Infrared detection technology (IR) is used within a range of applications to detect specific gases that absorb IR light at characteristic wavelengths.
An infrared light beam passes through a gas cloud and onto collection optics where it is split and sent through filters onto infrared sensors.
The “measuring” beam, with a frequency of around 3.3μm, is absorbed by hydrocarbon gas molecules, and the beam intensity is reduced. The “reference” beam (around 3.0μm) is not absorbed by the gas, so arrives at the receiver at full strength. The %LEL of gas present is determined by the difference in intensity between the beams measured by the photo-receiver.
IR sensors are utilised within a variety of markets from agriculture, to waste management, and are often made specific use of in environments which cause pellistor-based sensors to function incorrectly or in some cases fail. IR sensors tuned to 3.3 microns can detect many gas types containing hydrogen to carbon atomic bonds (C-H), whilst IR sensors tuned to 4.25 microns can detect gas types containing carbon dioxide (O=C=O).
Hazard
Agricultural carbon dioxide emissions are a concern for both our atmosphere and the workers within the sector. From fertiliser manufacture to food storage and packaging processes, carbon dioxide is regularly generated, stored, transported and used, and consequently poses ongoing gas risks. Commercial greenhouse control systems can be used to measure and control temperature and carbon dioxide concentrations. CO2 must also be monitored in pig and poultry farming during gas stunning processes. Anaerobic digestion, and the production of biogas, also requires early gas detection through carbon dioxide and methane sensing to keep the process safe. Methane emissions must be monitored from agriculture and dairy farming.
When waste is deposited in landfill a number of harmful gases are generated, including VOCs, methane and carbon dioxide. These gases are created through the action of microorganisms, which involves the evaporation of volatile organic compounds, chemical reactions between waste components and microbial action. It is now mandatory that landfill gases are removed from these sites to avoid the risk of an explosion. Gas detection devices can be incorporated into landfill gas processing systems in order to monitor these gases easily, and from there make informed decisions about their removal.
A lack of indoor air circulation can allow oxygen depletion and carbon dioxide accumulation to unsafe levels. Ensuring air is safe to breathe can be achieved by the use of gas detectors in a space, to measure the oxygen and carbon dioxide content of the room.
Pellistor sensors, also known as catalytic bead sensors, have drawbacks for 4 reasons, not working in zero oxygen, burning out in high volume concentrations of fuel, poisoning of their catalysts, and sensor ageing, and so infrared sensors were developed to address these drawbacks and can significantly enhance safety in conditions where pellistors would fail to report the presence of gas. IR sensors tend to be used to detect carbon dioxide and flammable gases and do so reliably in many environments. Some sensitive high-end gas analysers use IR to detect carbon monoxide, refrigerants, ammonia, and even sulphur dioxide.
In some settings pellistors are prone to poisoning, in which they have an irreversible loss of sensitivity, or inhibition, which is a reversible loss of sensitivity, by a range of chemicals. When poisoned, a pellistor produces no output when exposed to flammable gas, and therefore would not go into alarm when the environment becomes unsafe. Compounds containing silicon, lead, sulphur, and phosphates at just a few parts per million (ppm) can impair pellistor performance. Sooting, where pellistors are exposed to highly carbon loaded fuels, leads to carbon deposits on the active pellistor bead which can inhibit or even block the passage of gas to the bead. If exposed to high levels of flammable gas pellistors can encounter “sooting”, but this is not recommended as a cure because it causes another problem where the localised temperature differences throughout the pellistor bead cause it to crack. Thereafter that pellistor bead is dangerous to use.
IR sensors are unaffected by other gases and are suitable for both high gas concentrations and use in inert (oxygen free) backgrounds where catalytic pellistor sensors would perform poorly. Note: the desired concentration range of use must be checked against the sensor datasheet to avoid saturation issues.
IR sensors are not susceptible to poisoning making them ideal for detection of combustible gases in low-oxygen environments, such as fuel storage tanks during flushing with inert gas prior to maintenance, or which still contain high levels of fuel vapours.
Many IR sensor types also require less power than pellistors to operate, whereas pellistors always require high amounts of power to function.
Pellistors have a limited life span and can deliver inaccurate readings if calibrated to a single target gas type when another is present.
The fail-safe nature of IR sensors, which automatically alert you to any fault, provides an additional layer of safety.
