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Detection Measures


Detection may consist in supervising an unattended site, or in monitoring signals that cannot be perceived by attending employees, with the aim of producing an action before an accident escalates. In the case of hydrogen fires, detection can fulfil both requirements. The major hazards due to an unwanted release of hydrogen relate to the build up of explosive conditions. In this respect, hydrogen is potentially more hazardous than other conventional fuels (methane, propane) or their vapors (gasoline) in most confined situations because of its high flammability, wide detonability ranges and its low ignition energy (CracknellRF:2002). Although its high buoyancy means that the risks of an unwanted release are likely to decrease rapidly to acceptable levels in outdoor situations and/or in areas where there is adequate ventilation, the deployment of an adequate system for the detection of explosive atmospheres should always be taken into consideration as a possible safety measure.

Detection of Explosive Atmospheres

In regulatory terms, the issue of an explosive atmosphere is covered within the existing legislation for the safe use of flammable and explosive gases in general. Alongside other protection measures, the European Parliament and Council Directive 1999/92/EC on the Minimum Requirements for Improving the Safety and Health Protection of Workers potentially at risk from explosive atmospheres (Directive:1999:92:EC:2000) prescribes that “Where necessary, workers must be given optical and/or acoustic warnings and withdrawn before the explosion conditions are reached”. It follows that the necessity of installing a detection system should be estimated as part of a preliminary analysis of the operational hazards posed by the use of flammable gases. The point is further detailed in a subsequent Communication of the European Commission (COM:2003:515final) on the good practice for implementing the Directive, which states that “Concentrations in the vicinity of a plant can be monitored e.g. by means of gas alarms”. For such alarms to be used, the substances likely to be present, the location of the sources, maximum source strength and dispersion conditions must be known in sufficient detail and the instrument performance must be appropriate to the conditions of use, especially with regard to response time, alarm level and cross-sensitivity. Failure of individual functions of gas alarm systems should not lead to dangerous situations and the number and location of measuring points must be chosen to allow the anticipated mixtures to be detected quickly and reliably. Last but not least, gas alarms for use in hazardous areas must be approved and suitably marked as safe electrical equipment according to the European Directive 94/9/EC (Directive:94:9:EC:1994), which in turn is supported by a number of European standards prepared by CENELEC (EuropeanCommission:ATEXGuidelines:2007) annexes 5-7. While ensuring the safety of industrial operation in the presence of flammable gases is a well-recognized issue for which a number of established technologies can be used, there is a need to reconsider existing knowledge of hydrogen detection in the light of a future hydrogen economy. A wide variety of novel applications could be in sight, some of which may bring hydrogen much closer to the general public than it has ever been before, thus requiring hydrogen sensors to be as ubiquitous as computer chips in our society (DiMeoFJr:2000). Both, the U.S. Department of Energy (DoE) and the European Hydrogen Fuel Cell Platform (HFP), have been identifying new directions for hydrogen sensor development, envisaging innovation in both materials and concepts for applications ranging from large-area physical sensing to in-situ detection of leaks from portable devices (HFP:StrategicResearchAgenda:2005). Efficiency over a wide range of hydrogen (and oxygen) concentrations, low sensitivity to gaseous contaminants and poisoning are outstanding requirements, along with the possibility to efficiently integrate “intelligent” sensing devices into hydrogen systems, so that safety or emergency measures can be activated automatically where needed.

Detection Techniques

Several types of hydrogen sensors are in use, selected according to the operating conditions. Electrochemical, catalytic and thermal conductivity sensors are mainly used in industries where hydrogen risk is present. Semi-conductor-based sensors are most often used in research laboratories, whereas MEMS's (Micro Electro Mechanic System) are used in the aeronautic and space travel industries. The operating principles of commercially available sensorsand some other sensors which are under development are briefly described below.

