Infrared open path detector
Encyclopedia
Infrared open-path gas detectors send out a beam of infrared
light, detecting gas anywhere along the path of the beam. This linear 'sensor' is typically a few metres up to a few hundred metres in length. Open-path detectors can be contrasted with Infrared point sensor
s.
They are widely used in the petroleum
and petrochemical
industries, mostly to achieve very rapid gas leak detection
for flammable gases at concentrations comparable to the lower flammable limit
(typically a few percent by volume). They are also used, but so far to a lesser extent, in other industries where flammable concentrations can occur, such as in coal mining
and water treatment
. In principle the technique can also be used to detect toxic
gases, for instance hydrogen sulfide
, at the necessary parts-per-million concentrations, but the technical difficulties involved have so far prevented widespread adoption for toxic gases.
Usually, there are separate transmitter and receiver units at either end of a straight beam path. Alternatively, the source and receiver are combined, and the beam bounced off a retroreflector
at the far end of the measurement path. For portable use, detectors have also been made which use the natural albedo
of surrounding objects in place of the retroreflector. The presence of a chosen gas (or class of gases) is detected from its absorption
of a suitable infrared wavelength in the beam. Rain, fog etc. in the measurement path can also reduce the strength of the received signal, so it is usual to make a simultaneous measurement at one or more reference wavelengths. The quantity of gas intercepted by the beam is then inferred from the ratio of the signal losses at the measurement and reference wavelengths. The calculation is typically carried out by a microprocessor
which also carries out various checks to validate the measurement and prevent false alarms.
The measured quantity is the sum of all the gas along the path of the beam, sometimes termed the path-integral concentration of the gas. Thus the measurement has a natural bias (desirable in many applications) towards the total size of an unintentional gas release, rather than the concentration of the gas that has reached any particular point. Whereas the natural units of measurement for a Infrared point sensor
are parts-per-million (ppm) or the percentage of the lower flammable limit (%LFL), the natural units of measurement for an open path detector are ppm.metres (ppmm) or LFL.metres (LFLm). For instance, the fire and gas safety system on an offshore platform
in the North Sea
typically has detectors set to a full-scale reading of 5LFLm, with low and high alarms triggered at 1LFLm and 3LFLm respectively.
, so there is little incentive for applications that play to a point detector's strengths: where the point detector can be placed at the known location of the highest gas concentration, and a relatively slow response is acceptable. The open path detector excels in outdoor situations where, even if the likely source of the gas release is known, the evolution of the developing cloud or plume is unpredictable. Gas will almost certainly enter a extended linear beam before finding its way to any single chosen point. Also, point detectors in exposed outdoor locations require weather shields to be fitted, increasing the response time significantly. Open path detectors can also show a cost advantage in any application where a row of point detectors would be required to achieve the same coverage, for instance monitoring along a pipeline, or around the perimeter of a plant. Not only will one detector replace several, but the costs of installation, maintenance, cabling etc. are likely to be lower.
An important consideration for both types is a realistic assessment of their availability in service, distinguishing carefully between revealed and unrevealed downtime. The latter, where a detector appears to be working but is insensitive to gas, is especially serious. For instance, point detectors of the catalytic
type are prone to poisoning by silicones and H2S
, or by clogging of the gauze or sinter by water or ice. Point detectors of the infrared type are immune from the former but not the latter mechanism. Open path detectors suffer downtime from anything that blocks the path of the beam, such as people, vehicles or thick fog. However, they approach the ideal of eliminating unrevealed downtime because the loss of signal strength is easily made to generate a 'beam block' signal, distinct from zero gas or a hardware fault. Infrared detectors of both types, unlike passive sensors, invariably incorporate a microprocessor
capable of self-checking the circuitry, so unrevealed downtime due to hardware failures is largely eliminated. The possibility of (revealed) downtime of an open path detector due to fog can be minimised by limiting the beam path to moderate lengths. In fog-prone areas, such as the North Sea
, paths are often limited to 15 to 20 metres for this reason, even though the detectors are capable of much greater distances in clear air.
to aid the signal processing at the receiver:
The precise wavelength passbands
used must be isolated from the broad infrared spectrum. In principle any conventional spectrometer
technique is possible, but the NDIR technique with multilayer dielectric
filters and beamsplitters is most often used. These wavelength-defining components are usually located in the receiver, although one design has shared the task with the transmitter.
