Gas sensors are devices used to detect the presence of gases in an area. Such sensors can be used to detect a gas leak or other emissions and can help shut down the process automatically by interfering with a control system. This is done in many different ways, one way is by sounding an alarm, which warns the operator in the area and indicates the location of the gas leak. What makes gas sensors so significant, is that they can detect gases that are harmful to organic life, animals, and humans.
Gas sensors are used mostly to detect flammable, combustible and toxic gases, as well as oxygen depletion. These sensors can be found in locations such as oil rigs, to monitor the processes of manufacturing, and are used emerging technologies such as photovoltaics.
Gas leak detection is the process in which potentially hazardous gas leaks are identified by sensors, and an alarm is sent off to inform people of the danger. Another way of detection is the visual identity that can be carried out by using a thermal camera.
Gas detectors are classified as per their mechanism of operation such as oxidation, semiconductors, photoionization, catalytic, infrared, etc. They are classified into many categories:
- Portable gas detectors are used to monitor the atmosphere around us, they are usually worn on clothing, a belt or harness. Such detectors are mostly battery-driven and they transmit warnings through audible and visible signals, such as flashing lights and alarms.
- Fixed types of gas detectors are used to detect one or more types of gases. Such detectors are mounted near the process area of a plant, in control rooms, or an area of interest, such as a bedroom of a residence. Normally, industrial sensors are installed on a mild steel structure that is rigid where continuous monitoring can be carried out.
Types of Gas Sensors
Electrochemical gas sensor: Electrochemical gas sensors allow the gases to diffuse through a porous membrane to an electrode where it is either reduced or chemically oxidized. The determination of the amount of current produced is measured by the amount of gas that is oxidized at the electrode which indicates the concentration of the gas. Some customization in electrochemical gas detectors has been done by manufacturers, by changing the porous barrier to allow the detection of the concentration range for a certain gas. Moreover, since the diffusion barrier is a mechanical/ physical barrier, the detector tends to be more reliable and stable over the durability of the sensor and hence require less maintenance than any other early detector technology.
Catalytic Bead Sensor: These sensors are commonly used to measure combustible gases that present an explosion hazard when concentration levels are in between the upper explosion limit (UEL) and the lower explosion level (EL). The reference and active beads containing platinum wire coils are located on opposite arms of a Wheatstone bridge circuit and heated electrically, up to a few hundred-degrees Celsius. A catalyst present in the active bead allows combustible compounds to oxidize, thereby enhancing the heating of the bead even further and changing its electrical resistance. The result of the voltage difference between the passive and active beads is proportional to the concentration of all combustible gases and vapours present. The sampled gas can enter the sensor through a sintered metal frit, that provides a barrier to prevent an explosion when the instrument is carried into an atmosphere containing combustible gas. Pellistors measure all combustible gases, but they are more sensitive to smaller molecules that diffuse through the sinter more rapidly. The measurable concentration ranges are from a few hundred ppm to a few volume percents. Such sensors are robust and inexpensive, but require some minimum percentage of oxygen in the atmosphere to get tested and they can be poisoned or inhibited by compounds such as mineral acids, silicones, chlorinated organic compounds, and sulphur compounds.
Photoionization Sensors: These detectors use a high-photon-energy UV lamp to ionize chemicals in the sample gas. If the ionization energy of the compound is below that of the lamp photons, an electron will be ejected, and the resulting current is proportional to the concentration of the compound. The broad range of compounds can be detected at levels that range from a few ppb to several thousand ppm. Some detectable compound classes in order of decreasing sensitivity include olefins, alkyl iodides and aromatics, amines, sulphur compounds, ketones, alkyl bromides, organic esters, aldehydes and alkanes, and organic acids. Photoionization detectors are beneficial because of their simplicity and excellent sensitivity. The major limitation with these detectors is that their measurements are not compound-specific. Photoionization detectors are widely used for industrial hygiene and environmental monitoring. They are usually bench-type, miniature, hand-held clothing clipped PIDs.
Infrared Point Sensors: These sensors use radiation which passes through the volume of gas. In these sensors, energy from the sensor beam is absorbed in certain wavelengths, depending on the properties of a specific gas. For example, carbon monoxide absorbs wavelengths of about 4.4-4.5 micrometers. The energy of 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. The advantages of these sensors that they do not have to be placed into the gas to detect it and can be used for remote sensing. The infra-red sensors are used for detecting hydrocarbons and other infrared active gases such as carbon dioxide and water vapour. These sensors are commonly found in refineries, waste-water treatments, chemical plants, gas turbines, chemical plants, and other facilities where flammable gases are present and there is a possibility of explosion. The remote sensing can monitor large volumes of space. Infrared sensors are also being researched in the area of Engine Emissions. The sensor detects high levels of carbon monoxide and other abnormal gases present in vehicle exhaust.
Infrared Image Sensors: These sensors include both active and passive systems. For active sensing, infrared imaging sensors usually scan a laser view of an entire scene and track for the backscattered light at the absorption line of the wavelength of a specific target gas. Passive imaging sensors, on the other hand, measure spectral changes in every pixel of an image and explore individual spectral signatures that indicate the presence of target gases. The compound types that can be imaged are similar to the ones that can be detected with infrared point detectors, but the images can help identify the source of gas.
Semiconductor Sensors: These sensors are known as metal-oxide-semiconductor sensors. They detect gases by a chemical reaction that takes place when the gas comes in direct contact with the sensor. The common material used in semiconductor sensors is tin oxide. The electrical resistance in the sensor reduces when it encounters the monitored gas. The change in the resistance is used to calculate the gas concentration. Semiconductor sensors are usually used to detect oxygen, hydrogen, alcohol vapour, and poisonous gases such as carbon monoxide. Semiconductor sensors are commonly used in carbon monoxide sensors and breathalyzers. Semiconductor sensors must encounter the gas to detect it and they work for a smaller distance as compared to ultrasonic and infrared point detectors. Semiconductor sensors can detect various gases such as sulphur dioxide, hydrogen sulphide, carbon monoxide, and ammonia. These sensors have been widely used since the 1990s.
Ultrasonic Non-gas Detectors: These are not gas detectors; they detect the aural emission created when a pressurized gas expands in a low-pressure area through a small outlet (point of leakage). These devices use aural sensors to detect the changes in the background noise of their environment. As high-pressure gas leaks generate sound in the ultrasonic range of 25 kHz to 10 MHz, the sensors can easily differentiate the frequencies from the background aural noise (which is 20 Hz – 20 kHz). The ultrasonic gas leak detectors cannot measure the concentration of gas, but they can determine the leak rate of gas escaping. This is because the ultrasonic sound level depends on both the pressure and size of the gas leak.
Ultrasonic gas detectors are mostly used for remote sensing in the outdoors where weather conditions can easily disintegrate evading gas even before it reaches the leak detectors that require interaction with the gas to detect it and sound an alarm. Ultrasonic detectors are commonly found on offshore and onshore oil/gas platforms, gas turbine power plants, gas compressors, metering stations, and other facilities that have outdoor pipelines.
Holographic Gas Sensors: These sensors use light reflection to detect changes in a polymer film matrix that contain a hologram. As holograms tend to reflect light at certain wavelengths, any change in their composition can generate a colorful reflection that indicates the presence of a gas molecule. Holographic sensors require sources of light such as a CCD detector or an observer, or white light or lasers.