The FID chamber is first heated to 191°C. Once the FID reaches temperature, the valve is opened to allow FID fuel and air into the chamber, and the flame ignites automatically. The exhaust sample is injected in the sample port. The FID fuel, air, and exhaust sample are mixed together at the bottom of the detector’s flame jet, and are burned on the flame’s tip. As the hydrocarbons burn, they form positively charged ions. These ions are repelled by the jet’s nozzle head, which has a positive bias voltage. The positive ions are then attracted to the negatively charged collector plate, creating a current which is then measured. User-selectable measurement ranges ensure accuracy for a wide variety of test applications.
The SEMTECH® Dual FID differs slightly in that the exhaust sample is injected in a single input manifold and is split into two samples. The first half is directed to the THC channel FID chamber. The second half is directed to an NMHC oxidation catalytic chamber for removal of the NMHC from the sample. This leaves only methane in the sample, which is injected in the CH4 channel FID chamber. Actual THC and/or CH4 concentrations are calculated from this measurement.