At the receiver, the infrared signal strengths are measured by some form of infrared detector
. Generally photodiode
detectors are preferred, and are essential for the higher modulation frequencies, whereas slower photoconductive
detectors may be required for longer wavelength regions. The signals are fed to low-noise amplifier
s, then invariably subject to some form of digital signal processing
. The absorption coefficient
of the gas will vary across the passband, so the simple Beer–Lambert law cannot be applied directly. For this reason the processing usually employs a calibration table
, applicable for a particular gas, type of gas, or gas mixture, and sometimes configurable by the user.
spectrum, the wavelengths must lie within a spectral window so the air in the beam path is itself transparent. These wavelength regions have been used:
. It did not achieved large volume sales, mainly because of cost and doubts about long-term reliability with moving parts. Beginning in 1985, Shell Research in UK was funded by Shell Natural Gas to develop an open-path detector with no moving parts. The advantages of the 2.3 μm wavelength were identified, and a research prototype was demonstrated. This design had a combined transmitter-receiver with a corner-cube
retroreflector
at 50 m. It used a pulsed incandescent lamp, PbS
photoconductive detectors in the gas and reference channels, and an Intel 8031 microprocessor for signal processing. In 1987 Shell licenced this technology to Sieger-Zellweger (later Honeywell
) who designed and marketed their industrial version as the 'Searchline', using a retro-reflective panel made up of multiple corner-cubes. This was the first open-path detector to be certified for use in hazardous areas
and to have no moving parts. Later work by Shell Research used two alternately pulsed incandescent sources in the transmitter and a single PbS detectors in the receiver, avoiding zero drifts caused by the variable responsivity
of PbS detectors. This technology was offered to Sieger-Zellweger, and later licensed to PLMS. a company part-owned by Shell Ventures UK. The PLMS GD4001/2 in 1991 were the first detectors to achieve a truly stable zero without moving parts or software compensation of slow drifts. They were also the first infrared gas detectors of any kind to be certified intrinsically safe
. The Israeli company Spectronix (also Spectrex) made an important advance in 1996 with their SafEye, the first to use a flash tube source, followed by Sieger-Zellweger with their Searchline Excel in 1998. In 2001 the PLMS Pulsar, soon afterwards acquired by Dräger
as their Polytron Pulsar, was the first detector to incorporate sensing to monitor the mutual alignment of the transmitter and receiver during both installation and routine operation.
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
light, detecting gas anywhere along the path of the beam. This linear 'sensor' is typically a few metres up to a few hundred metres in length. Open-path detectors can be contrasted with Infrared point sensor
Infrared point sensor
A infrared point sensor is a point gas detector based on the nondispersive infrared sensor technology.-Principle:Dual source and dual receivers are used for self compensation of changes in alignment, light source intensity and component efficiency. The transmitted beams from two infrared sources...
s.
They are widely used in the petroleum
Petroleum industry
The petroleum industry includes the global processes of exploration, extraction, refining, transporting , and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline...
and petrochemical
Petrochemical
Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane....
industries, mostly to achieve very rapid gas leak detection
Gas leak detection
Gas leak detection is the process of identifying potentially hazardous gas leaks by means of various sensors. These sensors usually employ an audible alarm to alert people when a dangerous gas has been detected. Common sensors used today include Infrared Point Sensor, Ultrasonic gas detectors,...
for flammable gases at concentrations comparable to the lower flammable limit
Lower flammable limit
Lower flammability limit , usually expressed in volume per cent, is the lower end of the concentration range of a flammable solvent at a given temperature and pressure for which air/vapor mixtures can ignite. The flammability range is delineated by the upper and lower flammability limit. Outside...
