Gas Sensors

Photoionization Detectors (PID)

A broadband "is there something volatile in the air?" survey tool. Fast and sensitive to a huge range of vapors — but it's a detector of presence, not an identifier, and its blind spots are big.

OPS broadband VOC survey / trend monitoring TECH lamp selection, correction-factor math, interpreting an unknown

How it works

A PID shines high-energy ultraviolet light from a gas-discharge lamp onto the sampled air. When a molecule's ionization potential (IP) is lower than the lamp's photon energy, the UV knocks an electron off the molecule, creating charged ions. Two electrodes collect those ions as a tiny current proportional to how many molecules got ionized — i.e., roughly proportional to the vapor concentration. The molecule isn't destroyed; it recombines and flows out, so the PID is non-destructive and responds within seconds.

The reading is usually displayed in ppm (or ppb) of isobutylene equivalents — because isobutylene is the near-universal calibration gas (see below).

Ionization potential vs. lamp energy — the master concept

Everything a PID can and can't do comes down to one comparison: the lamp's energy (in electron-volts, eV) versus the target molecule's ionization potential (also in eV).

✓ Remember — the one rule of PIDs

If the molecule's IP is LOWER than the lamp energy, the PID can ionize (see) it. If the IP is HIGHER than the lamp, the PID is blind to it. A 10.6 eV lamp sees anything with an IP below ~10.6 eV, and nothing above it. That's the whole game.

Common PID lamps. Higher energy sees more compounds but at a cost.
Lamp	Sees (IP below lamp)	Trade-offs
9.8 eV	Fewer compounds — more selective; rejects some interferents	Longer-lived; used when you want to exclude higher-IP background.
10.6 eV	The workhorse — most aromatics, many solvents, ketones, larger hydrocarbons	Best balance of coverage and lamp life. The default field lamp.
11.7 eV	The most compounds — reaches some chlorinated solvents, formaldehyde, and others out of reach of 10.6 eV	Short lamp life; the window crystal is hygroscopic (absorbs moisture) and degrades/fogs quickly. Fragile, high-maintenance, needs frequent replacement.

Field Tip — don't reach for the 11.7 lamp by default

The 11.7 eV lamp is tempting because it "sees more," but its window is hygroscopic and it fails fast in real field humidity. Most hazmat work runs the 10.6 eV lamp. Use 11.7 deliberately for a specific higher-IP target, and expect to babysit and replace it.

What it's good for

Fast, sensitive broadband VOC survey — the go-to tool for "is anything volatile off-gassing here?" It sees many things at sub-ppm levels, often long before a cat bead or EC sensor would register.
Finding and tracking a plume / source — chase a rising number to its origin; watch it fall as you decon or ventilate.
Bounding an unknown at the survey stage — a big PID number in an "empty-looking" space tells you to slow down even before you know what it is.
Quantifying a KNOWN single chemical using its correction factor (see below).
Detecting many toxic vapors well below their LEL — the PID's ppm-level sensitivity catches health hazards a %LEL sensor misses entirely.

What it CANNOT do / cannot see

⚠ Warning — the PID is blind to several critical gases

Anything with an ionization potential above the lamp energy is invisible. With a standard 10.6 eV lamp, the PID cannot see:

Methane (IP ~12.6 eV) and natural gas / ethane — a PID reading of zero tells you nothing about a natural-gas leak.
Carbon monoxide (CO) and carbon dioxide (CO₂).
Hydrogen cyanide (HCN), hydrogen, and many small toxic molecules.
Most permanent gases (N₂, O₂), water vapor, and HCl/HF.

A zero on the PID does not mean the air is safe — it means nothing the lamp can ionize is present at that instant. Pair the PID with LEL, EC toxic sensors, and colorimetrics.

And the point that trips up everyone new to it:

✓ Remember — a PID is a survey tool, NOT an identifier

A PID gives you one number that lumps together everything ionizable in the air. It cannot tell benzene from acetone from a cocktail of ten solvents. Never announce a chemical identity from a PID reading. Use it to find and track hazards, then use identification tools (tubes, FTIR, GC-MS) to say what it is.

Correction factors & the isobutylene standard

PIDs are almost universally calibrated to isobutylene — a convenient, stable, non-toxic reference gas, and the default cal gas for RAE PIDs. The display reads in "isobutylene equivalents." A correction factor (CF), published by the manufacturer for hundreds of chemicals, converts that reading into the true concentration of a known single chemical:

true ppm = displayed ppm × correction factor

A CF less than 1 means the PID over-responds (reads high) — e.g., some aromatics.
A CF greater than 1 means the PID under-responds (reads low) — the true concentration is higher than displayed, sometimes several times higher. This is the dangerous direction: a modest PID number can hide a large amount of a high-CF chemical.