Electrochemical Sensors
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Fig. 1: Electrochemical hydrogen detector

The working principle is amperometry, i.e. the measurement of current driven by redox- (reduction-oxidation) reactions. The process is based on an electrochemical cell covered by a semi-permeable, selective membrane which enables the exclusive diffusion of hydrogen. The diffusion rate through this membrane is proportional to air temperature and to the partial pressure of hydrogen (and therefore to its concentration in air). Once it has diffused through the membrane, hydrogen comes into contact with the boundary layer between the membrane and the electrolyte which consists of sulphuric acid. The hydrogen is instantly ionised at the solid-liquid interface of a platinum catalytic electrode (working electrode). This ionisation enables a redox reaction with the second electrode (auxiliary electrode) consisting of platinum oxide. These reactions cause a potential difference between the electrodes which enables the hydrogen concentration to be determined by a non-linear correlation. The reaction products generate charge barriers which tend to restrict the reaction. To improve the stability and the reproducibility of the measurement, a third, chemically non-active electrode is added to the cell. A potentiostat (created by using an operational amplifier) is applied to maintain the potential of the working electrode at the same value as this third electrode, called the reference electrode. The lifetime of the amperometric cell is limited by a dry-out effect of the electrolyte which is strongly influenced by its exposure to certain operating conditions, especially raised temperatures. (AccorsiA:1994),(JamoisD:1997)

Catalytic Bead Sensors
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Fig. 2: Catalytic bead detector

The detection principle is based on combustion heat measurement of flammable gas at the surface of a metallic catalyser. This means that a pearl covered with a catalyst (called a pellistor or catalytic pearl) or even a platinum filament is heated by the Joule effect while its electric power consumption is measured. Combustion of gas molecules at the element surface causes an increase in its temperature and therefore a change in its resistance. This resistance modification creates an imbalance in the Wheatstone bridge where the measurement element is inserted. Hydrogen concentration in air is linearely correlated to the imbalance of the bridge. To overcome the influence of variations in temperature and room humidity, a second element, similar to the one used for the measurement, but with a non-catalytic surface, is inserted into the Wheatstone bridge. In the absence of combustible gas, each of the two elements undergoes identical resistance variations and the bridge remains balanced. (AccorsiA:1994),(JamoisD:1997),(MoliereM:2005)

Heat Conduction Sensors (Catharometers)
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Fig. 3: Catharometric detector

Heat conduction sensors use the high thermal conductivity of hydrogen gas. A material heated by the Joule effect is stabilised at a temperature which depends on the electrical power provided and thermal exchanges with the gaseous environment. A change in the composition of the atmosphere causes a change in the sensor temperature. The derivative of this temperature change, which varies the electrical resistance of the element, is linearly correlated to the concentration of hydrogen gas in air. For the measurement, a metallic wire conductor coated with chemically inert material is exposed to the gas probe. A second identical wire conductor is exposed to a reference atmosphere for temperature compensation. The electrical resistance variation is also measured using a Wheatstone bridge. Signals caused by the varying thermal conditions are weaker than the signals of catalytic sensors. (AccorsiA:1994)

Semiconductor Sensors
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Fig. 4: Semi-conductor detector

The support material of the redox-reaction is no longer a metal, but an n- or p-type semi-conductor of metal oxide (SnO2, ZnO, etc.). Its conductivity is caused by shortages of oxygen (oxide not exactly stoichiometric). These redox reactions, or simply adsorption reactions on the surface, change the material resistance by modifying the number of oxygen shortages. The material is heated, similar to the catalytic pearls, but the measurement is different: The resistance variation of the material itself is measured rather than that of the heating element, which is linked to hydrogen concentration by a non-linear correlation. (AccorsiA:1994)

FED Field Effect Gas Sensor
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Fig. 5: Field effect detector, FED

This sensor type is based on a metal oxide field effect transistor. Hydrogen diffuses into the transistor bulk, and its electrical properties change, dependent on the hydrogen concentration. Hydrogen presence induces an increase of the threshold voltage and a decrease of transconductance in an electrical connection as shown in Fig. 5. These transconductance changes are linked to hydrogen concentration by a non-linear correlation.

