Sensors help boost air quality for comfort and safety


Those working in confi ned spaces around industrial plants need to be sure they are not moving into areas with harmful levels of air contamination. Portable systems incorporating air quality sensors make sure they are informed.


In a variety of industrial situations, from wastewater systems to petrochemical refi ning and from mining to steel production, fi xed systems ensure adequate ventilation is available to clear the air of waste gases that can easily build up from chemical processes.


There are a number of sensor technologies suitable for detecting contaminants in air. However, a key issue for any form of chemical sensor compared to one designed for measuring physical parameters such as temperature or pressure is one of selectivity. There is a huge variety of chemical structures among gases. These structures provide ways to identify individual chemicals, or classes of them, among a mixture in the gas sample. This allows for the creation of highly tuned sensors with limited interference from other gases that may be present. The selectivity can be challenging though when the application calls for the ability to detect a range of harmful gases within a typical air mixture.


A commonly used technology for detecting specifi c gases, or those with common chemical structures, is that of the non-dispersive infrared (NDIR) sensor. This takes advantage of the selective absorption of light by different chemical bonds. Many of the gases of interest in air quality measurements show strong absorption of IR at specifi c wavelengths that are different to those of, for example, molecular oxygen or nitrogen. For example, the harmful oxidising gas ozone only contains oxygen atoms, but can be detected within a sample of air that contains O2


SGX Sensortech (an Amphenol company) manufactures both MiCS and NDIR sensors.


The differences give rise to bonds that absorb strongly at 9µm, whereas O2


bonds do not.


The characteristic IR absorption wavelength of O2


is much shorter at 0.763µm.


A number of the other gases of interest in air quality sensing show strong absorption bands in the range between 1µm and 10µm. For applications such as VOC detection, a typical technique is to look for the 3.3 to 3.5µm range of wavelengths often absorbed by the carbon-hydrogen bonds that are common to these molecules. NDIR sensors for VOCs, carbon dioxide, nitrogen oxides, CO or other gases work by passing the gas sample through a chamber illuminated by an IR source with a detector. The detector is protected by a fi lter to remove unwanted wavelengths, mounted opposite. By monitoring the output of the detector, software can gauge the concentration of the target gas within the chamber.


Miniaturised gas sensors are typically designed for single gas types, such as carbon dioxide or VOCs. To measure the presence of more than one type of gas, sensors will often be deployed in parallel, where they can detect contamination with high repeatability and accuracy. A more advanced technique is Fourier transform IR spectroscopy. This uses fi ngerprinting


because the bonds between the three atoms in ozone are different. The ozone molecule forms a scissor-like structure similar to that of the water molecule rather than the dumbbell-like form of O2


.


Reflective Gas Cell IR Lamp


IR Filtered Detectors


Active Reference


L Lamp Modulation Gas In / Out


In an NDIR sensor gas passing through a chamber is illuminated by an IR source with a detector.


28 Signals Out


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