(typically a few percent by volume). They are also used, but so far to a lesser extent, in other industries where flammable concentrations can occur, such as in coal mining
Coal mining
The goal of coal mining is to obtain coal from the ground. Coal is valued for its energy content, and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from iron ore and for cement production. In the United States,...
and water treatment
Water treatment
Water treatment describes those processes used to make water more acceptable for a desired end-use. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the...
. In principle the technique can also be used to detect toxic
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...
gases, for instance hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, at the necessary parts-per-million concentrations, but the technical difficulties involved have so far prevented widespread adoption for toxic gases.
Usually, there are separate transmitter and receiver units at either end of a straight beam path. Alternatively, the source and receiver are combined, and the beam bounced off a retroreflector
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
at the far end of the measurement path. For portable use, detectors have also been made which use the natural albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
of surrounding objects in place of the retroreflector. The presence of a chosen gas (or class of gases) is detected from its absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of a suitable infrared wavelength in the beam. Rain, fog etc. in the measurement path can also reduce the strength of the received signal, so it is usual to make a simultaneous measurement at one or more reference wavelengths. The quantity of gas intercepted by the beam is then inferred from the ratio of the signal losses at the measurement and reference wavelengths. The calculation is typically carried out by a microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
which also carries out various checks to validate the measurement and prevent false alarms.
The measured quantity is the sum of all the gas along the path of the beam, sometimes termed the path-integral concentration of the gas. Thus the measurement has a natural bias (desirable in many applications) towards the total size of an unintentional gas release, rather than the concentration of the gas that has reached any particular point. Whereas the natural units of measurement for a Infrared point sensor
Infrared point sensor
A infrared point sensor is a point gas detector based on the nondispersive infrared sensor technology.-Principle:Dual source and dual receivers are used for self compensation of changes in alignment, light source intensity and component efficiency. The transmitted beams from two infrared sources...
are parts-per-million (ppm) or the percentage of the lower flammable limit (%LFL), the natural units of measurement for an open path detector are ppm.metres (ppmm) or LFL.metres (LFLm). For instance, the fire and gas safety system on an offshore platform
Oil platform
An oil platform, also referred to as an offshore platform or, somewhat incorrectly, oil rig, is a lаrge structure with facilities to drill wells, to extract and process oil and natural gas, and to temporarily store product until it can be brought to shore for refining and marketing...
in the North Sea
North Sea oil
North Sea oil is a mixture of hydrocarbons, comprising liquid oil and natural gas, produced from oil reservoirs beneath the North Sea.In the oil industry, the term "North Sea" often includes areas such as the Norwegian Sea and the area known as "West of Shetland", "the Atlantic Frontier" or "the...
typically has detectors set to a full-scale reading of 5LFLm, with low and high alarms triggered at 1LFLm and 3LFLm respectively.
Advantages and disadvantages versus fixed-point detectors
An open path detector usually costs more than a single point detectorInfrared point sensor
A infrared point sensor is a point gas detector based on the nondispersive infrared sensor technology.-Principle:Dual source and dual receivers are used for self compensation of changes in alignment, light source intensity and component efficiency. The transmitted beams from two infrared sources...
, so there is little incentive for applications that play to a point detector's strengths: where the point detector can be placed at the known location of the highest gas concentration, and a relatively slow response is acceptable. The open path detector excels in outdoor situations where, even if the likely source of the gas release is known, the evolution of the developing cloud or plume is unpredictable. Gas will almost certainly enter a extended linear beam before finding its way to any single chosen point. Also, point detectors in exposed outdoor locations require weather shields to be fitted, increasing the response time significantly. Open path detectors can also show a cost advantage in any application where a row of point detectors would be required to achieve the same coverage, for instance monitoring along a pipeline, or around the perimeter of a plant. Not only will one detector replace several, but the costs of installation, maintenance, cabling etc. are likely to be lower.