Resistive Palladium Sensor
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Fig. 6: Resistive palladium sensor

This sensor type consists of a catalytically active palladium surface. Hydrogen is adsorbed, dissociates to hydrogen atoms and generates palladium hydride, which has a higher electrical resistance than the pure palladium. This resistance change, which is linearely correlated to hydrogen concentration, is then measured. (TanOK:1999)

MEMS – Micro Electro Mechanic Systems

Micro electro mechanic systems combine calculators and miniscule devices such as sensors, valves, gears, mirrors and actuators loaded on a semi-conductor chip. The “detector” chip comprises

  • two hydrogen detection devices, namely a Schottky palladium-chromium diode (PdCr) for low concentrations, and a resistive palladium sensor for high concentrations;
  • a temperature sensor and a heating element to control the temperature,
  • electronics which enable the treatment of the signals from the different devices present on the chip.
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Fig. 7: Schottky diode based H2 sensor

The operating principle of the Schottky diode is the following: the palladium enables the adsorption and the dissociation of the hydrogen molecule into hydrogen atoms. The hydrogen atoms diffuse through the palladium up to the PdCr interface and modify the surface charge. This change is detectable by measuring the voltage-current pair and is dependent on the hydrogen concentration by a non-linear correlation. In the case of a resistive sensor, the formation of palladium hydrides (caused by the adsorption and the dissociation of the hydrogen molecule into hydrogen atoms) increases the resistance compared to the pure palladium. (ChengSY:2003),(KimJ:2003),(ChenHI:2001)

Emerging Hydrogen Detection Technologies

Hydrogen detection technologies can generally be divided into non-optical and optical based technologies. The following section gives a survey on both, starting with non-optical technologies (TobiskaP:2001),(TanOK:1999). Recent technologies are

  • semiconductor
  • Schottky diode
  • palladium wire network
  • surface acoustic wave sensor on a nano-structured sensitive layer

The development of semi-conductor and Schottky diode sensors mainly aims to improve the selectivity of the different layers as well as testing new metallic substrate–deposit combinations. Although these technologies are available on the market, research continues in order to reduce drift and to increase selectivity. The operation of semi-conductor and Schottky diode technologies is described above. Among others the operating principles of palladium wire network based sensors and surface sound wave sensors on a nano-structured sensitive layer are described. Emerging optical technologies mainly use fibre optics combined with hydrogen-sensitive coatings to measure hydrogen concentrations. (BaoX:1995),(BillingtonR:1999),(GlennSellarR:2003)

Palladium Wire Network

These sensors consist of a network of 20 to 100 palladium nano or mesoscopic wires. These networks of palladium nano-wires are prepared by electro-deposition on a graphite surface and transferred onto a glass slide covered with a cyanacrylate film. The nano-wires are then connected on either side by silver contacts. These palladium nano-wires are in fact “broken” and do not conduct the current. In the presence of hydrogen, the palladium swells slightly, and the nanoscopic spaces or “breakages” are “repaired”, enabling the passage of electric current. The resistance change depends on the hydrogen concentration, in a concentration range from 2 to 10%. In order to operate these sensors require a permanent power connection, and may even need to be heated. (FavierF:2001),(MatsumiyaM:2003)

Surface Acoustic Wave Sensor on a Nano-Structured Sensitive Layer

A surface acoustic wave sensor is built around two interconnected transducers placed on the surface of a piezoelectric substrate. By connecting alternating current to the metallic conductors of the entrance transducer, an alternation of compressions and expansions occurs which generates a surface wave. This wave moves towards the second transducer to be converted back to an electric signal. During the transit between the two electrodes, it is possible to influence the wave by using a nano-structured sensitive net, which consists of the same palladium wire network mentioned in the previous paragraph. This network represents disruptions for the wave conducting surface and varies its physical characteristics (density, rigidity, electric conductivity, thickness) with absorbed hydrogen, which itself depends on the hydrogen concentration present. These disruptions lead to hydrogen concentration dependent phase speed and attenuation variations of the surface acoustic waves. Temperature dependencies can be minimized by comparison with the output signal of a second, identical sound wave sensor without an H2-sensitive Pd network.

