Having recently shared our video on pellistors and how they work, we thought it would make sense to also post our video about PID (photo-ionisation detection). This is the technology of choice for monitoring exposure to toxic levels of another group of important gases – volatile organic compounds (VOCs).
The sensor includes electrodes and a lamp, which is full of a noble gas (often krypton) as a source of high-energy ultraviolet light (UV). The UV light’s energy excites the neutrally charged VOC molecules, so removing an electron.
Having lost an electron, which is negatively (-) charged, the VOC molecule now has a positive (+) charge. The (+) molecule and the (-) electron collect at oppositely charged electrodes, resulting in a flow of current. The amount of current flow is directly proportionate to concentration of gas, and is converted to a parts-per-million readout on a detector display.
The amount of energy needed to remove an electron from a VOC molecule is called the ionisation potential (IP). The larger the molecule, or the more double or triple bonds the molecule contains, the lower the IP. Thus, in general, the larger the molecule, the easier it is to detect!
To learn more about the hazards of VOCs, read our blog from 27 Nov 2014, “What are VOCs”, our visit our page “How safe is safe? Understanding VOC safety thresholds” and download our white paper on the subject.