An important consideration for both types is a realistic assessment of their availability in service, distinguishing carefully between revealed and unrevealed downtime. The latter, where a detector appears to be working but is insensitive to gas, is especially serious. For instance, point detectors of the catalytic
Catalytic bead sensor
A catalytic bead sensor is a type of sensor that is used for gas detection.-Principle:The catalytic bead sensor consist of two coils of fine platinum wire each embedded in a bead of alumina, connected electrically in a Wheatstone bridge circuit. One of the pellistors is impregnated with a special...
type are prone to poisoning by silicones and H2S
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, or by clogging of the gauze or sinter by water or ice. Point detectors of the infrared type are immune from the former but not the latter mechanism. Open path detectors suffer downtime from anything that blocks the path of the beam, such as people, vehicles or thick fog. However, they approach the ideal of eliminating unrevealed downtime because the loss of signal strength is easily made to generate a 'beam block' signal, distinct from zero gas or a hardware fault. Infrared detectors of both types, unlike passive sensors, invariably incorporate a microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
capable of self-checking the circuitry, so unrevealed downtime due to hardware failures is largely eliminated. The possibility of (revealed) downtime of an open path detector due to fog can be minimised by limiting the beam path to moderate lengths. In fog-prone areas, such as the North Sea
North Sea oil
North Sea oil is a mixture of hydrocarbons, comprising liquid oil and natural gas, produced from oil reservoirs beneath the North Sea.In the oil industry, the term "North Sea" often includes areas such as the Norwegian Sea and the area known as "West of Shetland", "the Atlantic Frontier" or "the...
, paths are often limited to 15 to 20 metres for this reason, even though the detectors are capable of much greater distances in clear air.
Component parts
In principle any source of infrared radiation could be used, together with an optical system of lenses or mirrors to form the transmitted beam. In practice the following sources have been used, always with some form of modulationModulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
to aid the signal processing at the receiver:
- An incandescent light bulbIncandescent light bulbThe incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...
, modulated by pulsing the current powering the filament or by a mechanical chopperOptical chopperAn optical chopper is a mechanical device which periodically interrupts a light beam. Three types are available: variable frequency rotating disc choppers, fixed frequency tuning fork choppers, and optical shutters...
. For systems used outdoors, it is difficult for an incandescent source to compete with the intensity of sunlightSunlightSunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
when the sun shines directly into the receiver. Also, it is difficult to achieve modulation frequencies distinguishable from those that can be produced naturally, for instance by heat shimmerAtmospheric refractionAtmospheric refraction is the deviation of light or other things like humanelectromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude...
or by sunlight reflecting off waves at sea. - A gas-discharge lampGas-discharge lampGas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas, i.e. a plasma. The character of the gas discharge critically depends on the frequency or modulation of the current: see the entry on a frequency classification...
is capable of exceeding the spectral powerSpectral power distributionIn color science and radiometry, a spectral power distribution describes the power per unit area per unit wavelength of an illumination , or more generally, the per-wavelength contribution to any radiometric quantity .Mathematically, for the spectral...
of direct sunlight in the infrared, especially when pulsed. Modern open path systems typically use a xenon flashtubeFlashtubeA flashtube, also called a flashlamp, is an electric arc lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations. Flashtubes are made of a length of glass tubing with electrodes at either end and are filled with a gas that, when triggered, ionizes...
powered by a capacitorCapacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
discharge. Such pulsed sources are inherently modulated. - A semiconductor laserLaser diodeThe laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
provides a relatively weak source, but one that can be modulated at high frequency in wavelength as well as amplitude. This property permits various signal processing schemes based on Fourier analysis, of use when the absorption of the gas is weak but narrow in spectral linewidthSpectral linewidthThe spectral linewidth characterizes the width of a spectral line, such as in the electromagnetic emission spectrum of an atom, or the frequency spectrum of an acoustic or electronic system...
.
The precise wavelength passbands
Band-pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
used must be isolated from the broad infrared spectrum. In principle any conventional spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
technique is possible, but the NDIR technique with multilayer dielectric
Optical coating
An optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and...
filters and beamsplitters is most often used. These wavelength-defining components are usually located in the receiver, although one design has shared the task with the transmitter.
