INFICON MicroFID II Portable Flame Ionization Detector - Manual (Page 15)

074-579-P1A 15
Flame Ionization Detector Operation
The MicroFID II uses a flame ionization detector for the measurement of combustible organic compounds
in air at parts-per million levels. The permanent air gases (argon, carbon dioxide, nitrogen, oxygen, water
vapor, etc.) are not ionized by the flame.
When the MicroFID II is flamed on, the internal pump draws air in through the MicroFID II inlet. This
sample air provides the oxygen necessary for combustion in the hydrogen fed flame.
Figure 1. Flame Ionization Detector
When the proper ratio of hydrogen to air is present in the combustion chamber, the flame is started
automatically with a glow plug. A thermocouple is used to monitor the status of the flame.
When the sample passes through the flame, the combustible organic compounds in the sample will be
ionized. After the compounds have been ionized by the flame, the ionized particles are subjected to a
continuous electric field between the repeller electrode at the jet (+75V) and the collector electrode.
The ions move in the electric field, generating a current, which is proportional to the concentration of the
ionized molecules in the ionization chamber. An electrometer circuit converts the current to a voltage that
is then fed to the microprocessor.
After the sample passes through the flame and has become ionized, it is vented from the detector through
a flame arrestor. The flame arrestor prevents the flame from igniting any flammable gases present in the
sampling location.
Electrometer
Repeller Electrode
(Jet +75 Volts)
Collector Electrode
Sample In
Exhaust Out
Flame Arrestor
Flame Arrestor
Microprocessor
Combustion Chamber
Glow Plug Thermocouple