Fibre Optic with a Palladium Micro-Mirror
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Fig. 8: Fibre optical Pd micro-mirror sensor

This type of interferometric hydrogen sensor is based on a multi-modal fibre optic with a palladium micro-mirror. Hydrogen is absorbed by the palladium micro-mirror located at the end of the fibre. The optical and electric properties of the palladium change. Consequently, the reflected wave is modified whereas the incident wave remains the same. (BevenotX:2000)

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Fig. 9: Fibre optical WO3 mirror sensor
Fibre optic with a Tungsten Trioxide Mirror

Tungsten trioxide (WO3) shows hydrogen concentration dependent changes in its refractive index (BensonDK:1999), which leads to changes in the reflected light intensity. Resulting intensity variations of the reflected beam can be interferometrically detected.

Fibre Optic Coated with a Palladium Layer
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Fig. 10: Fibre optical Pd film sensor

The sensitive area is a fibre section covered with a thin palladium film. Light waves passing the fibre cause evanescent waves on the fibre core surface. Since the core of the fibre is covered with a palladium layer, the evanescent fields are altered. When hydrogen is absorbed by the palladium film, the refractive index of the Pd coating changes by reduction. This change in the refractive index modifies the absorption of the guided light, which can be detected by monitoring the light intensity via interferometer techniques like Fabry-Perot. (TabibAzarM:1999),(UttamchandaniD:1999),(KazemiAA:1999),(MaN:1999)

Bragg Network Fibre Optic
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Fig. 11: Fibre optical Pd lattice sensor

A Bragg network causes periodic or aperiodic disruption of the effective absorption ratio or of the effective refractive index of a fibre-optic cable. Predetermined light wavelengths are reflected by the Bragg network while all other wavelengths pass through it. In this sensor, which operates with UV light, a mechanical stress develops which is caused by the palladium layer when it absorbs hydrogen. This stress stretches or compresses the Bragg network and therefore the wavelengths or optical lengths of reflected or transmitted light. Where several Bragg networks with different lattice constants are used, several hydrogen sensors can be multiplexed on a single fibre. (SutapunB:1999)

Chemochromic Sensors
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Fig. 12: Chemochromic sensor

This type of non-electric indicator consists of a thin film coating or a pigment of a transition metal oxide such as tungsten oxide or molybdenum oxide with a catalyst such as platinum or palladium. The oxide is partially reduced in the presence of hydrogen in concentrations as low as 300 parts per million and changes from transparent to a dark color. The color change is fast and easily seen from a distance. In air, the color change reverses quickly when the source of hydrogen gas is removed, in the case of tungsten oxide, or is nearly irreversible, in the case of molybdenum oxide. The partially reduced transition metal oxide becomes semi conductive and increases its electrical conductivity by several orders of magnitude when exposed to hydrogen. The integration of this electrical resistance sensor with an RFID (Radio Frequency IDentification) tag may extend the ability of these sensors to record and transmit a history of the presence or absence of leaked hydrogen over long distances. Over long periods of exposure to the atmosphere, the indicator’s response may slow due to catalyst degradation. Current emphasis is on controlling this degradation. Chemochromic sensors and their derivatives like paints, tape, cautionary decals and coatings for hydrogen storage tanks may be used to complement conventional electronic hydrogen sensors, or as a low-cost alternative in situations where an electronic signal is not needed for visual human surveillance. (HoaglandW:2007)

