Open path IR detectors measure hydrocarbon concentration as a product of gas density and beam length, expressed in LEL·m (or ppm·m for trace leaks), across a single optical path that commonly runs 5–120 m between transmitter and receiver [S2][S5].
Point IR detectors report local concentration in %LEL at the sensor head, respond faster because there is no beam to align, and survive in oxygen-deficient or enriched atmospheres, which a catalytic bead cannot [S5]. For a full comparison of detector classes, see the gas detector and combustible gas detector reference pages.
Coverage Geometry and Beam Length
An open path system is a line-of-sight measurement: a focused IR beam from a transmitter crosses the protected area and lands on a receiver, and any hydrocarbon molecule between them absorbs specific wavelengths, dropping received energy in proportion to gas concentration integrated along the path [S2]. Because the reading is an integrated value, a small high-concentration cloud and a large dilute cloud at the same LEL·m read identically to the controller.
Open path units are typically calibrated for C1 to C8 hydrocarbon molecules, which covers methane, ethane, propane, butane, and the gasoline-range components most often handled in refining and petrochemical service [S3]. The same line-of-sight geometry that gives wide coverage is the reason open path units respond faster in still air than legacy point sensors: the entire beam volume is the sensing element [S7].
What Open Path Cannot Tell You
An open path detector cannot pinpoint the location of a leak because the leak can occur anywhere along the optical path, and the controller only knows that something between the two heads absorbed IR [S1][S5]. If a plant operator needs to know which flange, pump, or compressor seal is releasing gas, the open path beam only narrows it down to a 5–120 m segment.
For local leak source identification, engineers add fixed gas detector point sensors close to the flanges, valves, and pump housings; a point reading above alarm setpoint maps directly to that piece of equipment. The article on open path alarm setpoints in LEL·m walks through how that integrated reading is converted to warn and alarm thresholds.
Point vs Open Path Decision Matrix
The two technologies answer different questions, and the right answer in 2026 is usually to deploy both rather than pick one. The comparison below lines them up against the criteria that drive a refinery or LNG plant layout. [S4]
Coverage area: open path covers a continuous line of sight that can replace 3–8 point detectors along a fence line or tank rim [S2][S4]. Point sensors cover roughly a 1–5 m radius around the head, so a plant must install many to match open path footprint. Response behaviour: open path beams respond in seconds in still air because the entire beam volume is sensitive [S7]; point IR responds as fast or faster because the gas reaches a small, high-concentration sensing chamber. Leak localisation: point sensors win outright, since the alarm is at the head; open path can only report a beam-integrated LEL·m and cannot say where along the path the gas is [S1]. Atmosphere tolerance: both IR types operate in oxygen-deficient and oxygen-enriched service, unlike catalytic bead sensors, and both tolerate humidity and temperature swings without routine calibration [S5]. Mechanical risk: open path beam alignment is sensitive to vibration, structural sway, and thermal growth of the mounting structure, and misalignment causes sensitivity loss and fault trips; point sensors on rigid mounts are largely immune to this failure mode [S2].
Where Open Path Is the Right Tool
Open path detectors are best applied where the goal is area coverage and early warning of a growing cloud, not source identification. The use cases that show up repeatedly in 2025–2026 refinery, LNG, and tank-farm specifications are fence-line monitoring of process unit boundaries, compressor shelter perimeter coverage, storage tank dike monitoring, and loading-rack area coverage where a release would form a drifting vapour cloud before reaching a worker [S2][S6].
Modern open path designs use Differential Optical Absorption Spectroscopy (DOAS), which the OEM literature describes as spectral fingerprint analysis of the atmosphere, and which lets the detector distinguish hydrocarbon absorption from sunlight, water vapour, and other interferences [S2]. The dual-range design is common: ppm·m for small hydrocarbon leaks and LEL·m for catastrophic releases, with separate alarm setpoints for each band [S5].
Where Point Sensors Are the Right Tool
Point gas detectors belong wherever a known piece of equipment can leak and the operator needs to act on that specific asset. Flanges, valve packings, pump seals, compressor crankcases, and sample points are the standard targets, and the design rule of thumb is to place the head within 1–5 m of the likely release point so the local concentration rises above alarm before the gas disperses [S4][S6].
For workers entering a vessel or confined space, a portable gas detector carried by the crew provides personal protection that no fixed beam can. For routine area surveys across a unit with several leak sources, a multi-gas detector gives the LEL plus oxygen and toxic gas channels in one instrument. The selection criteria for the underlying open channel flowmeter-style integrated measurement concept, line-of-sight averaging over a defined path, is the same idea applied to flow and is the conceptual parent of the open path gas reading.
Failure Modes and Engineering Limits
Open path detector fault behaviour is dominated by beam obstruction and alignment drift, not by gas chemistry. Fog, snow, heavy rain, and bird or insect perching on a window can attenuate the beam and either mask a leak or cause a nuisance fault, which is why OEM guidance recommends rain and sun shrouds over both heads to keep the optics clean and to limit thermal cycling of the sealed enclosure [S2].
Catalytic bead point sensors carry a different failure mode: the platinum catalyst is poisoned by silicones, lead, sulphur compounds, and certain phosphate esters, and the bead needs oxygen to combust the sample, so it goes blind in a purged or oxygen-deficient enclosure [S5]. IR point sensors remove both failure modes at higher unit cost, which is why the 2025–2026 retrofit wave in refinery and gas-plant fixed systems has been IR point replacing catalytic bead, with open path added for the new perimeter coverage the point network cannot give [S5][S6].
Standards and Sourcing
Performance requirements for open path combustible gas detectors are published under ANSI/ISA-12.13.04, authored by the ISA 12.13 committee on combustible gas detection instruments, and that document is the engineering reference for beam-length, response-time, and poison-immunity claims in OEM datasheets [S5]. For point sensors, the parallel performance standards sit under the same ISA 12.13 family and govern %LEL accuracy, calibration gas verification, and poison tolerance. Cross-referencing the fixed gas detector and portable gas detector reference pages gives the side-by-side ratings that a 2026 spec needs to call out for hazardous-area classification and SIL rating.
The signal to watch over the next planning cycle is whether new builds are moving from catalytic bead to IR point as the default, then layering open path on top for area coverage, or whether some EPCs are skipping the point network and relying on open path plus CFD-modelled gas dispersion to localise leaks.