Gas Sensors
Flame Ionization Detectors (FID)
Measures total organic vapor by burning it in a hydrogen flame. Sees methane and the light hydrocarbons a PID can't — at the cost of carrying a hydrogen supply and more operational fuss.
TECH total-organic-vapor survey, landfill/pipeline work, PID-vs-FID selection
How it works
An FID burns the sampled air in a small hydrogen flame. Organic (carbon-containing) molecules combust and, in doing so, produce ions in the flame. A charged collector electrode picks up those ions as a current proportional to the number of carbon atoms being burned — so the FID responds to total organic vapor, roughly in proportion to carbon content. The more organic carbon flowing through the flame, the higher the reading (typically displayed as methane or hexane equivalents).
The key contrast with a PID: a PID relies on the molecule's ionization potential being below the lamp energy (so it misses methane), while an FID just burns whatever organic carbon is present — including methane and the other small hydrocarbons a PID can't touch.
An FID needs a small onboard cylinder of hydrogen to run its flame. That's a compressed flammable gas you now own and must manage — filling, transporting, storing, and operating it near a potentially flammable atmosphere. Follow the manufacturer's and your department's procedures for the hydrogen supply, and understand the ignition-source implications of operating a live flame instrument on scene.
What it's good for
- Total organic vapor survey including methane — the standard tool where light hydrocarbons dominate and a PID would read low or zero.
- Landfill, pipeline, and natural-gas work — methane and light-hydrocarbon leak surveys, subsurface gas migration, and gas-utility investigations, where methane is the whole point.
- Wide dynamic range and high sensitivity to organics, with a fairly uniform per-carbon response that makes it a good general organic-vapor quantifier.
- Complementary to the PID — running both, and comparing, helps distinguish "PID-visible" vapors (aromatics, larger molecules) from "FID-only" light hydrocarbons like methane.
What it CANNOT do / limitations
- Blind to inorganics. No carbon to burn = no signal. FIDs do not see CO, CO₂ (essentially), HCN, chlorine, ammonia, hydrogen, or the acid gases. It is strictly an organic-vapor tool.
- Doesn't identify. Like a PID, it's a single total-organics number, not a chemical name. (An FID becomes an identifier only when it's the detector at the end of a gas chromatograph.)
- Needs oxygen and a stable flame — very oxygen-poor or very high-concentration atmospheres can extinguish the flame; the reading drops out until re-lit.
- Response varies by compound — per-carbon response isn't perfectly uniform; heteroatom-substituted organics respond differently.
- Not a life-safety multi-gas replacement — it tells you nothing about O₂, toxics, or the inorganic flammables.
Operational complexity & failure modes
- Hydrogen management — refilling from a supply cylinder, leak-checking, and safe transport add logistics a PID doesn't have.
- Flame-out — the flame can be blown or starved out (high wind at the exhaust, O₂-poor sample, liquid/dust ingestion, very high VOC hit). No flame = no reading; you must re-ignite.
- Warm-up and startup — lighting and stabilizing the flame takes time; it's not an instant power-on-and-go instrument.
- Contamination of the burner/collector from dirty samples degrades performance.
- Temperature and humidity affect flame stability and baseline.
- Training burden — more of a technician-operated instrument than an operations-level survey meter.
PID vs. FID — comparison
| Factor | PID (photoionization) | FID (flame ionization) |
|---|---|---|
| Detection mechanism | UV lamp ionizes molecules with IP below lamp energy | Hydrogen flame burns organic carbon, making ions |
| Sees methane / light hydrocarbons? | No (IP too high for standard lamps) | Yes |
| Sees aromatics, larger VOCs? | Yes, very sensitive | Yes |
| Sees inorganics (CO, NH₃, Cl₂, H₂)? | No | No |
| Consumables / fuel | None (lamp) | Hydrogen cylinder |
| Startup / complexity | Power on, quick | Light & stabilize flame; more involved |
| Ignition-source concern | Low (UV, non-flame) | Live flame — manage on scene |
| Humidity effect | Quenches (reads low) | Affects flame; generally less humidity-sensitive |
| Typical use | General VOC survey, plume tracking | Landfill / pipeline / natural-gas & methane work |
If the FID reads high but the PID reads low/zero, you're likely looking at methane or light aliphatic hydrocarbons (landfill gas, natural gas). If both read, heavier/aromatic organics are present. The ratio between the two is a crude but useful clue about what family of organics you're dealing with — before any identification tool comes out.
Calibration & bump test schedule
- Bump/functional test before each day's use (ISEA position) — confirm flame lights and holds, the instrument responds to cal gas, and the alarm works.
- Full calibration per the manual (commonly methane in air), on the manufacturer's schedule; span with the specified reference and apply response factors for known targets.
- Verify the hydrogen supply is charged and leak-free as part of readiness checks.
- Recalibrate after flame-out troubleshooting, burner cleaning, or erratic readings.
Field care & storage
- Manage the hydrogen cylinder per procedure — secure storage, leak checks, safe refilling.
- Keep the sample path clean and dry; avoid liquid/dust ingestion that fouls or extinguishes the flame.
- Allow warm-up and let cold instruments acclimate; protect the exhaust from high wind during use.
- Store discharged/serviced per manual for the fuel system between deployments.
Common rookie mistakes
- Expecting the FID to see inorganics (CO, ammonia, chlorine, hydrogen) — it only sees organic carbon.
- Forgetting the flame is an ignition source and the hydrogen supply is a flammable gas to manage.
- Not noticing a flame-out (reading drops to baseline) and interpreting it as "clean."
- Treating the total-organics number as an identification.
- Neglecting hydrogen supply readiness and finding an empty cylinder on scene.
Representative instruments
Portable FIDs are generic examples such as the TVA-1000 series (dual PID/FID), Photovac MicroFID, and similar organic-vapor analyzers. Some instruments combine a PID and FID in one body so you can read both simultaneously. FIDs are also the detector inside many gas chromatographs. FIDs are not part of a typical RAE fleet — expect this capability to come from a gas utility, environmental contractor, regional team, or partner agency. Brands are illustrative; your model and manual govern.
The FID's superpower is seeing methane and light hydrocarbons that the PID misses — invaluable for landfill, pipeline, and natural-gas work. Its costs are a hydrogen fuel supply, a live flame, and more operator skill, and it's blind to every inorganic. Pair it with a PID to sort the organic-vapor family; pair both with your multi-gas for life safety.
Next: chemical-specific spot checks — Colorimetric Tubes & Chip Systems →