Gas detector

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Portable gas detector

A gas detector is a device which detects the presence of various gases within an area, often as part of a safety system. This type of equipment is used to detect a gas leak and interface with a control system so a process can be automatically shut down. A gas detector can also sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave the area. This type of device is important because there are many gases that can be harmful to organic life, such as humans or animals.

Gas detectors can be used to detect combustible, flammable and toxic gases, and oxygen depletion. This type of device is used widely in industry and can be found in a variety of locations such as on oil rigs, to monitor manufacture processes and emerging technologies such as photovoltaic. They may also be used in firefighting.

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 sensors, ultrasonic sensors, electrochemical gas sensors, and semiconductor sensors. More recently, infrared imaging sensors have come into use. All of these sensors are used for a wide range of applications, and can be found in industrial plants, refineries, wastewater treatment facilities, vehicles, and around the home.

History[edit]

Gas leak detection methods became a concern after the effects of harmful gases on human health were discovered. Before modern electronic sensors, early detection methods relied on less precise detectors. Through the 19th and early 20th centuries, coal miners would bring canaries down to the tunnels with them as an early detection system against life threatening gases such as carbon dioxide, carbon monoxide and methane. The canary, normally a very songful bird, would stop singing and eventually die if not removed from the presence of these gases, signaling the miners to exit the mine quickly.

Before the development of electronic household carbon monoxide detectors in the 1980s and 1990s, carbon monoxide presence was detected with a chemically infused paper that turned brown when exposed to the gas. Since then, many electronic technologies and devices have been developed to detect, monitor, and alert the leakage of a wide array of gases.

As the cost and performance of electronic gas sensors improved, they have been incorporated into a wider range of systems. Their use in automobiles was initially for engine emissions control, but now gas sensors may also be used to insure passenger comfort and safety. Carbon dioxide sensors are being installed into buildings as part of demand-controlled ventilation systems. Sophisticated gas sensor systems are being researched for use in medical diagnostic, monitoring, and treatment systems, well beyond their initial use in operating rooms. Gas monitors and alarms for carbon monoxide and other harmful gases are increasingly available for office and domestic use, and are becoming legally required in some jurisdictions.

Originally, detectors were produced to detect a single gas, but modern units may detect several toxic or combustible gases, or even a combination of both types.[1]Newer gas analyzers can break up the component signals from a complex aroma to identify several gases simultaneously.[2]

Types[edit]

Gas detectors can be classified according to the operation mechanism (semiconductors, oxidation, catalytic, infrared, etc.). Gas detectors come packaged into two main form factors: portable devices and fixed gas detectors.

Portable detectors are used to monitor the atmosphere around personnel, and are worn on clothing or on a belt/harness. These gas detectors are usually battery operated. They transmit warnings via a series of audible and visible signals such as alarms and flashing lights, when dangerous levels of gas vapors are detected.

Fixed type gas detectors may be used for detection of one or more gas types. Fixed type detectors are generally mounted near the process area of a plant or control room, or an area to be protected, such as a residential bedroom. Generally, industrial sensors are installed on fixed type mild steel structures, and a cable connects the detectors to a SCADA system for continuous monitoring. A tripping interlock can be activated for an emergency situation.

Electrochemical[edit]

Electrochemical gas detectors work by allowing gases to diffuse through a porous membrane to an electrode where it is either chemically oxidized or reduced. The amount of current produced is determined by how much of the gas is oxidized at the electrode,[3] indicating the concentration of the gas. Manufactures can customize electrochemical gas detectors by changing the porous barrier to allow for the detection of a certain gas concentration range. Also, since the diffusion barrier is a physical/mechanical barrier, the detector tended to be more stable and reliable over the sensor's duration and thus required less maintenance than other early detector technologies.

However, the sensors themselves are subject to corrosive elements or chemical contamination, and may last only 1–2 years before a replacement is required.[4] Electrochemical gas detectors are used in a wide variety of environments such as refineries, gas turbines, chemical plants, underground gas storage facilities, and more.

