There are many different types of gas sensors out there which are used to detect gaseous molecules at the parts per million (ppm) or the parts per billion (ppb) level. These applications can range from environmental monitoring of gases, carbon monoxide detectors and the detection of gas in industrial environments. The use of fiber optics in various sensing applications has also become a popular choice. In this article, we look at the areas where gas sensing and optical fibers meet and how these sensors are used to detect gaseous molecules.
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Fiber Optic Sensors
Fiber optic sensors are a class of sensors where an optical fiber, and its subsequent components such as Bragg diffraction gratings, are either used as the intrinsic sensing element (i.e. the whole sensor itself), or they act as means of carrying information (via an optical cable) from a remote sensor to another location where the data is collected. As they work on optics, i.e. light, no electronics are required in the long-range transmission of sensor data, hence their viability for remote sensing applications. They are also immune to electromagnetic interference.
Fiber optic sensors, and fiber optical cables, are a form of waveguide. In this instance, they guide light from one point to another point. So, as the active sensing element detects a change in the local environment—in the case of gas sensors, it is when the sensing material interacts with the gaseous molecules—the intensity of the transmitted light changes. When this change is detected by the detector at the other end of the cable, calibrations within the data output software can then be used to output the concentration of gaseous molecules detected.
Light is known to propagate in all directions from a source. Fiber optics use a special type of low refractive index cladding (often made of glass or a polymer composite), that keeps the light on its intended path (by reflecting it along the intended path) and stops it from propagating outside of the fiber. This is vital because any loss of light leads to changes in the intensity and the output signal, which can lead to false results. This enables fiber optic sensors to be highly accurate over long and short distances.
Detecting Gaseous Molecules with Fiber Optic Sensors
Given the inherent stability of fiber optics and their ability to be made very thin, they have a lot of applications oil, gas and industrial processing. As there are many different types of gaseous molecules that can be detected, this leads to the use of fiber optic sensors in many different environments, as well as differences in the materials that can be used, the sensor fabrication processes, and a variation in the sensing mechanisms from application to application.
Here, we focus on the different areas where fiber optic sensors are used to detect different gaseous molecules, including some of the main industries and environments you may find them in. But, some of the thin film sensing elements used in these fiber optic sensors can range from sol-gel materials, to transition metal complexes and metal oxide films, gamma-irradiated materials, porous materials, doped materials, plasmonic materials and silicate-based materials.
Two of the main gaseous molecules which are detected within these environments are ammonia—to check if there has been a leak during the production process which may pose a hazard to the operator’s health—and hydrogen gas—specifically within the oil and gas industry where large amounts of hydrogen gas can build up in the downhole bores. The build-up of large amounts of hydrogen gas can be explosive, so fiber optic sensors are employed to keep an eye on the levels of hydrogen gas in these environments and ensure that the highest levels of safety are maintained.
Whilst these are two key areas, the use of fiber optic gas sensors is very widespread and a large amount of gaseous chemicals can be detected using fiber optic sensors. Because fiber optics work without electrical components, they can be used in harsh chemical environments without the sensing capabilities becoming compromised. Aside from hydrogen gas and gaseous ammonia, fiber optic sensors can also be used to detect a range of gaseous hydrocarbons, oxygen, ozone, carbon monoxide, carbon dioxide, nitrous oxide, hydrofluoric acid (HF gas), various alcohols, acetone, butyl acetate, acetic acid, and various chemical warfare agents.
Another aspect to fiber optic gas sensing is humidity (or relative humidity in most cases). Whilst it is not a mechanism for detecting harmful and toxic chemicals, the detection of humidity is related to gas sensing because it is the detection of gaseous water molecules within the local atmospheric environment.
Sources and Further Reading:
- Halliburton: https://www.halliburton.com/content/dam/ps/public/pinnacle/contents/Brochures/web/fiber-optics.pdf
- U.S. Department of Energy Office of Scientific and Technical Information: https://www.osti.gov/servlets/purl/938805
- Keyence: https://www.keyence.com/ss/products/sensor/sensorbasics/fiber/info/
- “Fiber-Optic Sensors Based on Surface Plasmon Resonance: A Comprehensive Review”- Sharma A. K. et al, IEEE Sensors Journal¸ 2007, DOI: 10.1109/JSEN.2007.897946
- “Fiber Optic Detection of Ammonia Gas”- Kalvoda L. et al, Acta Polytechnica, 2006
- “Fiber-Optic Chemical Sensors and Biosensors”- Wolfbeis O. S., Analytical Chemistry, 2004, DOI: 10.1021/ac040049d
- “Fiber optics assisted ammonia gas detection property of gamma irradiated magnesium tetraborate”- Mohandoss R., Sensors and Actuators A: Physical¸ 2019, DOI: 10.1016/j.sna.2018.11.012