Survey on typical hydrogen sensor properties

Table 1: typical hydrogen sensor properties

Sensor Type
Standard / Special
Hydrogen Concentration Range Cross Sensitivity / Selectivity Accuracy Long-Term Stability Response Time Warm-up Time Power Consumption Costs
Electrochemical 10,000 ppm CO /
high selectivity
10% - < 1 min 0.5 h 1 mW low
Catalytic bead 100% LEL Hydrocarbons, Combustible Gases & Vapours /
low selectivity
10% o < 0.5 min 5 min 1 W medium
Heat Conduction / Catharometer 100% Vol. CH, CO2, He, Ar, Ne, SF6 /
high selectivity
0.5% + < 0.5 min 1 min 10 min medium
Semiconductor 100% LEL low selectivity 5% -- < 0.5 min 5 min 25 mW very low
Field Effect Transistor 30,000 ppm high selectivity 10% - < 10 s 1 min - medium
Ultrasonic 100% Vol. low selectivity 10% o 1 µs 1 s - medium
Gas Chromatograph 50% Vol. very low /
very high
10% - 1 min 3 h - very high
Mass Spectroscopy 100 ppm
-100% Vol.
very low /
very high
10% + 10 ms 6 h - very high
MEM's 10ppm
-100% Vol.
low selectivity 10% - - - - high

Detection Layout

As a colourless, odourless and tasteless gas, hydrogen cannot be detected by human senses. Means should therefore be provided to detect the presence of hydrogen in places where leaks and/or accumulations may occur. The hydrogen detection system should be compatible with other systems such as those for fire detection and fire suppression. Hydrogen detection devices themselves should not be a source of ignition and the response times of these devices should be as rapid as possible. Some important performance factors to be considered when selecting a hydrogen sensor for a particular application include:

  • Response time
  • Detection range
  • Durability / lifetime
  • Calibration / maintenance
  • Cross sensitivity / specificity
  • Area coverage

The correct location of reliable sensors is crucial for timely detection and warning of hydrogen leaks before an explosive mixture is formed. Recommended locations (ISO/TR 15916, 2004 (ISO:TR:15916:2004E)) for sensors include the following:

  • Locations where hydrogen leaks or spills are possible
  • Hydrogen connections that are routinely separated (for example, hydrogen refuelling ports)
  • Locations where hydrogen could accumulate
  • Building air intake ducts, if hydrogen could be carried into the building
  • Building exhaust ducts, if hydrogen could be released inside the building

A generally accepted and commonly used concentration level for alarm activation is 1 % hydrogen (volume fraction) in air, which is equivalent to 25 % of the lower flammability limit. This level should normally provide adequate time for an appropriate response to be initiated, such as a system shutdown, evacuation of personnel or other measures where necessary. In designing a reliable hydrogen detection and monitoring system, the following recommendations have been made by NASA 1997 (NASA:NSS:1740:16:1997):

  • Evaluate and list all possible sources to be monitored (valves, flanges, connections, bellows, etc.) and provide valid justification for sources not monitored.
  • Evaluate the expected response time of the leak detection system to ensure compatibility with the responding safety system.
  • Provide visual and audible alarms as necessary when the worst allowable condition (red line) is exceeded. The allowable condition must still be in the safe range, but a warning indicates a problem.
  • Provide portable detectors for field operations or isolated areas and permanently installed detectors for remote-automated operations.
  • Utilize a program to maintain and periodically recalibrate detectors to ensure acceptable performance.
  • Determine the number and distribution of sampling points in the hydrogen detection system based on the possible leak rate, ventilation amount, and area size. Consideration should be given to methods of routing hydrogen to the detector.

At a European level, and to the knowledge of the authors, no EN standard or recommendation for detection layouts specific to hydrogen systems has been made publically available so far. However, an obligation is posed under the ATEX directive (Directive:94:9:EC:1994) for the necessary instructions for detection or alarm devices for monitoring the occurrence of explosive atmospheres to be provided in the appropriate places. The European Standard EN50073 (EN:50073:1999) supporting the Directive provides details of the criteria for the selection, installation and placement of combustible gas sensors, which are essentially coherent with the information laid down in the previous paragraphs of the EN 50073. The international standard IEC 61779-6 (EN:61779-6:1999), very similar to the EN 50073, also provides a two-pages document in the annex that summarizes the above points in the form of a typical environmental and application check-list.

Maintenance of Detectors

A detector includes two elements: a sensor and a transducer. The sensor is the sensitive element responsible for converting a physical value (e.g. gas concentration) into a useful output signal. The transducer turns the output signal into meaningful information displayed by the user interface. Sensor or / and transducer ageing may cause a drift with time. Maintenance is therefore essential to maintain the high performance level required for a safety applications.

