Infrared (IR) Point Gas Detectors

What Is an Infrared Gas Detector?

Combustible gas detectors are the field elements of gas detection systems that allow a leak in a hazardous area to be recognized before it can form an ignitable mixture. The result is usually expressed as a percentage of the lower explosive limit (LEL); 100% LEL corresponds to the lowest concentration at which the gas forms an ignitable mixture with air. Alarm thresholds are defined in the lower part of this scale to gain time for a response.

An infrared (IR) gas detector works on the principle that hydrocarbon molecules absorb infrared light at certain wavelengths. Unlike catalytic sensors, the measurement does not rely on a combustion reaction at the sensor surface; it therefore does not need oxygen to be present and is not affected by the poisoning mechanisms typical of catalytic elements. Exposure to high gas concentrations does not cause a permanent loss of sensitivity in the optical measuring elements either.

The detector described on this page is a point type: the gas must reach the optical measuring zone of the detector by diffusion. For this reason it is installed close to likely leak sources such as compressors, pump seals, flanges and valve groups. For perimeter monitoring of large areas it can be combined with open path gas detectors. Hydrogen and other gases that do not absorb infrared light cannot be measured this way.

How Point Infrared Gas Measurement Works

The detector calculates the concentration by comparing the absorption of the gas along the optical path at two different wavelengths. The measurement proceeds in the following steps:

1.      Gas diffusion into the measuring zone: Ambient gas enters the optical measuring zone by diffusion through the environment shield or environment cap. The environment shield protects the measuring zone from rain, dust and direct air flow but lengthens the response time slightly: T90 is 2 s with the shield and under 1 s without it.

2.      Heating the optical surfaces: The optical surfaces are heated so that moisture and condensation cannot disturb the light path. This design is particularly important for measurement continuity on offshore and high-humidity sites.

3.      Absorption at measurement and reference wavelengths: In the solid-state optical assembly, light from the IR source passes through the optical path containing the gas and reaches the receivers. The measurement wavelength coincides with the band absorbed by the C–H bonds of hydrocarbons, while the reference wavelength lies in a region unaffected by the gas. As the gas concentration rises, only the signal in the measurement channel decreases.

4.      Ratio calculation and temperature compensation: The ratio between the two channels largely compensates for changes that affect both channels together, such as source aging, optical contamination and temperature. A temperature-compensated algorithm converts this ratio into a %LEL value via the calibration curve defined at the factory.

5.      Self-monitoring and signal output: The detector continuously cross-checks the condition of its IR sources and receivers, applies internal corrections when needed and verifies its key functions without interruption. The measured value is transmitted via 4–20 mA and HART, and status information via Modbus RTU over RS-485.

Duct-Mounted Applications

For in-duct measuring points such as ventilation ducts and air intake lines, long-probe versions with 15 cm and 35 cm extensions are available. These versions can be installed with AISI 316 duct flange kits compatible with DIN 150 and DIN 200 PN6, JIS 150 and JIS 200 5K and JIS 105 10K, or with a clamping-type stainless steel kit; a separate gas test kit can be used for the duct flange.

Gas-Free Function Testing and Bump Testing

For routine checks, a gas-free tester providing the equivalent of 50% LEL for methane helps verify the alarm chain without bringing test gas to site. When a function test with gas (bump test) is required, a test gas nozzle, handheld, short or medium bump test housings and a remote bump test kit can be used. The absence of field calibration adjustment does not remove the need for periodic function tests and proof tests.

The response of an infrared combustible gas detector to different hydrocarbons can differ from its response to the calibration gas. Where several gas types are present in a plant, the choice of calibration gas and the alarm thresholds should take these differences into account; detection performance is governed by gas dispersion and by where the detector sits relative to the leak source.

Key Features

·         Dual-wavelength optical measurement: Using measurement and reference wavelengths together limits the influence of optical contamination and source aging on the reading.

·         No calibration adjustment in the field: The detector is calibrated at the factory; an internal cross-check mechanism aims to hold the calibration inside a narrow tolerance band, and no calibration adjustment is made on site.

·         5-year proof test interval: A long proof test interval helps simplify maintenance planning for safety instrumented functions.

·         Gas-free function testing: A dedicated test device allows function checks without bringing test gas to site.

·         Heated optics: Driving off moisture from the optical surfaces helps prevent condensation-related measurement deviations and faults.

·         Fast response: A response time of T90 = 2 s with the environment shield and T90 < 1 s without it is provided.

·         Wide ambient range: Operation from –40 °C to +75 °C and a relative humidity range of 0–99% suit indoor and outdoor installations.

·         Vibration-resistant design: Vibration testing to IEC 60079-29 confirmed operation within the specified tolerance without loss of function, fault signals or false alarms.

