This page sets out what each detector physically does, the compounds and concentrations each handles best, and how the choice plays out in LDAR programmes, remediation surveys and screening work — closing with where the PTM100 and the FID-only PTM200 sit.
How each detector works
An FID burns the sample in a hydrogen flame; carbon-bearing compounds ionise as they burn, and the ion current is proportional to carbon mass. The response is broad across hydrocarbons, stable and quantitative, which is why reference methods for leak measurement are written around it. Its costs are the flame itself: hydrogen fuel, ignition management and no response to compounds without carbon-hydrogen bonds.
A PID floods the sample with ultraviolet light; compounds whose ionisation energy falls below the lamp energy ionise and are counted. It needs no fuel, starts instantly and reaches lower concentrations for many aromatics and solvents — but its response varies by compound, it cannot see methane or other high-ionisation-energy gases, and lamp fouling in dirty streams is a real maintenance item.
Where each wins
Choose FID where the number must stand as a hydrocarbon measurement — LDAR component readings, NMHC work, high concentrations and hot streams; the PTM200's high-temperature FID with catalytic furnace exists for exactly the regulatory NMHC question. Choose PID for low-ppm screening, aromatics and solvents, indoor air and any work where carrying hydrogen is impractical.
Choose both — the PTM100 pattern — where a survey mixes duties: FID for the defensible component readings, PID for sensitive screening between them, sharing one sample path, one handset and one data trail.
Practical ownership
FID ownership is fuel logistics and flame discipline; the PTM100 automates ignition, monitors flame temperature and displays hydrogen pressure and remaining time to soften both. PID ownership is lamp cleaning and response-factor bookkeeping, softened by a built-in compound library — the PTM100 carries more than 200 gases. Either way, calibration gas and a daily function check are the routine that keeps survey data defensible.
Response factors in daily practice
The factor tables are where the comparison becomes arithmetic. An FID calibrated on methane reads most hydrocarbons within a modest band of truth — acceptable for emission work with published corrections. A PID calibrated on isobutylene reads benzene near-honestly, toluene and xylene within a small factor, and light alkanes so weakly that raw readings understate them several-fold. Surveyors therefore plan with the plant's compound list open: which compounds dominate, what factors apply, and whether the corrected readings still resolve the thresholds the programme enforces. An instrument whose best-case correction leaves the decision threshold inside its error band is the wrong instrument for that compound, whatever its other virtues.
Weather, water and the field environment
Field conditions vote in this comparison. FID flames dislike wind, rain and oxygen-starved samples, and the instrument is a small system to keep lit, fuelled and pumped. PIDs shrug off wind but drink humidity: saturated air quenches response and fogs lamps, and dirty atmospheres coat optics between cleanings. Tropical plants, steamy process areas and winter fieldwork each tilt the choice, and experienced survey teams carry the cleaning kit, the spare lamps or the hydrogen discipline as second nature — the instrument's entourage is part of the instrument, and it should be specified and budgeted as such.
Programmes that run both, and why
Mature LDAR and remediation programmes often field both principles with a routing rule: PID for breadth-first screening where its family fits, FID for the regulated numbers and the methane-bearing streams, each instrument's readings recorded with its principle named. The rule survives because the two failures are mirror images — the PID's blindness is total but predictable, the FID's completeness is real but operationally expensive — and routing by question keeps each instrument inside its competence. Procurement that forces a single winner usually saves the smaller sum and spends the larger one in re-surveys.
Calibration gases and the morning routine
The two principles wake up differently. The FID's morning is a small ceremony: hydrogen and zero air confirmed, flame lit, methane span checked, pump and line proven — five competent minutes that also serve as the instrument's daily health examination. The PID's morning is shorter — isobutylene span, lamp window inspected, humidity noted — with the ceremony deferred to lamp cleaning and factor selection. Programmes should staff and schedule for the ceremony they actually bought; the FID rushed is a liar with a flame, and the PID unexamined is a optimist with a dirty lamp. Neither routine is a defect; both are the price of the physics, quoted honestly here so the comparison is made with eyes open.
Cost per reliable reading
Total the programmes, not the instruments: FID carries hydrogen logistics, flame consumables and heavier maintenance against near-universal response; PID carries lamps, cleaning and correction-factor management against instant, flame-free screening. Per reliable reading — a number the programme can defend — the costs converge more often than the purchase prices suggest, and the divergence that remains is explained by the compound list and the thresholds, which is to say by the questions being asked. The comparison's final form is therefore a sentence, not a score: for these compounds, at these thresholds, under these conditions, this principle produces defensible readings at this cost. Anything shorter is advertising.
Terms the comparison depends on
Response factor — the multiplier that converts an instrument's reading on a compound from its reading on the calibration gas toward truth. Correction factor — the PID dialect of the same idea, tabulated per compound per lamp. Ionisation energy — the molecular property that decides whether a PID lamp can see a compound at all. Lamp energy — the photon energy of the PID's ultraviolet source, 10.6 electron-volts in the common form. Flame ionisation — the combustion of sample carbon in a hydrogen flame, whose ion current the FID counts. Isobutylene — the PID's conventional calibration gas, the 'equivalent' its readings are denominated in.
Method-defined parameter — a quantity that exists only as a specified instrument measures it, which is what most VOC totals honestly are. Leak definition — the concentration at a component interface that a programme declares a leak, triggering its repair clock. First repair attempt — the programme's initial tightening or adjustment, verified by re-measurement. Delay of repair — the governed postponement for components that cannot be freed without a shutdown. Screening value — the field reading, factor-corrected, that programmes compare against their leak definitions. Carbon counting — the FID's near-universal census of organic carbon, methane included, the property that built its regulatory career.
Choosing for the regulator's method
Where a programme answers to a prescribed method — and emission LDAR usually does — the method's instrument is the default and the argument is over: reference methods were written around FID for reasons of completeness that no convenience overturns. The live choices gather at the method's edges: PID for the pre-survey that makes the FID's days efficient, PID for the compounds and sites the method does not reach, imaging for discovery across both. Programmes that respect the prescription and exploit the edges get compliance and intelligence from the same budget, which is the comparison's most valuable outcome — better than either instrument winning.
A field day with both
Picture the survey as it is actually run on a mature programme. The morning opens with imaging or a PID sweep of the unit, the surveyor moving fast, marking the components that speak. The FID then walks the marked list and the regulated sample, dwelling, recording the numbers the programme's thresholds are written in, factors applied from the plant's own table. By afternoon the data clerk holds two datasets that agree where they should — the sweep's geography and the sniffer's quantities — and the repair list writes itself with a confidence neither dataset earns alone. That choreography is the comparison's final answer: not a victor but a division of labour, rehearsed until it looks like one instrument with two hands.
| Point | FID | PID |
|---|---|---|
| Principle | Hydrogen flame ionisation of carbon compounds | Ultraviolet photoionisation |
| Sees | Broad hydrocarbons, incl. methane | Compounds below lamp ionisation energy; not methane |
| Strength | Quantitative, method-standard response | Low-ppm sensitivity, instant start, no fuel |
| Costs | Hydrogen supply, flame management | Lamp fouling, compound-dependent response |
| Home ground | LDAR readings, NMHC, hot/high streams | Screening, aromatics, indoor and low-level work |
| On this site | PTM200 (FID, NMHC) and PTM100 (FID channel) | PTM100 (PID channel); PID channels in portable platforms |
- Reference leak measurement is FID territory
- Low-level screening is PID territory
- Dual-detector instruments remove the either/or decision
- Calibration routine decides data defensibility more than detector choice
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