Regarding maintenance, detectors should be:

  • regularly cleaned, especially the head of the detector, to allow gas to reach the sensitive element,
  • regularly inspected for possible malfunctions, visible damage or other deterioration,
  • calibrated (zero and sensitivity adjusting) with a standard gas in accordance with the procedure outlined in the instruction handbook.

Maintenance intervals depend on both the context of use and the type of detector involved (detection technique, portable or fixed detector…). The best way to determine the maintenance interval for a detector is based on experience gained through the use of this detector. For new installations, it may be wise to carry out maintenance frequently at first (perhaps weekly), increasing the time intervals (to, perhaps, monthly) as confidence grows on the basis of the maintenance records of the installation concerned. Information on the maintenance protocol should be found in the user manual. IEC 61508 (EN:61508-1:2001) also deals with the need for periodic maintenance.

Detection of Hydrogen Flames

Hydrogen burns with very pale blue flames and emits neither visible light in day time (solar radiation can overpower the light from a hydrogen flame) nor smoke (it produces water when it burns in air) unless, for example, sodium salt is added or dust particles are entrained and burned along with the combustible mixture. Compared to hydrocarbon combustion, hydrogen flames radiate significantly less heat and so human physical perception of this heat does not occur until direct contact is made with the flame. A hydrogen fire may therefore remain undetected and propagate despite human direct monitoring in areas where hydrogen can leak, spill or accumulate and form potentially combustible mixtures. Hydrogen fire detectors ensure that immediate action is taken in these situations. Hydrogen fire detectors can be either stationary for continuous monitoring of remote operations or portable for field operations.

Expected Performance of Hydrogen Fire Detectors.

For an efficient and reliable use, a hydrogen fire detector should fulfil the following criteria:

  • it should detect every true incident and avoid false ones
  • it should be specific and pick up hydrogen fire signals among many different signals that become even more numerous when detector sensitivity is increased
  • it should have a limited response time especially if it triggers a safety action
  • if possible it should have an automatic periodic check up

In terms of performance, its ability

  • to detect a hydrogen flame at a sufficient distance, and
  • to detect small flames

should be considered when installing a hydrogen flame detector.

For instance, NASA (NASA:NSS:1740:16:1997) indicates that a fire detection system should at least be capable of detecting, at a minimum distance of 4.6 m, the flame from the combustion of 5.0 l/min of gaseous hydrogen at NTP flowing through a 1.6 mm orifice to produce a 20 cm high flame.

Possible Means to Detect a Hydrogen Fire

A hydrogen fire can potentually be detected by using thermal detectors (such as rate-of-temperature-rise or overheat detectors) to pick up radiative, convective or conductive heat. These reliable detectors of various types are suitable for hydrogen fire detection as long as they are located very near to where the fire breaks out. Other common fire detector types such as those with ionising cells, are not appropriate for detecting hydrogen fires.

Though hydrogen fires tend to emit radiation over a broad range and are not characterised by extreme peaks, hydrogen fire detectors can also rely on UV and IR light detection. Beside the radiation itself, hydrogen flames can be indirectly visible by their strong heat effect and turbulence - “heat ripples” - of the surrounding atmosphere. Optical flame detectors detect specific spectral radiation emitted during the combustion process by the various chemical species (ions, radicals, molecules) that are either intermediates or final products of combustion. Chemical species emit radiation at wavelengths characteristic to the particular species.

  • The hydroxyl radical (OH) and water are the main emitting chemical species in the hydrogen combustion process. These species emit radiation at specific spectral bands, according to their electronic structure and the typical energy (translation, vibration, rotation) of the process.
  • OH (being an active intermediate with an available free electron) emits strongly in the UV spectral band at the 0.306 & 0.282 µm peak and additional weaker emission peaks at 0.180 - 0.240 µm. It also emits infrared energy in the near IR band (vibration and rotation of the molecule) with several peaks within the 1-3 µm spectral band.
  • H2O emits mainly in the near IR band (vibration and rotation) with strong peaks at 2.7; 1.9 & 1.4 µm, ranging from the highest to the lowest intensity.