At the receiver, the infrared signal strengths are measured by some form of infrared detector
Infrared detector
An infrared detector is a photodetector that reacts to infrared radiation. The two main types of detectors are thermal and photonic.The thermal effects of the incident IR radiation can be followed through many temperature dependent phenomena....
. Generally photodiode
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
detectors are preferred, and are essential for the higher modulation frequencies, whereas slower photoconductive
Photoconductivity
Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation....
detectors may be required for longer wavelength regions. The signals are fed to low-noise amplifier
Low-noise amplifier
Low-noise amplifier is an electronic amplifier used to amplify possibly very weak signals . It is usually located very close to the detection device to reduce losses in the feedline. This active antenna arrangement is frequently used in microwave systems like GPS, because coaxial cable feedline is...
s, then invariably subject to some form of digital signal processing
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
. The absorption coefficient
Attenuation coefficient
The attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter. A large attenuation coefficient means that the beam is quickly "attenuated" as it passes through the medium, and a small...
of the gas will vary across the passband, so the simple Beer–Lambert law cannot be applied directly. For this reason the processing usually employs a calibration table
Lookup table
In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...
, applicable for a particular gas, type of gas, or gas mixture, and sometimes configurable by the user.
Operating wavelengths
The choice of infrared wavelengths used for the measurement largely defines the detector's suitability for a particular applications. Not only must the target gas (or gases) have a suitable absorptionAbsorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
spectrum, the wavelengths must lie within a spectral window so the air in the beam path is itself transparent. These wavelength regions have been used:
- 3.4 μm region. All hydrocarbonHydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s and their derivatives absorb strongly, due to the C-H stretch mode of molecular vibrationMolecular vibrationA molecular vibration occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant translational and rotational motion...
. It is commonly used in infrared point detectorsInfrared point sensorA infrared point sensor is a point gas detector based on the nondispersive infrared sensor technology.-Principle:Dual source and dual receivers are used for self compensation of changes in alignment, light source intensity and component efficiency. The transmitted beams from two infrared sources...
where path lengths are necessarily short, and for open-path detectors requiring parts-per-million sensitivity. A disadvantage for many applications is that methane absorbs relatively weakly compared to heavier hydrocarbons, leading to large inconsistencies of calibration. For open-path detection of flammable concentrations the absorption for non-methane hydrocarbons is so strong that the measurement saturates, a significant gas cloud appearing 'black'. This wavelength region is beyond the transmission range of borosilicate glassBorosilicate glassBorosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
, so windows and lenses must be made of more expensive materials and tend to be small in apertureApertureIn optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
.
- 2.3 μm region. All hydrocarbonHydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s and their derivatives have absorption coefficientsAttenuation coefficientThe attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter. A large attenuation coefficient means that the beam is quickly "attenuated" as it passes through the medium, and a small...
appropriate for open path detection at flammable concentrations. A useful advantage in practical applications is that the detector's response to many different gases and vapours is relatively uniform when expressed in terms of the lower flammable limitLower flammable limitLower flammability limit , usually expressed in volume per cent, is the lower end of the concentration range of a flammable solvent at a given temperature and pressure for which air/vapor mixtures can ignite. The flammability range is delineated by the upper and lower flammability limit. Outside...
. Borosilicate glassBorosilicate glassBorosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
retains useful transmission in this wavelength region, allowing large aperture optics to be produced at moderate cost.
- 1.6 μm region. A wide range of gases absorb in the near-infrared. Typically the absorption coefficientsAttenuation coefficientThe attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter. A large attenuation coefficient means that the beam is quickly "attenuated" as it passes through the medium, and a small...
are relatively weak, but light molecules show narrow, individually resolvedSpectral resolutionThe spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum...
spectral lines rather than broad bands. This results in relatively large values of the gradientGradientIn vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
and curvatureCurvatureIn mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...
of the absorption with respect to wavelength, enabling semiconductor laser-basedLaser diodeThe laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
systems to distinguish gas molecules very specifically; for instance hydrogen sulfideHydrogen sulfideHydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, or methaneMethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
to the exclusion of heavier hydrocarbons.
