11.3 Photoionization and Flame Ionization Detectors
Key Takeaways
- A photoionization detector (PID) uses an ultraviolet lamp to ionize chemicals whose ionization energy is below the lamp’s electron-volt rating; common lamps are 10.6 eV, with 11.7 eV and 9.8 eV also in service.
- A PID reading is parts per million as isobutylene-equivalent unless a manufacturer response factor is applied. The meter does not identify the chemical.
- Methane’s ionization energy is about 12.6 eV, above even an 11.7 eV lamp, so a PID does not see methane. Some chlorinated species also need a higher-energy lamp or a different method.
- A flame ionization detector (FID) burns a hydrogen flame and is strong on hydrocarbons, including some PID-blind compounds such as methane; it is not a meter for inorganic gases and the flame is an ignition-source and fuel-logistics problem.
- Use a PID to find and map a volatile organic plume, then confirm identity with shipping papers, SDS, colorimetric methods, or laboratory analysis — never by treating the ppm number as a chemical name.
11.3 Photoionization and Flame Ionization Detectors
Quick Answer: A photoionization detector (PID) uses an ultraviolet (UV) lamp to ionize chemicals whose ionization energy (IE) is below the lamp’s electron-volt (eV) rating. The everyday lamp is 10.6 eV; 11.7 eV and 9.8 eV lamps also exist. The display is ppm as isobutylene-equivalent unless you apply a response factor. A PID does not identify the chemical and does not see methane. A flame ionization detector (FID) uses a hydrogen flame, is strong on hydrocarbons including some PID-blind compounds, and is not for inorganic gases. Find the plume with the PID or FID; confirm identity with papers, a Safety Data Sheet (SDS), colorimetric methods, or a lab.
NFPA 470 11.2.2 names the PID among required detection technologies. OSHA 1910.120(q)(6)(iii)(B) still wants verification, not a mystery number on a screen. Technician academies add the FID because some hydrocarbons that never trip a 10.6 eV PID will burn in a hydrogen flame. Chapter 12 will give you detector tubes as another confirmation tool. This section is why ppm on a PID is not a name, and why CGI versus PID versus FID is a comparison the exam expects you to draw, not blur.
PID: a UV lamp, not a laboratory
The lamp emits photons at a fixed energy (9.8, 10.6, or 11.7 eV are the common field lamps). If a molecule’s IE is lower than that energy, the photon can knock off an electron. The instrument counts those ions and scales the current to a ppm display. If the IE is higher than the lamp, that molecule is invisible to this PID, even at a dangerous concentration.
10.6 eV is the workhorse: it sees a large set of volatile organic compounds (VOCs) and some inorganics whose IEs sit just under 10.6 (hydrogen sulfide is about 10.5 eV, so a 10.6 lamp can respond to H2S — which is not a reason to retire the electrochemical H2S sensor). 9.8 eV is more selective (fewer compounds, less clutter). 11.7 eV reaches some higher-IE species, including some chlorinated solvents a 10.6 lamp misses, but 11.7 lamps are shorter-lived, more humidity-sensitive, and not the default stick in the cache.
Methane has an IE of about 12.6 eV. Neither 10.6 nor 11.7 ionizes it. A natural-gas leak can fill a room, the CGI can climb in % LEL, and the PID can sit at 0.0 ppm. That is not a broken PID. It is physics. Carbon monoxide (~14 eV) is also PID-blind; that is why the four-gas still has a CO cell.
Some chlorinated species (for example methylene chloride, carbon tetrachloride) have IEs that straddle or exceed 10.6 eV. A 10.6 lamp may under-see or miss them. “The PID is quiet” is not proof the chlorinated solvent is gone.
Isobutylene-equivalent ppm and response factors
Almost every field PID is calibrated with isobutylene. The number you read is “as if this vapor were isobutylene.” A response factor (RF) or correction factor from the manufacturer converts that equivalent toward a known target:
true ppm (estimate) ≈ isobutylene-equivalent reading × RF (follow your manufacturer’s definition; some tables are written as the inverse).
If you do not know the chemical, you cannot honestly convert. You can still use the PID as a relative plume hunter: walk upwind to a zero, then walk the gradient until the number rises, mark the hot side, and tell command “VOC, isobutylene-equivalent, lamp 10.6 eV.” That sentence is competent 11.2.2 documentation. “The product is 47 ppm of X” without an identity or an RF is fan fiction.