·         Multiple outputs and status indicator: A 4–20 mA output configurable as source or sink, HART, RS-485 Modbus RTU and a multi-colored LED indicator on the front of the detector are provided together.

·         Miswiring protection: The electronics are designed to tolerate being powered up with the terminal wires accidentally reversed; inrush current is limited to 1 A.

·         Long-probe options for duct mounting: Versions with 15 cm and 35 cm extensions allow flange mounting for gas measurement inside ducts.

Technical Specifications

The following are typical technical data for the point infrared gas detector covered on this page. Target gas, LEL reference standard, output configuration and approval option are defined at ordering; final values should be confirmed for the selected configuration.

Response time depends on whether the environment shield is used; in the arctic configuration, power consumption is higher and start-up takes longer below –20 °C. The accuracy value applies to the calibration gas; the reading may differ for other hydrocarbons.

Key Advantages

·         Measurement resistant to poisoning: Because the measurement does not rely on a catalytic surface reaction, there is no risk of sensitivity loss caused by substances such as silicones or sulfur compounds that can affect catalytic sensors.

·         Operation independent of oxygen: Optical measurement does not depend on the ambient oxygen level, so combustible gas measurement can continue in inerted or oxygen-depleted areas.

·         Reduced maintenance workload: No field calibration adjustment, a 5-year proof test interval and gas-free function testing reduce field maintenance time and test gas logistics.

·         Fast and stable alarms: A short T90 time and ratio-based measurement support early leak recognition and stable evaluation of alarm thresholds.

·         Resistance to harsh site conditions: A 316 stainless steel housing, IP66/IP67 and NEMA 4X protection, heated optics and vibration resistance provide a construction suited to offshore and coastal facilities.

·         Remote condition monitoring: Status information transmitted over Modbus RTU helps operators monitor detectors from the control room and plan maintenance based on actual need.

·         Functional safety suitability: With the optional SIL 2 certificate to IEC 61508, the detector can be included in safety instrumented functions after assessment; loop-level verification is still required.

Application Areas

Point infrared gas detectors are used for measurement close to leak sources in hazardous areas where hydrocarbon gases may be present:

Onshore and Offshore Oil and Gas Facilities

Leak monitoring near separators, compressors and pump packages in process facilities and on pipelines.

Platforms, FPSO and FLNG Vessels

Provides continuous measurement on floating production and liquefaction units under salt-laden and vibrating conditions, supported by heated optics and a stainless steel housing.

Refineries

Used in process units to monitor hydrocarbon leaks, primarily methane and propane, at %LEL level.

Chemical and Petrochemical Plants

Forms a point gas detection layer at flammable gas feed lines, around reactors and in compressor buildings.

Power Generation Facilities

Provides leak detection at natural gas supply stations, gas turbine enclosures and fuel preparation areas; duct-mounted versions can be used in ventilation lines. Flue gas oxygen measurement for combustion control is handled as a separate requirement with in-situ oxygen analyzers.

Tank Storage and Offloading Facilities

Positioned close to likely leak sources around tanks, in pump rooms and at offloading points.

Gas Filling and Distribution Terminals

Used to monitor propane and methane leaks at filling racks, compressor stations and regulator stations.

Gas Transport and Pipelines

Provides point gas detection at compressor stations, valve rooms and metering stations.

How to Select an Infrared Gas Detector

The effectiveness of a point gas detector depends on measuring the right gas at the right location. Before a technical assessment, the following points should be clarified:

Target Gas and Measurement Requirement

·         Predominant hydrocarbon in the plant and choice of calibration gas: methane or propane

·         Where several hydrocarbons are present, response differences relative to the calibration gas

·         LEL reference standard: NFPA (2017) or IEC/ISO

·         Handling risks that cannot be measured with the IR method, such as hydrogen and toxic gases, with separate technologies

Mounting Point and Accessories

·         Distance to the leak source and gas density relative to air — high-level mounting for lighter-than-air methane, low-level mounting for heavier-than-air propane

·         Free-standing mounting or duct mounting with a 15 cm / 35 cm extension

·         Need for a pole mounting kit (2-inch or 3–4 inch), sunshade, mosquito net, environment shield or environment cap

·         Use of an aspiration kit where aspirated sampling is required

Ambient Conditions

·         Whether the ambient temperature stays within –40 to +75 °C

·         Arctic configuration for cold climates and the longer start-up time below –20 °C

·         Choice of environment shield according to exposure to rain, dust and direct wind, and its effect on response time

Testing and Maintenance Strategy

·         Alignment of the proof test period with the safety requirements

·         Preference for a gas-free tester, a remote bump test kit or a bump test housing

·         Need for a calibration certificate

System Integration and Approvals

·         Configuration of the 4–20 mA output as source or sink

·         Need for condition monitoring via HART and Modbus RTU

·         ATEX, IECEx, INMETRO or US–Canadian explosion-proof approval depending on the project country

·         Requirement for a SIL 2 certificate

Point detectors provide more complete protection when they are planned together with open path detectors for wide-area monitoring and, in areas with a fire risk, with IR3 flame detectors or UV/IR flame detectors. Safety-related LEL measurement should be assessed as a need separate from process measuring instruments such as process gas chromatographs or oxygen and combustibles transmitters.