History
The first open-path detector offered for routine industrial use, as distinct from research instruments built in small numbers, was the Wright and Wright 'Pathwatch' in the US, 1983. Acquired by Det-tronics in 1992, the detector operated in the 3.4 μm region with a powerful incandescent source and a mechanical chopperOptical chopper
An optical chopper is a mechanical device which periodically interrupts a light beam. Three types are available: variable frequency rotating disc choppers, fixed frequency tuning fork choppers, and optical shutters...
. It did not achieved large volume sales, mainly because of cost and doubts about long-term reliability with moving parts. Beginning in 1985, Shell Research in UK was funded by Shell Natural Gas to develop an open-path detector with no moving parts. The advantages of the 2.3 μm wavelength were identified, and a research prototype was demonstrated. This design had a combined transmitter-receiver with a corner-cube
Corner reflector
A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves back directly towards the source, but shifted . Unlike a simple mirror, they work for a relatively wide-angle field of view. The three intersecting surfaces often have...
retroreflector
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
at 50 m. It used a pulsed incandescent lamp, PbS
Lead(II) sulfide
Lead sulfide is an inorganic compound with the formula Pb. It finds limited use in electronic devices. PbS, also known as galena, is the principal ore and most important compound of lead....
photoconductive detectors in the gas and reference channels, and an Intel 8031 microprocessor for signal processing. In 1987 Shell licenced this technology to Sieger-Zellweger (later Honeywell
Honeywell Analytics
Honeywell Analytics is a producer of gas detector based in Poole in the United Kingdom.-Origins of the Organisation:Originally incorporated as EIC-Sieger in 1959 and later re-branded as J&S Sieger Ltd in 1961, the company revolutionised the fledgling gas detection industry, when its founder, Joshua...
) who designed and marketed their industrial version as the 'Searchline', using a retro-reflective panel made up of multiple corner-cubes. This was the first open-path detector to be certified for use in hazardous areas
Electrical Equipment in Hazardous Areas
In electrical engineering, a hazardous location is defined as a place where concentrations of flammable gases, vapors, or dusts occur. Electrical equipment that must be installed in such locations is especially designed and tested to ensure it does not initiate an explosion, due to arcing contacts...
and to have no moving parts. Later work by Shell Research used two alternately pulsed incandescent sources in the transmitter and a single PbS detectors in the receiver, avoiding zero drifts caused by the variable responsivity
Responsivity
Responsivity measures the input–output gain of a detector system. For a system that responds linearly to its input, there is a unique responsivity. For nonlinear systems, the responsivity is the local slope ....
of PbS detectors. This technology was offered to Sieger-Zellweger, and later licensed to PLMS. a company part-owned by Shell Ventures UK. The PLMS GD4001/2 in 1991 were the first detectors to achieve a truly stable zero without moving parts or software compensation of slow drifts. They were also the first infrared gas detectors of any kind to be certified intrinsically safe
Intrinsic safety
Intrinsic safety is a protection technique for safe operation of electronic equipment in explosive atmospheres and under irregular operating conditions. The concept was developed for safe operation of process control instrumentation in hazardous areas, particularly North Sea gas platforms...
. The Israeli company Spectronix (also Spectrex) made an important advance in 1996 with their SafEye, the first to use a flash tube source, followed by Sieger-Zellweger with their Searchline Excel in 1998. In 2001 the PLMS Pulsar, soon afterwards acquired by Dräger
Dräger
The Drägerwerk AG is a German company based in Lübeck which makes breathing and protection equipment, gas detection and analysis systems, and noninvasive patient monitoring technologies. Customers include hospitals, fire departments and diving companies....
as their Polytron Pulsar, was the first detector to incorporate sensing to monitor the mutual alignment of the transmitter and receiver during both installation and routine operation.