Short answer: in most facilities, open-path and point gas detectors are not interchangeable options but two measurement methods that answer different questions. A point detector reports the concentration at its own location, in %LEL (or ppm for toxic gases), once the gas actually reaches it; it is strong at known, confined leak sources, in enclosed spaces and in ducts. An open-path detector reports the total effect of gas along the beam between a transmitter and a receiver, in LEL·m (or ppm·m for toxic gases); it relies on the probability that a cloud crosses the beam in large, open areas where the leak location and wind direction are uncertain.
That answer holds under three conditions: the target gas must be detectable by the chosen measuring principle (infrared cannot detect hydrogen); an open path needs a continuous, stable line of sight; a point detector needs a location the gas will actually reach. Detector count, layout, spacing, alarm setpoints, voting logic and bypass rules are outside the scope of this article. They are decided through HAZOP, a gas dispersion study and a design review by a competent team; this article only prepares input for the selection and coverage discussion.
They do not measure the same quantity: %LEL vs. LEL·m
A point reading is a concentration: what percentage of the lower explosive limit is present in the detector's measuring zone? An open-path reading is concentration multiplied by distance: the sum of the gas encountered along the beam. For that reason, alarm thresholds and logged values of the two detector types cannot be compared directly; without knowing the size and position of the cloud, you cannot say whether a given LEL·m value or a given %LEL value indicates the more serious situation.
The table below is an illustrative example scenario, not a real facility, design or recommended threshold. It assumes a 40 m open-path beam and a uniform concentration inside the cloud.
Example scenario: what two detectors would see on the same site (hypothetical, for illustration)
Situation | Point reading | Open-path reading | What it shows |
Cloud 3 m thick at 25% LEL; point detector inside the cloud | 25% LEL | 0.25 × 3 = 0.75 LEL·m | Both detectors see the gas, but the numbers are different quantities. |
Same cloud drifts past 10 m from the point detector | 0 (gas does not reach the sensor) | 0.75 LEL·m | Gas not reaching the sensor is the basic limitation of point detection. |
Dilute cloud 15 m wide at 5% LEL | 5% LEL if inside | 0.05 × 15 = 0.75 LEL·m | An open-path reading cannot distinguish a small, dense cloud from a wide, dilute one. |
Heavier-than-air gas pools at grade, beam mounted higher | Reads if at a suitable height | 0 (gas does not cross the beam) | If beam height ignores gas behavior, the open path is blind too. |
The 0.75 LEL·m in rows one and three, averaged over a 40 m beam, equals roughly 1.9% LEL. That figure shows neither the real concentration inside the cloud nor whether the cloud forms a hazardous volume. Open-path thresholds only make sense together with expected cloud size, beam length and leak scenarios; point thresholds only make sense together with the conditions under which gas reaches that point.
Site conditions that change the decision
For open path, line of sight is everything
· Beam block: vehicles, cranes, people, dense steam or temporary scaffolding can interrupt the beam. Open-path devices typically report this as a separate status signal for loss of monitoring rather than as gas, but no gas is monitored on that path while it lasts. How the device signals it is in the data sheet; how the control system handles it is a design decision.
· Fog, rain, snow and dust: attenuate the beam, and dirty windows do the same. The attenuation level up to which measurement continues depends on the model and must be confirmed in the data sheet against site conditions.
· Alignment and structural movement: if the structure carrying the transmitter or receiver moves with vibration, wind load or thermal expansion, the beam can drift off the receiver over long distances. Rigidity of the mounting structure is part of the selection.
· Sun and heat sources: review how external infrared sources such as sunlight, flares and hot surfaces affect the device, and whether the beam looks toward them.
For point detection, gas reaching the sensor is everything
· Gas density: lighter-than-air gases (e.g., methane) tend to rise, heavier-than-air gases (e.g., propane) tend toward grade and pits; cold vapors can initially behave as heavier than air. Mounting elevation is considered accordingly.
· Ventilation and airflow: ventilation direction in enclosed spaces, prevailing wind and obstructions in open areas determine whether gas reaches the sensor.
· Response time: the response time in a data sheet describes what happens after gas reaches the sensor; it does not include transport time from the source to the sensor.
· Sensing principle: catalytic bead sensors rely on a surface combustion reaction, need oxygen and are at risk of poisoning by substances such as silicones or sulfur compounds and of inhibition by some halogenated compounds. Infrared sensors are not affected by these mechanisms but cannot detect gases without infrared absorption (e.g., hydrogen). The same limitation applies to infrared open-path devices.
· Calibration gas and LEL reference: a detector's response to different hydrocarbons can vary with the calibration gas. In addition, the LEL reference behind the %LEL scale (e.g., NFPA or IEC/ISO values) can be selected in the device configuration, so the reference used should be recorded.
Selection and site data matrix
The matrix below summarizes the data to collect before a design review or a request for quotation, and how each item affects the two detector types. It does not make the decision; it makes visible which questions the decision depends on.
