accumulation), and fuel storage areas
- Food and beverage: CO2 monitoring in fermentation areas and refrigeration plant rooms to detect asphyxiation risk
Key facts
- The XNX / 4-20mA Gas Transmitter symbol represents a loop-powered field instrument that outputs a 4–20 mA analogue signal proportional to gas concentration (4 mA = 0% range, 20 mA = 100% range), per IEC 60381-1 and ISA-5.1.
- The 'live zero' convention (4 mA at zero gas concentration) allows the receiving system to distinguish a valid zero reading from a broken wire (0 mA), a critical safety feature in gas detection loops.
- The transmitter symbol has four terminals: + (24 V loop supply), - (loop return carrying the 4–20 mA signal), Alarm A (relay contact 1), and Alarm B (relay contact 2); alarm relays de-energise on gas detection (fail-safe configuration).
- XNX transmitters accept interchangeable sensor cartridges (catalytic bead, electrochemical, IR) allowing the same transmitter housing to detect different gases by swapping the sensor module.
- In hazardous areas, 4–20 mA gas transmitters must be certified to ATEX/IECEx (IEC 60079 series) for use in Zone 1 or Zone 2 explosive atmospheres; intrinsically safe (Ex ia) or flameproof (Ex d) versions are used depending on the zone.
- The instrument tag for a gas transmitter in ISA-5.1 / P&ID convention is AT (Analyser Transmitter) or GT (Gas Transmitter) followed by a loop number; the loop number links the field device to the control system I/O list and loop diagram.
- 4–20 mA current loops are inherently immune to resistive voltage drops in long cable runs: the same current (and therefore the same reading) flows regardless of cable resistance, unlike voltage-output sensors affected by IR drop.
Diagrams that use this symbol
Frequently asked questions
What does the XNX / 4-20mA gas transmitter symbol mean in a schematic?
The XNX / 4-20mA Gas Transmitter symbol represents a field-mounted gas detection instrument that measures gas concentration and outputs a 4–20 mA analogue current signal proportional to the measurement range (4 mA = 0%, 20 mA = 100%). The symbol also indicates two alarm relay outputs that activate at programmable concentration thresholds.
What does the XNX gas transmitter symbol look like in a P&ID?
In a P&ID (ISA-5.1 convention), the gas transmitter appears as a circle divided horizontally: the upper half shows the variable code (AT for Analyser Transmitter or GT for Gas Transmitter) and the lower half shows the loop number. In electrical schematics it is a rectangular block with four labelled terminals: + (24 V), - (Loop), Alarm A, and Alarm B.
What does 4-20mA mean on a gas transmitter?
4–20 mA is the analogue output range of the gas transmitter: 4 mA represents 0% of the gas concentration range (zero gas) and 20 mA represents 100% of the full-scale range (e.g., 100% LEL for flammable gas). The minimum value being 4 mA rather than 0 mA is called the 'live zero' and allows the receiving system to detect a broken wire (0 mA) rather than confusing it with a valid zero-gas reading.
What are the pin names on the XNX gas transmitter symbol?
The XNX gas transmitter symbol has four pins: + (24 V DC loop supply positive), - (Loop return, carrying the 4–20 mA signal), Alarm A (first alarm relay contact, trips at the lower concentration threshold), and Alarm B (second alarm relay contact, trips at the higher concentration threshold). Alarm relays are normally energised and de-energise on gas detection for fail-safe operation.
What standard governs the 4-20mA gas transmitter symbol in P&ID drawings?
The instrument symbol for gas transmitters in P&ID drawings is governed by ISA-5.1 (ANSI/ISA-5.1-2009) for North American practice and ISO 10628-2 for international P&ID conventions. The 4–20 mA signal level standard is defined in IEC 60381-1. Hazardous-area certification is governed by IEC 60079 (ATEX/IECEx) or NEC Article 505 (North America).
What gases does an XNX-type transmitter detect?
XNX-type 4–20 mA transmitters are designed to accept interchangeable sensor cartridges, allowing detection of flammable gases (methane, propane, hydrogen) using catalytic bead sensors, toxic gases (H2S, CO, Cl2, NH3) using electrochemical cells, and oxygen enrichment or depletion using galvanic oxygen sensors. The specific gas detected is determined by the installed sensor cartridge, not the transmitter housing.
Why do alarm relays on a gas transmitter de-energise (rather than energise) on gas detection?
Gas transmitter alarm relays are wired as normally energised (energised = safe condition) and de-energise when gas is detected or when power is lost. This fail-safe design ensures that a power failure, broken wire, or transmitter fault also triggers the alarm — preventing the dangerous scenario where a real gas leak is masked by an equipment fault. This is a fundamental safety requirement under IEC 61508 / IEC 61511 functional safety standards.
Related symbols
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- Ground / Earth symbol
- IR Transmitter LED symbol
- Neutral Rail symbol
- Power Amplifier Block symbol
- Power Rail symbol
Physical breadboard, PCB, and panel mapping
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