Infrared point[edit]

Main article: Infrared point sensor

Infrared (IR) point sensors use radiation passing through a known volume of gas; energy from the sensor beam is absorbed at certain wavelengths, depending on the properties of the specific gas. For example, carbon monoxide absorbs wavelengths of about 4.2-4.5 μm.[5] (This wavelength is approximately a factor of 10 larger than that of visible light, which ranges from .39 μm to .75 μm for most people). The energy in this wavelength is compared to a wavelength outside of the absorption range; the difference in energy between these two wavelengths is proportional to the concentration of gas present.[6]

This type of sensor is advantageous because it does not have to be placed into the gas itself in order to detect it, but can be used for remote sensing. Infrared point sensors can be used to detect hydrocarbons,[7] and other infrared active gases such as water vapor and carbon dioxide. IR sensors are commonly found in wastewater treatment facilities, refineries, gas turbines, chemical plants, and other facilities where flammable gases are present and the possibility of an explosion exists. The remote sensing capability allows large volumes of space to be monitored.

Engine emissions are another area where IR sensors are being researched for use. The sensor would be able to detect high levels of carbon monoxide or other abnormal gases in vehicle exhaust, and even be integrated with vehicle electronic systems to notify drivers.[8]

Infrared imaging[edit]

Main article: Thermographic camera

Infrared imaging sensors include both active and passive systems. For active sensing, IR imaging sensors typically scan a laser across the field of view of a scene and look for backscattered light at the absorption line wavelength of a specific target gas. Passive IR imaging sensors, on the other hand, measure spectral changes at each pixel in an image and look for specific spectral signatures which indicate the presence of target gases.[9] The types of compounds which can be imaged are the same as those which can be detected with infrared point detectors, but the images may be helpful in identifying the source of a gas.

Semiconductor[edit]

Semiconductor sensors detect gases by a chemical reaction that takes place when the gas comes in direct contact with the sensor. Tin dioxide is the most common material used in semiconductor sensors,[10] and the electrical resistance in the sensor is decreased when it comes in contact with the monitored gas. The resistance of the tin dioxide is typically around 50 kΩ in air but can drop to around 3.5 kΩ in the presence of 1% methane.[11] This change in resistance is used to calculate the gas concentration. Semiconductor sensors are commonly used to detect hydrogen, oxygen, alcohol vapor, and harmful gases such as carbon monoxide.[12] One of the most common uses for semiconductor sensors is in carbon monoxide sensors. They are also used in breathalyzers.[13] Because the sensor must come in contact with the gas in order to detect it, semiconductor sensors work over a smaller distance than infrared point or ultrasonic detectors.

Ultrasonic[edit]

Ultrasonic gas detectors use acoustic sensors to detect changes in the background noise of its environment. Since most high-pressure gas leaks generate sound in the ultrasonic range of 25 kHz to 10 MHz, the sensors are able to easily distinguish these frequencies from background acoustic noise which occurs in the audible range of 20 Hz to 20 kHz.[14] The ultrasonic gas leak detector then produces an alarm when there is an ultrasonic deviation from the normal condition of background noise. Despite the fact that ultrasonic gas leak detectors cannot measure gas concentration, the device is still able to determine the leak rate of an escaping gas because the ultrasonic sound level depends on the gas pressure and size of the leak.[15] Ultrasonic gas detectors are mainly used for remote sensing in outdoor environments where weather conditions can easily dissipate escaping gas before allowing it to reach gas leak detectors that require contact with the gas in order to detect it and sound an alarm. These detectors are commonly found on offshore and onshore oil/gas platforms, gas compressor and metering stations, gas turbine power plants, and other facilities that house a lot of outdoor pipeline.

Holographic[edit]

Holographic gas sensors use light reflection to detect changes in a polymer film matrix containing a hologram. Since holograms reflect light at certain wavelengths, a change in their composition can generate a colorful reflection indicating the presence of a gas molecule.[16] However, holographic sensors require illumination sources such as white light or lasers, and an observer or CCD detector.