Hazardous Area Approvals and Conformity Standards

The approval options available at ordering and the conformity standards specified for this infrared gas detector are listed below.

For each project, check the valid certificate copies for hazardous area marking details (gas group, temperature class) and for the scope of any gas detection performance approval. The calibration certificate and the SIL 2 certificate are not supplied by default; if required, they must be requested separately with the order.

Infrared Gas Detection Solutions from TLY Enerji

The effectiveness of point gas detectors depends on measuring the right gas, at the right location and against the right reference. On projects, TLY Enerji first reviews the hydrocarbon inventory of the plant, likely leak sources, the density of the gases relative to air and the ambient conditions; the choice of calibration gas, LEL reference standard, mounting accessories and environmental protection elements is based on this data.

We provide engineering support in connecting the 4–20 mA, HART and Modbus RTU outputs of the detectors to the gas detection control system, PLC or DCS; entering alarm levels and fault states into the signal list; planning function checks with gas during commissioning; and preparing proof test and bump test programs. We also offer project teams a technical opinion when layout alternatives combining point and open path detectors are assessed.

Frequently Asked Questions

What is an infrared gas detector?

An infrared gas detector is a fixed gas detection device that works on the principle that hydrocarbon gases absorb infrared light at certain wavelengths, and measures their concentration in %LEL. Because it measures the gas at its mounting location, it is referred to as a point detector. It is used in oil and gas, chemical, power and storage facilities for early recognition of methane or propane leaks.

What is the difference between IR and catalytic gas detectors?

Catalytic sensors burn the gas on a heated catalyst surface and measure the resulting temperature change; they therefore need oxygen and can be poisoned by certain substances. Infrared detectors measure the light absorption of the gas instead; they do not need oxygen, are not affected by poisoning and do not suffer a permanent loss of sensitivity at high concentrations. On the other hand, they cannot detect gases without IR absorption, such as hydrogen.

Can an infrared gas detector detect hydrogen?

No. The hydrogen molecule shows no significant absorption in the infrared region, so it cannot be measured with the IR method. The detector described on this page is configured at the factory for methane or propane and targets hydrocarbon gases. In areas with a hydrogen leak risk, other detection technologies should be evaluated; for hydrogen fires, flame detectors with hydrogen fire detection can be considered separately.

Does an infrared gas detector need calibration?

The detector is calibrated at the factory and no calibration adjustment is made in the field. Its IR sources and receivers are continuously cross-checked, with internal corrections applied when necessary. Nevertheless, periodic function tests with a gas-free tester or test gas must be carried out to verify the safety function, and proof tests should be planned on the basis of the 5-year interval and the plant’s safety requirements.

What is the response time of an IR gas detector?

Response time is specified as T90 and is 2 seconds with the environment shield and under 1 second without it. The environment shield protects against rain, dust and direct wind but slightly slows the gas on its way to the measuring zone. Because the response time does not include the time the gas needs to reach the detector, the proximity of the mounting point to the leak source is decisive for the overall detection time.

At what height should a gas detector be mounted?

Mounting height depends on the density of the target gas relative to air. Lighter-than-air methane tends to accumulate above the leak source, whereas heavier-than-air propane collects near the ground and in pits. The detector should be positioned according to this behavior and close to the leak source, taking ventilation flows, obstructions and maintenance access into account. The final layout should be determined by the plant’s risk assessment.

How does an IR gas detector perform in cold climates?

The detector can operate down to –40 °C, and its heated optics prevent condensation from forming on the optical surfaces. Above –20 °C the start-up time is under 2 minutes, while in colder conditions it can extend to 20 minutes. For very cold sites, an arctic configuration with an average power consumption below 5 W can be selected; power supply and cabling should be planned for this value.

Which outputs does the IR gas detector provide for the control system?

The measured value is transmitted through a 4–20 mA analog output that can be configured as source (default) or sink, with HART digital communication on this signal. Condition monitoring data can be read with Modbus RTU through the RS-485 serial port. A multi-colored LED indicator on the front of the detector shows the status quickly on site, and the electronics are protected against miswiring.