A Basic Guide to Photoionisation Detectors

PID stands for “Photo-Ionisation Detector”. A PID is a sensor that can be incorporated into a hand-held, personal, or fixed detector for detecting a broad range of volatile organic compounds (VOCs) and some inorganic compounds from parts-per-billion (ppb) to thousands of parts-per-million (ppm).

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What Is A VOC And What Makes It A Volatile Detectable By PID?

Volatile organic compounds (VOCs) are a wide range of naturally and synthetically occurring chemicals. They are described as volatile because they evaporate in conditions found on earth, releasing molecules into the atmosphere. VOCs are also extremely useful as they form the building blocks of many synthetic materials (plastics, rubbers, glues, paints etc.). VOCs also play a key part in pharmaceutical manufacturing and are a great fuel for transport and heating.

For successful detection of a VOC by PID the following 3 general criteria must be met:

  1. Have less than 10 Carbon atoms
  2. Have a boiling point of less than 250 °C. A boiling point between 250 and 300 °C is only marginally detectable by PID and a boiling point over 300 °C is not volatile enough for detection
  3. Have a vapour pressure greater than 0.00004 (4.0 x10-5) mbar or 0.00005 (5.0 x10-5) mm Hg at 25 °C

PIDs And Oxygen

PIDs are fully functional in applications where it is necessary to measure toxic compounds without oxygen present. This is useful as most electrochemical sensors and pellistor-type LEL sensors require oxygen to function.

Even though workers use breathing apparatus in such situations, it may still be necessary to measure VOCs.

VOCs are not only harmful through breathing but can be a hazard through skin exposure, or even have the potential for an explosion should air and oxygen suddenly intrude. Some chemical processes are also run in oxygen free atmospheres (inert atmospheres typically with N2 present) and PID can be used to monitor for VOCs in these applications. However, if you are using a PID in an oxygen free environment, it should also be zeroed and calibrated on oxygen free gas.

Oxygen is also incredibly useful when using PID, as a very small amount is ionised to create ozone (O3) which helps clean any residual contaminants from within the sensor chamber.

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For further information, see TA-02 which has an extensive list of chemicals and response factors. If you can’t find the compound you require, please email info@ionscience.com or telephone: +44 (0) 1763 208 503 with the chemical name and CAS number as we will endeavour to assist you with further details.

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Everything you need to know about A Basic Guide To Photoionisation Detectors

Our White Paper Article is available to download below, the information provide you with key information and discuss the facts and common questions asked regarding PIDs. We also highlight which lamp is best to use when detecting VOCs within your industries.

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