Selection and site data matrix for open-path and point gas detectors
Site / process data | Effect on open path | Effect on point detection | Question for the design review |
Target gases and mixture | Gas must be detectable by the measuring principle; range and calibration depend on the gas | Sensing principle is chosen per gas (catalytic, infrared or other) | Are all flammable and toxic gases listed? Is any gas undetectable by the chosen principle? |
Gas density relative to air and temperature | Beam height is chosen for the level the cloud will follow | Mounting level is chosen for where gas accumulates | Is a heavy-gas or cold-vapor scenario covered in the dispersion study? |
Leak sources and their locations | If sources are scattered and uncertain, the chance of the beam crossing the cloud matters | If the source is known and confined, close placement is meaningful | Which risk study do the leak scenarios come from? |
Openness of the area, wind and ventilation | Can provide coverage for dilute clouds in open, windy areas | Airflow can carry gas away from the sensor | Are prevailing wind and ventilation direction data available? |
Line of sight and traffic | Vehicles, cranes, people or steam block the beam; monitoring is interrupted | Not directly affected; mechanical protection and access matter | How will beam block be handled in the control system? |
Fog, rain, snow, dust | Attenuate the signal; monitoring continuity may be affected | Protective accessories may be needed; check their effect on response time | What is the worst visibility and contamination condition on site? |
Mounting structure and vibration | Rigid structure required; movement disturbs alignment | Less sensitive; check vibration rating in the data sheet | Which structure will carry transmitter and receiver, and is its movement known? |
External heat and light sources | Review effect of sun, flares and hot surfaces | Review proximity to heat sources against ambient temperature limits | Which way does the beam look; is there a flare or hot surface nearby? |
Poisoning or inhibiting substances, low oxygen | Not decisive for the optical principle | Decisive if a catalytic sensor is chosen | Are silicones, sulfur compounds, halogenated substances or inert atmospheres present? |
Maintenance access and testing | Window cleaning, alignment checks and functional test method | Access for test gas or gas-free testing; test connection for duct installations | How will functional and proof testing be done, how often, and with what access? |
Hazardous area and safety target | Ex marking must suit the zone; the safety function is defined separately | Same | What Ex protection type, gas group, temperature class and target SIL are required? |
Where each method does not fit
· An open path alone does not give the leak's position along the beam or the real concentration in the cloud; if location matters, consider it together with point detection.
· In loading areas, crane routes or zones with heavy steam release, where the beam is frequently blocked, open-path monitoring continuity can drop.
· However sensitive a point detector is, it cannot see gas that never reaches it; on its own it does not provide perimeter coverage in large open areas.
· Infrared detectors, point or open path, are not suitable for gases without infrared absorption such as hydrogen; other measuring principles are evaluated for these hazards.
· A safety gas detector does not replace analyzers used for process control or gas quality, and vice versa.
· Range, response time and accuracy values in a data sheet apply to that model under defined conditions; they are not the general performance of the technology or a detection outcome on site.
Ex certification, measuring performance and the safety function are separate questions
ATEX or IECEx marking shows that a detector is designed not to become an ignition source in the stated hazardous zone; it does not show that the detector will detect gas correctly and in time. Measuring performance is addressed by separate standards. IEC 60079-29-0:2025 sets general requirements and test methods for flammable, oxygen and toxic gas detection equipment, covers open-path flammable gas equipment as a distinct type, and excludes equipment used only for process monitoring or control. According to the IEC catalog page, it replaces the 2016 edition of IEC 60079-29-1 and IEC 60079-29-4:2009 for open-path detectors. Guidance on selection, installation, use and maintenance is given in IEC 60079-29-2.
Integrity of the safety function is a third question. IEC 60079-29-3 gives guidance on fixed gas detection systems used in safety-related applications under IEC 61508 and IEC 61511, and its catalog abstract states that measuring performance standards do not address the integrity of the safety function. A detector's SIL capability statement does not mean the loop of detector, logic solver and final element meets its target SIL; loop-level verification is done separately. Which edition of a standard applies to a project is set by the project specification and applicable regulations.
Data to prepare for a quotation or design review
· Plot plan, hazardous area classification drawing and list of leak sources
· Target gases, composition range, operating temperature and pressure, gas density relative to air
· Gas dispersion study, HAZOP and fire and gas coverage study results, if available
· For candidate open paths: transmitter-to-receiver distance, mounting structures, obstructions, vehicle and crane movements
· For point locations: enclosed space or duct details, ventilation direction and maintenance access
· Ambient temperature range, fog, snow, dust and salt-laden atmosphere, nearby flares and hot surfaces
· Required Ex protection type, gas group, temperature class, target SIL and project approval requirements
· Control system interface (4–20 mA, HART, Modbus), handling of fault, beam block and maintenance states, and test strategy
Depending on project scope, TLY Enerji can provide technical support in comparing open-path and point detector options against this data, in device selection and in control system integration. Detector layout and alarm philosophy remain decisions of the facility's risk assessment and the responsible design team.
Frequently asked questions
Can an open-path detector replace point detectors?
Usually not. An open path relies on a cloud crossing the beam over a large area; a point detector reports concentration at a specific location and is meaningful in enclosed spaces, ducts and at known leak sources. Many facilities evaluate both methods together; the combination is decided through risk assessment.
Can an LEL·m threshold be converted to a %LEL threshold?
Not without knowing the cloud size. LEL·m is concentration multiplied by the cloud's thickness along the beam; the same value can come from a small, dense cloud or a wide, dilute one. Setting thresholds is the design team's task, together with leak scenarios and dispersion assumptions.
Does an open-path detector alarm when the beam is blocked?
Open-path devices typically report beam block as a separate loss-of-monitoring status rather than a gas alarm. How the device signals it is defined in the data sheet; how the control system handles it is defined in the alarm philosophy.
Is an infrared point detector always more suitable than a catalytic one?
No. The infrared principle has advantages regarding poisoning and oxygen dependence, but it cannot detect gases without infrared absorption, such as hydrogen. The sensing principle is chosen based on the target gas, poisoning substances present and the maintenance strategy.
Related pages
· open-path combustible gas detectors: Transmitter-receiver design, LEL·m output and product selection criteria.
· infrared (IR) point gas detectors: Point infrared measuring principle and mounting accessories.
· UV/IR flame detectors: For areas that need flame detection alongside gas detection.
· triple IR (IR3) flame detectors: A flame detection option for open areas with hydrocarbon fire risk.