The carbon ions formed are measured by an electrical field. A small ion current flows when the charged carbons arrive at one of two electrodes and become neutralized. The FID gives some response to virtually all organic compounds. The OVA is easy to use but is limited by its supply of hydrogen as well as its battery charge. This instrument measures total organics and does not speciate individual compounds.
Molecules absorb the UV light leading to ionization and release of a certain amount of energy.
The ions move in the electrical field generating a current proportional to the chemical concentration in the detector cell. An electrometer circuit converts the current to a voltage, which is sent to the microprocessor for display as a concentration. The PID is more user friendly than the OVA requiring less frequent calibration. There are no hydrogen requirements, but it is battery powered. Sensitivity and accuracy may fluctuate depending on the type of compound being monitored.