Gas leaks are a common phenomenon though not always leading to hazardous accidents do pose a threat to the surrounding environment unless constantly monitored. Gas sensors, employed to detect these, have a wide range of application from common household to oil refineries, pharmaceutical manufacturing in clean rooms to rugged underground excavation sites. Often outfitted to an audio or visual output to signal the level of atmospheric contamination, gas detectors are of two types - portable and fixed.
Fixed gas sensors are usually used at industrial applications fitted to a mild steel structure near process areas or control rooms and anyplace where toxic gas is to be monitored. Data, transmitted through cables to the SCADA system determine the operation of the fail-safe system.
Depending on the principles of operation gas sensors are classified into the following technologies:
- Catalytic Bead
- Infrared point
- Infrared Imaging
According to a report published on the growth of global industrial sensor market, classified on technology adapted, for the years 2017-2021 by Technavio, A large chunk of the market is reserved for electrochemical followed by Semiconductor, infrared, Photoionization (PID) and solid state and ionic in that order
Description of the most common gas sensors
Principle : Gas is passed through a porous membrane onto an electrode where it undergoes chemical reactions of either reduction or oxidation resulting in the formation or depletion of electrons respectively producing current, hence the name. Magnitude of the current determines the percentage of gas in the atmosphere.
- They can be tuned to detect the gases depending on the membrane selection.
- More stable due to the use of non-movable physical components
- Stability gives them an edge over corrosive environments.
- Their environment renders them unusable after 1-2 years depending on the corrosive factors.
- Not suitable for all gases. Mostly electrochemically active gases like CO2, H2 can be detected
- Does not work in inert atmosphere
- Temperature range is limited
Application: Refineries, underground storage facilities, chemical plants.
Principle : Sensing layer made of metal oxide when in contact with reactive gas undergoes reactions that alter the resistance that translates to the concentration of gas exposed.
- Detecting a wide variety of gasses such as sulfurous gases (H2 S, SO2 ), carbon monoxide (CO), nitrogen oxide (NOx ), ammonia (NH3 ), and hydrocarbons (Cx Hy ) and volatile organic compounds (VOCs).
- Very less power consumption
- Detection limit is restricted by area, as the gas needs direct contact with the sensing element.
- Does not work in inert atmosphere.
- Contact with poison like halide, silicones, liquids compromise the working.
- Close range detection as in breath analyser.
- Air quality (AQ) monitoring
Principle : Gas molecules are broken down by high energy photons (UV Rays) creating a positive charged ions and electrons temporarily. The ion concentration is directly proportional to the gas concentration
- Used for portable devices.
- Highly sensitive
- Not selective of the gas. No idea of the gas detected.
- Does not perform in humid conditions
- High maintenance
Application: Portable applications
Detailed description and working principle of the sensors will be dealt separately