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A hydrogen sensor is a gas detector that detects the presence of hydrogen. They contain micro-fabricated point-contact hydrogen sensors and are used to locate leaks. They are considered low-cost, compact, durable, and easy to maintain as compared to conventional gas detecting instruments.^[1]

Key Issues

There are five key issues with hydrogen detectors:^[2]

Reliability: Functionality should be easily verifiable.
Performance: Detection 0.5% hydrogen in air or better
Response time < 1 second.
Lifetime: At least the time between scheduled maintenance.
Cost: Goal is $5 per sensor and $30 per controller.

Additional requirements

Measurement range coverage of 0.1%–10.0% concentration^[3]
Operation in temperatures of -30°C to 80°C
Accuracy within 5% of full scale
Function in an ambient air gas environment within a 10%–98% relative humidity range
Resistance to hydrocarbon and other interference.
Lifetime greater than 10 years

Types of microsensors

There are various types of hydrogen microsensors, which use different mechanisms to detect the gas. Palladium is used in many of these, because it selectively absorbs hydrogen gas and forms the compound palladium hydride.^[4] Palladium-based sensors have a strong temperature dependence which makes their response time too large at very low temperatures.^[5] Palladium sensors have to be protected against carbon monoxide, sulfur dioxide and hydrogen sulfide.

Optical fibre hydrogen sensors

Several types of optical fibre surface plasmon resonance (SPR) sensor are used for the point-contact detection of hydrogen:

Fiber Bragg grating coated with a palladium layer - Detects the hydrogen by metal hindrance.
Micromirror - With a palladium thin layer at the cleaved end, detecting changes in the backreflected light.
Tapered fibre coated with palladium - Hydrogen changes the refractive index of the palladium, and consequently the amount of losses in the evanescent wave.

Other types of hydrogen sensors

Electrochemical hydrogen sensor - low (ppm) levels of hydrogen gas can be sensed using electrochemical sensors which comprise an array of electrodes packaged so as to be surrounded by a conductive electrolyte and gas ingress controlled with a diffusion limited capillary.
MEMS hydrogen sensor - The combination of nanotechnology and microelectromechanical systems (MEMS) technology allows the production of a hydrogen microsensor that functions properly at room temperature. One type of MEMS-based hydrogen sensor is coated with a film consisting of nanostructured indium oxide (In₂O₃) and tin oxide (SnO₂).^[6] A typical configuration for mechanical Pd-based hydrogen sensors is the usage of a free-standing cantilever that is coated with Pd.^[7]^[8] In the presence of H₂, the Pd layer expands and thereby induces a stress that causes the cantilever to bend. Pd-coated nano-mechanical resonators have also been reported in literature, relying on the stress-induced mechanical resonance frequency shift caused by the presence of H₂ gas. In this case, the response speed was enhanced through the use of a very thin layer of Pd (20 nm). Moderate heating was presented as a solution to the response impairment observed in humid conditions.^[9]
Thin film sensor - A palladium thin film sensor is based on an opposing property that depends on the nanoscale structures within the thin film. In the thin film, nanosized palladium particles swell when the hydride is formed, and in the process of expanding, some of them form new electrical connections with their neighbors. The resistance decreases because of the increased number of conducting pathways.^[2]^[10]
Thick film sensor - Thick film hydrogen sensors rely on the fact that palladium hydride's electrical resistance is greater than the metal's resistance. The absorption of hydrogen causes a measurable increase in electrical resistance.
Chemochromic hydrogen sensor - Reversible and irreversible chemochromic hydrogen sensors, a smart pigment paint that visually identifies hydrogen leaks by a change in color. The sensor is also available as tape.^[11]^[12]
Diode based Schottky sensor - A Schottky diode-based hydrogen gas sensor employs a palladium-alloy gate. Hydrogen can be selectively absorbed in the gate, lowering the Schottky energy barrier.^[13] A Pd/InGaP metal-semiconductor (MS) Schottky diode can detect a concentration of 15 parts per million (ppm) H₂ in air.^[14] Silicon carbide semiconductor or silicon substrates are used.
Metallic La-Mg2-Ni which is electrical conductive, absorbs hydrogen near ambient conditions, forming the nonmetallic hydride LaMg2NiH7 an insulator.^[15]

Sensors are typically calibrated at the manufacturing factory and are valid for the service life of the unit.

Enhancement

Siloxane enhances the sensitivity and reaction time of hydrogen sensors.^[4] Detection of hydrogen levels as low as 25 ppm can be achieved; far below hydrogen's lower explosive limit of around 40,000 ppm.