These detection techniques assume that no interfering shield is placed between the flame and the UV / IR detector. Though optical techniques are available to pick up these various wavelengths, the main challenge consists in ditinguishing hydrogen flame signals from other potential sources that emit signals with a similar frequency and intensity.

UV Detectors

UV systems are preferable to IR systems because they are extremely sensitive. In addition, the probability of encountering an interfering signal is lower as long as UV detectors are shaded from sun light. Drawbacks are on the one hand the cost and on the other hand the reduced efficiency with liquid hydrogen flames, as fog blocks UV rays. The same applies whenever fog is present. False alarms can be triggered by random UV sources such as lightning or arc welding.

The ability of the detector to discriminate sunlight-induced UV radiation from hydrogen flames to avoid false alarms is the main challenge. Various techniques can be applied:

  • Optical filter: Cutting any wavelength above 0.29 µm to retain only those wavelengths attributable to a hydrogen fire accident. Even on a sunny day, the atmosphere filters sunray wavelengths below the proposed threshold of 0.29 µm. As a drawback, this solution also cuts down nearly 2/3 of the UV band and therefore decreases the detector acuity.
  • Concomitant cells: Two separate cells are used to monitor the same zone. One of the cells mostly analyses the visible spectrum where the sunlight signal is predominant in comparison with the hydrogen flame signal whereas the other cell focuses on the UV band. The UV signal from the UV cell is only taken into account if it diverges from the signal of the concomitant cell.
  • Signal characteristics: The flickering behaviour of a flame generates characteristically modulated AC components of the UV signal, which can be analyzed. This technique may not be compatible with a fast response need.
  • Operational area adaption: if parasitic signals are known to be minor, a positive signal may be assumed whenever a given threshold is reached.
IR Detectors
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Fig. 13: Atmospheric IR Transmission

It was mentioned above that fog may hinder UV transmission to the sensor cell. IR detectors are not affected by this issue. In addition, hydrogen flames emit sufficient IR for their detection with IR sensors. The main challenge remains the same as before: i.e. discrimination between IR related to a hydrogen fire and IR from the sun, any light sources or any hot materials. IR sources powered with alternating electric currents can be filtered due to their own 100 Hz modulated signal. However, neither hot bodies nor sunlight display a modulated signal that can be picked up and filtered. A solution consists in focussing on the 1.7 µm wavelength that corresponds to a peak emission of steam, bearing in mind that the atmosphere absorbs sun-emitted IR wavelengths between 1.81 & 1.88 µm and between 2.55 & 2.9 µm. The 1.7 µm wavelength is the only one of the three IR peaks mentioned above that falls within the IR filtering spectrum of the atmosphere. Figure 13 is taken from (NASA:NSS:1740:16:1997) and clarifies this situation comparing atmospheric transmission with hydrogen-air flame emission in the IR range.

THERMAL Detectors

Thermal detectors, e.g. temperature sensors, detect the heat of the flame. Such detectors need to be located very close to or at the site of a fire and are not specific to hydrogen flames.


Imaging systems mainly are available in the thermal IR region and do not provide continuous monitoring with alarm capability. A trained operator is required to interpret whether the image being viewed is a flame. UV imaging systems require special optics and are very expensive.

Brooms, Aerosol,...

Rescue services or maintenance teams use brooms as a simple method to locate small fires. The intent is that dry corn straw or sage grass broom easily ignites when it comes into contact with a hydrogen flame. Also non flammable objects or dust particles in a hydrogen flame cause the flame to emit radiation in the visible spectrum. Dirt and dry fire extinguishers have been used for this purpose, but extreme caution needs to be taken with such practice due to the required proximity to the flame.

However, it must be underlined that it is still a challenge for surveillance sensor developers to distinguish hydrogen-related signals from parasitic ones. To prevent false alarms and related automatic actions, in critical cases it si still an option to apply human analysis and actions in preference automatic ones.


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