For a given line of process pipe, tank farm perimeter, or offshore platform deck, one open path IR gas detector can replace between roughly 5 and 20 fixed point detectors, with a single beam commonly specified from 5 m up to 120-200 m depending on manufacturer [S1][S2][S4].
That geometry is what flips the cost comparison: a single open path unit is almost always more expensive than one point detector, but the per-metre and per-square-metre coverage cost drops once the protected length exceeds a few tens of metres, which is the normal case in oil and gas, petrochemical, and water-treatment layouts [S4][S5][S7].
Coverage geometry: how one beam replaces a row of points
Open path IR gas detectors send a beam of infrared light between a transmitter and receiver (or between a combined head and a retroreflector), and any gas in the line of sight absorbs a specific IR wavelength that is compared against a reference wavelength to suppress fog, rain, and beam blockage faults [S5]. The natural unit of the reading is therefore ppm.m or LFL.m, the path-integrated concentration, not a local ppm at a single point. Typical offshore alarm setpoints sit at 1 LFL.m (low) and 3 LFL.m (high) on a 5 LFL.m full scale [S5]. Honeywell publishes a 200 m maximum path length for its open path IR product line, and Autronica's AutroPath HC600 Exde 4-20 mA (now obsolete but representative) is rated for detection between two points up to 120 m apart at IP67 [S1][S2].
By contrast, a fixed point gas detector typically samples a radius of about 1-5 m around its head before gas concentration falls below the lower flammable limit, and exposed outdoor units need a weather shield that adds lag to the already slower diffusion response [S5]. For monitoring a 100 m pipeline segment, the published math is therefore roughly 1 open path head versus 10-20 point heads for comparable coverage, with the open path also responding before the cloud finds any single chosen point [S4][S5].
Per-unit cost versus per-metre installed cost
The unit price gap is real and is the single biggest source of specifier confusion. The Wikipedia entry on infrared open path detectors is blunt: an open path detector usually costs more than a single point detector, so there is little incentive for applications that play to a point detector's strengths, such as a known leak location and an acceptable slow response [S5]. Honeywell's own pros-and-cons sheet repeats the same line: open path gives wider area coverage (up to 200 m) but carries high initial cost [S1]. GDS Corp. and MSA both frame the same trade as a payback problem: higher capex, lower total installed and life-cycle cost [S6][S7].
Per metre of linear coverage, the math flips. Block Engineering states that a single open path sensor can protect the same space as a handful of point sensors at a fraction of the cost, and that point-sensor networks are expensive because each head is expensive, demands regular maintenance, and consumes filters and other consumables [S4]. On a per-metre basis, treating a 120 m open path head as covering the same linear risk as roughly 10 point detectors, the open path unit lands in the same order of magnitude as 1-2 point detectors plus their weather shields, cabling, and calibration labour [S4][S5]. The published case studies (GDS 2016, MSA 2020) consistently report that the total installed cost of an open path system ends up similar to or lower than a point detector network once commissioning, cabling, and the avoided consumable replacements are included [S6][S7].
Decision matrix: which topology fits the application
Three criteria separate the two architectures: protected length, response time, and the number of consumables the maintenance budget can absorb. [S5]
Open path IR wins on protected length per dollar above roughly 30-50 m of linear risk, because the transmitter-receiver pair replaces a chain of point heads along a pipeline, tank wall, or process module [S4][S5]. Open path also wins on response time in outdoor, open-air releases, because the cloud passes through the beam before it reaches any one chosen point, and the LFL.m reading directly biases toward the size of the release rather than the local concentration [S5]. Open path also loses its consumables problem: there are no air pumps, filters, or electrochemical cells to replace, which is the recurring cost line that dominates point-detector life-cycle cost [S4].
Point IR detectors win where the leak source is well known, the protected volume is small (cabinet, analyzer shelter, local pump), the response can be slow, and weather is not a concern, because a single well-placed point gas detector is cheaper to buy and easier to recalibrate in place [S1][S5]. They are also the only practical option for toxic gases at ppm level, since open path IR has not yet seen widespread toxic-gas adoption due to the technical difficulty of measuring ppm.m of H2S and similar species along a long beam [S5].
Limitations and failure modes that erode the cost case
Open path does not give up its coverage cheaply. The Wikipedia entry and Honeywell both flag the same failure modes: fog, rain, snow, and beam blockage can attenuate the received signal, which is why the receiver logic compares the measurement wavelength against one or more reference wavelengths and runs validation checks to suppress false alarms [S1][S5]. Misalignment between transmitter and receiver after a thermal event, vibration, or maintenance walk-by is the single most common field complaint, and the longer the path, the tighter the alignment tolerance per metre [S5]. Point detectors avoid the alignment problem but inherit their own: weather shields slow the response, and electrochemical cells drift and need periodic replacement [S4][S5].
Specifiers should also price the retroreflector or second head, the conduit run between them, and the alignment jig if the path is over about 50 m; those line items are where the open path capex advantage can disappear on short runs. The same trade-off shows up across related protection choices, for example the line-type versus spot-type heat detector head selection in Line-Type vs Spot-Type Heat Detector Heads, where the per-metre and per-head economics shift at roughly the same length thresholds.
Standards, ratings, and sourcing
Open path IR detectors in hazardous areas are typically certified to the ATEX/IECEx Ex d e (flameproof enclosure, increased safety terminal box) pattern, with Autronica's HC600 rated IP67 for offshore and onshore hydrocarbon service [S2]. The measurement principle (dual-wavelength IR absorption with reference channel) is the de facto industry baseline for flammable hydrocarbon detection along a beam, and path-integral units (ppm.m, LFL.m) are the norm in offshore safety systems [S5]. Point IR detectors for flammable gas use the same dual-wavelength technique locally and report in %LFL or ppm, which is the second reason the two architectures are complements rather than substitutes on a large site [S2][S5].
Honeywell, Autronica, MSA, GDS Corp., and Block Engineering all publish the same direction of travel on cost: higher unit price for open path, lower total installed and life-cycle cost once the protected length is in the tens of metres and the point count would otherwise run into double digits [S1][S2][S4][S6][S7]. The signal to track next is the toxic-gas open path roadmap, since that is the one segment where the per-metre cost case is still tilted toward point electrochemical and photoionization detectors until the ppm.m measurement problem is solved at production cost [S5].
Detailed specification references: open channel flowmeter, and dust detector.