Gas detectors can be classified according to the operation mechanism (
semiconductor, oxidation, catalytic, photoionization, 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 either hand-held or worn on clothing or on a belt/harness. These gas detectors are usually battery operated. They transmit warnings via 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
supervisory control and data acquisition (SCADA) system for continuous monitoring. A tripping interlock can be activated for an emergency situation.
Electrochemical 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, 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 detectors tend to be more stable and reliable over the sensor's duration and thus required less maintenance than other early detector technologies. However, the sensors are subject to corrosive elements or chemical contamination and may last only 1–2 years before a replacement is required. Electrochemical gas detectors are used in a wide variety of environments such as refineries, gas turbines, chemical plants, underground gas storage facilities, and more.
Catalytic bead Catalytic bead (
pellistor) sensors are commonly used to measure combustible gases that present an explosion hazard when concentrations are between the
lower explosion limit (LEL) and upper explosion limit (UEL). Active and reference beads containing platinum wire coils are situated on opposite arms of a Wheatstone bridge circuit and electrically heated, up to a few hundred degrees C. The active bead contains a catalyst that allows combustible compounds to oxidize, thereby heating the bead even further and changing its electrical resistance. The resulting voltage difference between the active and passive beads is proportional to the concentration of all combustible gases and vapors present. The sampled gas enters the sensor through a sintered metal frit, which provides a barrier to prevent an explosion when the instrument is carried into an atmosphere containing combustible gases. Pellistors measure essentially all combustible gases, but they are more sensitive to smaller molecules that diffuse through the sinter more quickly. The measurable concentration ranges are typically from a few hundred ppm to a few volume percent. Such sensors are inexpensive and robust, but require a minimum of a few percent oxygen in the atmosphere to be tested and they can be poisoned or inhibited by compounds such as silicones, mineral acids, chlorinated organic compounds, and sulfur compounds.
Photoionization Photoionization detectors (PIDs) use a high-
photon-energy UV lamp to ionize chemicals in the sampled gas. If the compound has an ionization energy below that of the lamp photons, an electron will be ejected, and the resulting current is proportional to the concentration of the compound. Common lamp photon energies include 10.0
eV, 10.6 eV and 11.7 eV; the standard 10.6 eV lamp lasts for years, while the 11.7 eV lamp typically last only a few months and is used only when no other option is available. A broad range of compounds can be detected at levels ranging from a few parts per billion (ppb) to several thousand parts per million (ppm). Detectable compound classes in order of decreasing sensitivity include: aromatics and alkyl iodides; olefins, sulfur compounds, amines, ketones, ethers, alkyl bromides and silicate esters; organic esters, alcohols, aldehydes and alkanes; hydrogen sulfide, ammonia, phosphine and organic acids. There is no response to standard components of air or to mineral acids. Major advantages of PIDs are their excellent sensitivity and simplicity of use; the main limitation is that measurements are not compound-specific. Recently PIDs with pre-filter tubes have been introduced that enhance the specificity for such compounds as
benzene or
butadiene. Fixed, hand-held and miniature clothing-clipped PIDs are widely used for industrial hygiene,
hazmat, and
environmental monitoring.
Infrared point 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. 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. and other infrared active gases such as
water vapor and
carbon dioxide. IR sensors are commonly found in waste-water 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. The sensor would detect high levels of carbon monoxide or other abnormal gases in vehicle exhaust and even be integrated with vehicle electronic systems to notify drivers. The types of compounds that can be imaged are the same as those that can be detected with infrared point detectors, but the images may be helpful in identifying the source of a gas.
Semiconductor Semiconductor gas sensors, most commonly
metal–oxide–semiconductor (MOS) sensors, detect gases by a chemical reaction that takes place when the gas comes in direct contact with the surface of the sensing element, which is usually heated to temperatures between 200 °C and 500 °C.
Tin dioxide is the most common material used in semiconductor gas sensors, and the electrical resistance in the sensor is decreased when it comes in contact with the monitored gas. The resistance of the tin dioxide layer, typically in the range of 10 to 500 kΩ in air, can drop to a small fraction of this value in the presence of a reducing gas. This change in resistance (often together with signals of other sensors, e.g. humidity sensor values, for cross-sensitivity compensation) is used to calculate the gas concentration. Semiconductor gas sensors are commonly used to detect hydrogen, alcohol vapor, and harmful gases such as carbon monoxide. One of the most common uses for semiconductor gas sensors is in carbon monoxide sensors. They are also used in
breathalyzers. Such interferences can be reduced using algorithmic optimizations and temperature-cycled operation.
Ultrasonic Ultrasonic gas leak detectors are not gas detectors per se. They detect the acoustic emission created when a pressured gas expands in a low pressure area through a small orifice (the leak). They 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. The ultrasonic gas leak detector then produces an alarm when there is an ultrasonic deviation from the normal condition of background noise. Ultrasonic gas leak detectors cannot measure gas concentration, but the device is 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. However, holographic sensors require illumination sources such as white light or
lasers, and an observer or
CCD detector. == Calibration ==