References

^ Qu, Xi Dong (2005). "MOS Capacitor Sensor Array for Hydrogen Gas Measurement" (PDF). Simon Fraser University. Retrieved 2008-10-21.
^ ^a ^b Pitts, Ronald. "Interfacial Stability Of Thin Film Hydrogen Sensors" (PDF). National Renewable Energy Laboratory. Retrieved 2008-10-21. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ NREL-Hydrogen Sensor Testing oct 2008
^ ^a ^b "Hydrogen sensors are faster, more sensitive". Innovations Report. 2005-05-31. Retrieved 2008-10-21.
^ Guemes, J. Alfredo (May 2005). "Comparison of three types of fibre optic hydrogen sensors within the frame of CryoFOS project". 17th International Conference on Optical Fibre Sensors. 5855: 1000. Bibcode:2005SPIE.5855.1000G. doi:10.1117/12.623731. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Alverio, Gustavo. "A Nanoparticle-based Hydrogen Microsensor". University of Central Florida. Retrieved 2008-10-21.
^ Baselt, D.R. "Design and performance of a microcantilever-based hydrogen sensor". Sensors and Actuators B. Retrieved 2013-02-26.
^ Okuyama, Sumio. "Hydrogen Gas Sensing Using a Pd-Coated Cantilever". Japanese Journal of Applied Physics. Retrieved 2013-02-26.
^ Henriksson, Jonas. "Ultra-low power hydrogen sensing based on a palladium-coated nanomechanical beam resonator". Nanoscale Journal. Retrieved 2013-02-26.
^ "Hydrogen Detection Systems". Makel Engineering. Retrieved 2008-10-21.
^ Chemochromic hydrogen sensor
^ FSEC Chemochromic Sensor
^ Schottky energy barrier
^ Silicon carbide^{[dead link]}
^ Hydrogenation-induced insulating state in the intermetallic compound LaMg2Ni.

External links

[1] Qu, Xi Dong (2005). "MOS Capacitor Sensor Array for Hydrogen Gas Measurement" (PDF). Simon Fraser University. Retrieved 2008-10-21.

[Pitts-2] Pitts, Ronald. "Interfacial Stability Of Thin Film Hydrogen Sensors" (PDF). National Renewable Energy Laboratory. Retrieved 2008-10-21. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[3] NREL-Hydrogen Sensor Testing oct 2008

[innovations-4] "Hydrogen sensors are faster, more sensitive". Innovations Report. 2005-05-31. Retrieved 2008-10-21.

[5] Guemes, J. Alfredo (May 2005). "Comparison of three types of fibre optic hydrogen sensors within the frame of CryoFOS project". 17th International Conference on Optical Fibre Sensors. 5855: 1000. Bibcode:2005SPIE.5855.1000G. doi:10.1117/12.623731. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[6] Alverio, Gustavo. "A Nanoparticle-based Hydrogen Microsensor". University of Central Florida. Retrieved 2008-10-21.

[7] Baselt, D.R. "Design and performance of a microcantilever-based hydrogen sensor". Sensors and Actuators B. Retrieved 2013-02-26.

[8] Okuyama, Sumio. "Hydrogen Gas Sensing Using a Pd-Coated Cantilever". Japanese Journal of Applied Physics. Retrieved 2013-02-26.

[9] Henriksson, Jonas. "Ultra-low power hydrogen sensing based on a palladium-coated nanomechanical beam resonator". Nanoscale Journal. Retrieved 2013-02-26.

[10] "Hydrogen Detection Systems". Makel Engineering. Retrieved 2008-10-21.

[11] Chemochromic hydrogen sensor

[12] FSEC Chemochromic Sensor

[13] Schottky energy barrier

[14] Silicon carbide^{[dead link]}

[15] Hydrogenation-induced insulating state in the intermetallic compound LaMg2Ni.

