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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 Sensors, and Semiconductor Sensors. 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
Since the time people have had the knowledge that certain gases were dangerous and harmful to their health, the need to detect these gas leaks has been around. Over the years many methods had been used in order to detect the presence of some of these gases. In the 1800s and early 1900s, coal miners would bring canaries down to the tunnels with them as an early detection system against life threatening gases such carbon monoxide and methane. In the case that these gases were present, the canary, normally a very songful bird, would stop singing and eventually die, signaling the miners to exit the mine quickly. Also in the 1980s and 90s, before carbon monoxide detectors and before it was known that carbon monoxide was a deadly household killer, the way to detect carbon monoxide was a piece of chemically infused paper that turned brown when it came into contact with the gas. Since then, many technologies and devices have been developed to detect, monitor, and alert the leakage of a wide array of gases.

Electrochemical Detectors
Electrochemical gas detectors work by allowing gases to diffuse through a porous membrane to an electrode where it is either oxidized or reduced. The amount of current produced is determined by how much of the gas is oxidized at the electrode. The sensor is then able to determine 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 tends to be more stable and reliable over the sensor's duration and thus requires less maintenance than other types of detectors. However, the sensors themselves are subject to corrosive elements 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.

Infrared Point Detectors
Infrared point (IR) sensors use radiation passing through a volume of gas to detect leaks. Energy from the radiation is absorbed as it passes through the gas at certain wavelengths. The range of wavelengths that is absorbed depends on the properties of the specific gas. Carbon monoxide absorbs wavelengths of about 4.2-4.5 μm, for example. This is approximately a factor of 10 larger than the wavelength 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. This type of sensor is advantageous because it does not have to be placed in the gas itself in order to detect it. Infrared point sensors can be used to detect hydrocarbons, compounds composed of hydrogen and carbon atoms, and other infrared active gases such as water vapor and calcium fluoride. 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. Engine emissions are another area where IR sensors are being researched for use. The sensor would be able to detect high levels of carbon dioxide in the vehicles’ exhaust, and even be integrated with the vehicles’ electronic systems to notify drivers.3

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

Ultrasonic Detectors
Ultrasonic gas detectors use acoustic sensors to detect changes in the background noise of its environment. Since most gas leaks occur in the ultrasonic range of 25 kHz to 10 MHz, the sensors are able to easily distinguish these frequencies from background 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. Despite the fact that Ultrasonic gas leak detectors don’t measure gas concentration, the device is still able to determine the leak rate of an escaping gas.8 By measuring its ultrasonic sound level, the detector is able to determine the leak rate, which depends on the gas pressure and size of the leak. The bigger the leak, the larger its ultrasonic sound level will be. Ultrasonic gas detectors are mainly used for 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.

Household Safety
There are many different sensors that can be purchased to detect hazardous gases around the house. Carbon monoxide is a very dangerous gas that robs the lungs of oxygen, killing hundreds of people worldwide each year. It is an odorless, colorless gas, making it impossible for humans to detect it. Carbon monoxide detectors can be purchased for around $20-60. 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 $35-100.