Calibration[edit]

All gas detectors must be calibrated on a schedule. Of the two form factors of gas detectors, portables must be calibrated more frequently due to the regular changes in environment they experience. A typical calibration schedule for a fixed system may be quarterly, bi-annually or even annually with some of the more robust units. A typical calibration schedule for a portable gas detector is a daily "bump test" accompanied by a monthly calibration.[17] Almost every portable gas detector requires a specific calibration gas which is available from the manufacturer. In the US, the Occupational Safety and Health Administration (OSHA) may also set minimum standards for periodic recalibration.[citation needed]

Challenge (bump) test[edit]

Because a gas detector is used for employee/worker safety, it is very important to make sure it is operating to manufacturer's specifications. Australian standards specify that a person operating any gas detector is strongly advised to check the gas detector's performance each day, and that it is maintained and used in accordance with the manufacturers instructions and warnings.[18]

A challenge test should consist of exposing the gas detector to a known concentration of gas to ensure that the gas detector will respond, and that both the audible and visual alarms activate. It is also important inspect the gas detector for any accidental or deliberate damage by checking that the housing and screws are all intact to prevent any liquid ingress, and that the filter is clean, all of which can affect the functionality of the gas detector. The basic calibration or challenge test kit will consist of: Calibration Gas / Regulator / Calibration Cap and hose (generally supplied with the gas detector at time of purchase) and a case for storage and transport. Because 1 in every 2,500 untested instruments will fail to respond to a dangerous concentration of gas, many large businesses will utilize an automated test/calibration station for use to bump test and calibrate their gas detectors daily.[19]

Oxygen concentration[edit]

Oxygen deficiency gas monitors are used for employee and workforce safety. Cryogenic substances such as liquid nitrogen (LN2), liquid helium (He), and liquid argon (Ar) are inert and can displace oxygen (O2) in a confined space if a leak is present. A rapid decrease of oxygen can provide a very dangerous environment for employees, who may not notice this problem before they suddenly lose consciousness. With this in mind, an oxygen gas monitor is important to have when cryogenics are present. Laboratories, MRI rooms, pharmaceutical, semiconductor, and cryogenic suppliers are typical users of oxygen monitors.

Oxygen fraction in a breathing gas is measured by electro-galvanic fuel cell sensors. They may be used stand-alone, for example to determine the proportion of oxygen in a nitrox mixture used in scuba diving,[20] or as part of feedback loop which maintains a constant partial pressure of oxygen in a rebreather.[21]

Hydrocarbons and VOCs[edit]

Detection of hydrocarbons can be based on the mixing properties of gaseous hydrocarbons – or other volatile organic compounds (VOCs) – and the sensing material incorporated in the sensor. The selectivity and sensitivity depends on the molecular structure of the VOC and the concentration; however it is difficult to design a selective sensor for a single VOC. Many VOC sensors detect using a fuel-cell method.

VOCs in the environment or certain atmospheres can be detected based on different principles and interactions between the organic compounds and the sensor components. There are electronic devices that can detect ppm concentrations despite not being particularly selective. Others can predict with reasonable accuracy the molecular structure of the volatile organic compounds in the environment or enclosed atmospheres[22] and could be used as accurate monitors of the Chemical Fingerprint and further as health monitoring devices.

Solid-phase microextraction(SPME)techniques are used to collect VOCs at low concentrations for analysis.[23]

Considerations for detection of hydrocarbon gases /risk control[edit]

  • Methane is lighter than air (possibility of accumulation under roofs)
  • Ethane is slightly heavier than air (possibility of pooling at ground levels / pits)
  • Propane is heavier than air (possibility of pooling at ground levels / pits)
  • Butane is heavier than air (possibility of pooling at ground levels / pits)

Combustible[edit]

Other[edit]

Household safety[edit]

There are several different sensors that can be installed to detect hazardous gases in a residence. Carbon monoxide is a very dangerous, but odorless, colorless gas, making it difficult for humans to detect. Carbon monoxide detectors can be purchased for around US$20–60. Many local jurisdictions in the United States now require installation of carbon monoxide detectors in addition to smoke detectors in residences.