Humidity, dust, and a dirty lamp window change the photon output and the ion current. Fog, steam, and a lamp that has not been cleaned per the manufacturer produce false highs, false lows, and drift. Bump or span the PID with isobutylene on the same schedule the AHJ uses for the four-gas, and clean the lamp when the manual says so — not with whatever solvent is in the engine bag.
FID: hydrogen flame, hydrocarbons, and logistics
A flame ionization detector burns a small hydrogen flame. Organic compounds that enter the flame produce ions; the meter counts them. Compared with a PID:
- Strength: many hydrocarbons, including methane, ethane, and some other PID-blind aliphatics, show up on an FID.
- Weakness: inorganic gases (chlorine, hydrogen chloride, ammonia as an inorganic, sulfur dioxide, and similar) are not what an FID is for.
- Cost of that strength: you must carry hydrogen fuel, keep the flame lit, and treat the flame as an ignition source in an atmosphere that may already be near LEL. That is a logistics and IAP problem, not a footnote.
An FID is still not an identifier. It is a carbon-counting plume tool. Use it when the suspected product is a light hydrocarbon a PID will not see, or when the cache issues FID for leak-survey work. Do not light a hydrogen flame as a substitute for a CGI in a space that is already 10% LEL on a four-gas.
CGI vs PID vs FID — pick the tool for the question
| Instrument | What it “sees” | Units | Hard limits |
|---|---|---|---|
| Catalytic CGI (four-gas LEL) | Flammable gases/vapors the bead can oxidize | % of LEL of the calibration gas | Needs oxygen; not a toxic meter; correction factors; poisons; 0% LEL ≠ non-toxic |
| PID | Molecules with IE below the lamp eV (many VOCs; some others) | ppm, usually isobutylene-equivalent | Does not identify; misses methane and some chlorinated / high-IE species; humidity and dirty lamps lie; RF required for a named concentration |
| FID | Organic / hydrocarbon compounds the hydrogen flame ionizes, including some PID-blind lights | Typically ppm carbon-count / methane-equivalent depending on cal | Not for inorganics; hydrogen fuel; ignition source; still not an identifier |
Memory hook: CGI answers “will it flash?” PID and FID answer “is there organic vapor at toxic-range ppm?” None of them answers “what is the exact chemical?” until you attach papers, SDS, tubes, or a lab.
Find the plume, then confirm identity
Technician use of a PID on a leak is a search pattern, not a naming ceremony:
- Bump / span with isobutylene; note lamp eV.
- Zero upwind in clean air.
- Walk upwind / uphill / upstream geometry from Section 11.1, waiting for pump and T90 if you are drawing through a hose.
- Map where the isobutylene-equivalent reading rises. That is the plume or the pool edge, not a UN number.
- Confirm with shipping papers, facility SDS, container marks, colorimetric tubes (Chapter 12.1) chosen for the suspected family, or laboratory / GC when the IAP needs a name a court or a hospital will believe.
A PID that is climbing while the CGI is still 0% LEL is a success, not a contradiction: you are in the ppm toxic band, below the percent-LEL fire band. That is exactly why the ionization detector exists.
Scenario: quiet PID, loud CGI — or the reverse
Scene A. A natural-gas odor at a regulator. Four-gas LEL is 18% (AHJ SOP: that is above a common 10% LEL hold). PID 10.6 eV reads 0.0. Correct reading of the tools: methane is CGI-visible and PID-blind. Withdraw or upgrade per the IAP, control ignition, and do not decide the leak is imaginary because the PID is quiet.
Scene B. An unidentified solvent tote, no fire. CGI 0% LEL, PID 10.6 eV at 120 ppm isobutylene-equivalent in a low spot. Correct reading: organic vapor is present at ppm; the CGI is not supposed to see that far below LEL; identity is still unknown so you do not convert 120 ppm into a PEL comparison until papers, SDS, or tubes name the product. Find the plume, hold the team to the IAP, confirm the name.
If you remember one sentence: a PID reports isobutylene-equivalent ppm for whatever its lamp can ionize and does not name the chemical; an FID’s hydrogen flame sees some PID-blind hydrocarbons; neither replaces a CGI for fire, and neither replaces papers, SDS, or tubes for identity.
How does a photoionization detector produce a field reading, and what units does that reading use unless a response factor is applied?
A 10.6 eV PID reads 0.0 ppm while a catalytic CGI shows 18 percent of LEL at a suspected natural-gas leak. What is the correct interpretation?
Which statement correctly compares a PID and an FID and states how technicians should use a PID on an unidentified solvent release?