Field Tip — where the CF numbers come from (RAE fleets)

For RAE PIDs, the standard correction-factor reference is RAE Systems Technical Note TN-106 — the published list of CFs (per lamp energy) for hundreds of chemicals. Keep a current copy with the instrument or in the tech reference library, and make sure you're reading the column for your lamp (9.8 / 10.6 / 11.7 eV — 10.6 eV is the RAE standard). CFs from one manufacturer's list don't automatically transfer to another maker's PID.

⚑ Common Rookie Mistake

Applying a correction factor to an unknown or a mixture. CFs only work when you already know exactly which single chemical you're measuring. On an unknown, the number is raw isobutylene-equivalent — treat it as a relative indicator (rising/falling), give yourself margin for a possible high-CF compound, and don't back-calculate a "real" concentration you can't justify.

Humidity, fouling & interference

Humidity quenching: water vapor absorbs UV and scatters ions, so high humidity typically makes a PID read low (quenching) — a real hazard can be under-reported in steamy or wet environments.
Fogging / condensation on the lamp window from humidity can produce erratic readings or drift.
High readings from benign sources: the PID's broad sensitivity means everyday products set it off — cleaning products, solvents, hand sanitizer, exhaust, off-gassing plastics, fresh paint, even some foods. A high number is not automatically a dangerous chemical; correlate with context.
Dust and aerosols can foul the lamp window and electrodes, and mist can carry contamination onto optics.
Very high concentrations can temporarily saturate the detector and dirty the lamp, needing clean-air recovery and possibly cleaning.

Degradation & failure modes

Lamp window fouling is the #1 maintenance issue — a film of contamination on the UV window cuts light and causes low readings and drift. The window needs periodic cleaning (per manual, with the correct cleaning compound) and the lamp eventually needs replacement.
Dirty sensor drift — contaminated electrodes/optics cause slow baseline drift; a failed bump or unstable zero is the tell.
Lamp aging / failure — output falls over time (fastest for 11.7 eV); a dead lamp reads zero.
Moisture damage — especially the hygroscopic 11.7 eV window.

Calibration & bump test schedule

Bump test before each day's use (ISEA position) with isobutylene — verifies lamp, electronics, pump, and alarm.
Full calibration per the manufacturer/AHJ — commonly monthly, extendable with a documented bump program; PIDs used hard (dirty environments) may need more frequent zero/span.
Clean the lamp window on the manufacturer's schedule and whenever readings drift or bumps fail; recalibrate after cleaning or lamp replacement.
Zero in genuinely clean air — a PID zeroed near exhaust or product will read low the rest of the incident.

Field Tip — AutoRAE docking stations

Departments running RAE fleets typically automate the daily bump and scheduled calibration with AutoRAE 2 docking stations: drop the instrument in the cradle and the dock runs the bump/cal, logs the result, and flags failures. Use it — automated, documented testing is exactly what the ISEA position calls for. But the dock doesn't see everything: still eyeball the inlet, external filter, and sample path, and verify the pump pulls properly (block the inlet and confirm the pump-stall alarm) before use. A cracked inlet fitting or clogged filter can pass a docked cal and still starve the sensor in the field.

Field care & storage

Keep the lamp window and sensor clean and dry; use inlet filters and a moisture trap in wet work.
Carry spare lamps and cleaning supplies for extended operations, especially if running 11.7 eV.
Avoid drawing in liquid, mist, or heavy dust.
Store with the lamp protected; let cold instruments acclimate to avoid window condensation.

Common rookie mistakes

⚑ Common Rookie Mistakes

Treating a PID zero as "all clear" — it's blind to methane/natural gas, CO, CO₂, and HCN.
Announcing a chemical identity from a PID number — it's a survey tool, not an identifier.
Applying a correction factor to an unknown/mixture and reporting a false "real" concentration.
Forgetting humidity quenching and under-reading a hazard in wet/steamy conditions.
Panicking over a high reading from a benign source (cleaner, exhaust) without correlating context.
Neglecting lamp-window cleaning, then chasing drift and failed bumps.

Representative instruments

In a RAE fleet, the PIDs you'll handle are the MiniRAE 3000 (dedicated handheld ppm PID) and ppbRAE 3000 (parts-per-billion sensitivity for low-level work), the PID channel on the MultiRAE multi-gas, and the UltraRAE 3000 — a compound-specific PID that adds a separation tube (e.g., for benzene) in front of the lamp so it can report one chemical instead of total VOCs. The AreaRAE platform carries a PID for wireless area monitoring. Equivalent non-RAE instruments exist (e.g., Ion Science Tiger, Dräger X-pid / X-am with PID). Lamp choice (9.8 / 10.6 / 11.7 eV, with 10.6 standard) is set for the mission. Brands are illustrative; your model and manual govern.

✓ Remember

The PID is your fast, sensitive survey instrument — brilliant for finding and tracking volatile vapors, useless for naming them, and blind to methane, CO, CO₂, and HCN. Run it alongside your LEL and EC sensors, correct only for known chemicals, and keep that lamp window clean.

Next: the total-organics cousin that can see methane — Flame Ionization Detectors (FID) →