Handheld flammable gas detectors can be used to trace leaks from natural gas lines, propane tanks, butane tanks, or any other combustible gas. These sensors can be purchased for US$35–100.

Industrial applications[edit]

Research[edit]

The European Community has supported research called the MINIGAS project that was coordinated by VTT Technical Research Center of Finland.[24] This research project aims to develop new types of photonics-based gas sensors, and to support the creation of smaller instruments with equal or higher speed and sensitivity than conventional laboratory-grade gas detectors.[24]

Manufacturers[edit]

See also[edit]

References[edit]

  1. ^ How Gas Detectors Work
  2. ^ Wali, Russeen (2012). "An electronic nose to differentiate aromatic flowers using a real-time information-rich piezoelectric resonance measurement". Procedia Chemistry. doi:10.1016/j.proche.2012.10.146. 
  3. ^ Detcon, http://www.detcon.com/electrochemical01.htm
  4. ^ United States Patent 4141800: Electrochemical gas detector and method of using same, http://www.freepatentsonline.com/4141800.html
  5. ^ Muda, R., 2009
  6. ^ Muda, R., 2009
  7. ^ International Society of Automation, http://www.isa.org/Template.cfm?Section=Communities&template=/TaggedPage/DetailDisplay.cfm&ContentID=23377
  8. ^ Muda, R., 2009
  9. ^ Edward Naranjo and Shankar Baliga and Philippe Bernascolle, "IR gas imaging in an industrial setting," in Thermosense XXXII, Proc. SPIE 76610K (2010). http://dx.doi.org/10.1117/12.850137
  10. ^ Figaro Sensor, http://www.figarosensor.com/products/general.pdf
  11. ^ Vitz, E., 1995
  12. ^ General Monitors, http://www.generalmonitors.com/downloads/literature/combustible/IR2100_DATA.PDF
  13. ^ Vitz, E., 1995
  14. ^ Naranjo, E., http://www.gmigasandflame.com/article_october2007.html
  15. ^ Naranjo, E., http://www.gmigasandflame.com/article_october2007.html
  16. ^ http://pubs.acs.org/doi/abs/10.1021/la102693m
  17. ^ Moore, James. "Calibration: Who Needs It?". Occupational Health and Safety Magazine. 
  18. ^ Colhoun, Jacquie. "Who is responsible for bump/challenge testing your gas detector". 
  19. ^ Bump test saves lives
  20. ^ Lang, M.A. (2001). DAN Nitrox Workshop Proceedings. Durham, NC: Divers Alert Network. p. 197. Retrieved 2009-03-20. 
  21. ^ Goble, Steve (2003). "Rebreathers". South Pacific Underwater Medicine Society Journal 33 (2): 98–102. Retrieved 2009-03-20. 
  22. ^ MartíNez-Hurtado, J. L.; Davidson, C. A. B.; Blyth, J.; Lowe, C. R. (2010). "Holographic Detection of Hydrocarbon Gases and Other Volatile Organic Compounds". Langmuir 26 (19): 15694–9. doi:10.1021/la102693m. PMID 20836549. 
  23. ^ Lattuati-Derieux, Agnès; Bonnassies-Termes, Sylvette; Lavédrine, Bertrand (2004). "Identification of volatile organic compounds emitted by a naturally aged book using solid-phase microextraction/gas chromatography/mass spectrometry". Journal of Chromatography A 1026 (1–2): 9–18. doi:10.1016/j.chroma.2003.11.069. PMID 14870711. 
  24. ^ a b Matthew Peach, Optics.org. "Photonics-based MINIGAS project yields better gas detectors." Jan 29, 2013. Retrieved Feb 15, 2013.