How PID Works

PhoCheck+ measures volatile organic compounds in air by photoionisation detection (PID), which is shown schematically in the figure below. Test gas  is drawn into the chamber from the PhoCheck+ probe. It is caused by the chamber design to move across the face of a lamp window, emitting light particles, or photons (shown by arrows) of high UV energy. Essential for the detection of a molecule XY is photoionisation , which occurs when a photon is adsorbed by the molecule, generating two electrically charged fragments, or ions, , one positively charged, X+, and one negatively charged, Y-. An electric field between two metal grids, called electrodes, attracts ions towards them. At the electrodes, ions are neutralised by movement of a tiny electric current, the photoionisation current, which is proportional to the concentration of XY, so that it can be amplified and displayed as a gas concentration. PhoCheck+ contains a third, fence electrode, patent protected, to ensure that the amplified current does not include a significant current due to other causes, such as water condensation on the chamber walls. Exhaust gas  is removed from the chamber by means of a pump inside the PhoCheck+ housing.