A PID measures volatile organic compounds (VOCs) in air by photoionisation detection (PID).

A lamp emits photons of high energy UV light and photoionisation occurs when a photon is absorbed by a molecule generating two electrically charged ions, one positively charged, and one negatively charged.

An electric field generated between electrodes attracts these ions and the resulting current is proportional to the concentration of the VOC.

The MiniPID includes a third fence electrode (patented) to ensure that the amplified current does not include significant contributions due to other current sources such as water condensation on the chamber walls.