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 ===Other types of hydrogen sensors===
 *Electrochemical hydrogen sensor - low (ppm) levels of hydrogen gas can be sensed using electrochemical sensors which comprise an array of electrodes packaged so as to be surrounded by a conductive electrolyte and gas ingress controlled with a diffusion limited capillary.
-*MEMS hydrogen sensor - The combination of [[nanotechnology]] and [[microelectromechanical systems]] (MEMS) technology allows the production of a hydrogen microsensor that functions properly at room temperature. One type of MEMS-based hydrogen sensor is coated with a film consisting of nanostructured [[indium(III) oxide|indium oxide]] (In<sub>2</sub>O<sub>3</sub>) and [[tin oxide]] (SnO<sub>2</sub>).<ref>{{cite web|url=http://nsfreunano.research.ucf.edu/YearBook/Titans/2004/alvero.html|title=A Nanoparticle-based Hydrogen Microsensor|last=Alverio|first=Gustavo|publisher=University of Central Florida|accessdate=2008-10-21}}</ref> A typical configuration for mechanical Pd-based hydrogen sensors is the usage of a free-standing cantilever that is coated with Pd.<ref>{{cite web|url=http://www.sciencedirect.com/science/article/pii/S0925400502003155|title=Design and performance of a microcantilever-based hydrogen sensor|last=Baselt|first=D.R.|publisher=Sensors and Actuators B|accessdate=2013-02-26}}</ref><ref>{{cite web|url=http://jjap.jsap.jp/link?JJAP/39/3584/|title=Hydrogen Gas Sensing Using a Pd-Coated Cantilever|last=Okuyama|first=Sumio|publisher=Japanese Journal of Applied Physics|accessdate=2013-02-26}}</ref> In the presence of H<sub>2</sub>, the Pd layer expands and thereby induces a stress that causes the cantilever to bend. Pd-coated nano-mechanical resonators have also been reported in literature, relying on the stress-induced mechanical resonance frequency shift caused by the presence of H<sub>2</sub> gas. In this case, the response speed was enhanced through the use of a very thin layer of Pd (20&nbsp;nm). Moderate heating was presented as a solution to the response impairment observed in humid conditions.<ref>{{cite web|url=http://goo.gl/oA9m4|title=Ultra-low power hydrogen sensing based on a palladium-coated nanomechanical beam resonator|last=Henriksson|first=Jonas|publisher=Nanoscale Journal|accessdate=2013-02-26}}</ref>
+*MEMS hydrogen sensor - The combination of [[nanotechnology]] and [[microelectromechanical systems]] (MEMS) technology allows the production of a hydrogen microsensor that functions properly at room temperature. One type of MEMS-based hydrogen sensor is coated with a film consisting of nanostructured [[indium(III) oxide|indium oxide]] (In<sub>2</sub>O<sub>3</sub>) and [[tin oxide]] (SnO<sub>2</sub>).<ref>{{cite web|url=http://nsfreunano.research.ucf.edu/YearBook/Titans/2004/alvero.html|title=A Nanoparticle-based Hydrogen Microsensor|last=Alverio|first=Gustavo|publisher=University of Central Florida|accessdate=2008-10-21}}</ref> A typical configuration for mechanical Pd-based hydrogen sensors is the usage of a free-standing cantilever that is coated with Pd.<ref>{{cite web|url=http://www.sciencedirect.com/science/article/pii/S0925400502003155|title=Design and performance of a microcantilever-based hydrogen sensor|last=Baselt|first=D.R.|publisher=Sensors and Actuators B|accessdate=2013-02-26}}</ref><ref>{{cite web|url=http://jjap.jsap.jp/link?JJAP/39/3584/|title=Hydrogen Gas Sensing Using a Pd-Coated Cantilever|last=Okuyama|first=Sumio|publisher=Japanese Journal of Applied Physics|accessdate=2013-02-26}}</ref> In the presence of H<sub>2</sub>, the Pd layer expands and thereby induces a stress that causes the cantilever to bend. Pd-coated nano-mechanical resonators have also been reported in literature, relying on the stress-induced mechanical resonance frequency shift caused by the presence of H<sub>2</sub> gas. In this case, the response speed was enhanced through the use of a very thin layer of Pd (20&nbsp;nm). Moderate heating was presented as a solution to the response impairment observed in humid conditions.<ref>{{cite web|url=http://pubs.rsc.org/en/Content/ArticleLanding/2012/NR/c2nr30639e#!divAbstract|title=Ultra-low power hydrogen sensing based on a palladium-coated nanomechanical beam resonator|last=Henriksson|first=Jonas|publisher=Nanoscale Journal|accessdate=2013-02-26}}</ref>
 *Thin film sensor - <!--Is this supposed to be a subcategory of the above? If so, the text should make it clear how they are related--> A palladium [[thin film]] sensor is based on an opposing property<!--???--> that depends on the nanoscale structures within the thin film. In the thin film, nanosized palladium particles swell when the hydride is formed, and in the process of expanding, some of them form new electrical connections with their neighbors. The resistance decreases because of the increased number of conducting pathways.<ref name="Pitts" /><ref>{{cite web|url=http://www.makelengineering.com/dir/Technologies/Hydrogen%20Detection/Hydrogen.htm|title=Hydrogen Detection Systems|publisher=Makel Engineering|accessdate=2008-10-21}}</ref>
 *Thick film sensor - Thick film hydrogen sensors rely on the fact that palladium hydride's [[electrical resistance]] is greater than the metal's resistance. The absorption of hydrogen causes a measurable increase in electrical resistance.