What are response factors?

Different VOCs respond differently to a PID at the same concentration. A response factor relates the PID reading to the actual concentration of a specific compound. Without applying response factors, PID readings tend to over- or under-state compounds depending on chemistry.

How often should a PID be calibrated?

Manufacturer guidance varies, but routine practice is bump-testing at the start of each working day and full span calibration on a defined interval — weekly or monthly depending on use intensity. Calibration records are essential for any work where readings will be cited as evidence.

Direct-reading VOC instruments

PID monitors — broad VOC detection, with discipline

Photoionisation detectors are the workhorse of field VOC screening. They are fast, broad and indicative — and they need to be operated with an understanding of what their readings actually represent.

Laboratory glassware representing VOC analysis context

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Principle

How photoionisation works

The UV lamp's photon energy determines which compounds the PID can detect — and which it cannot.

10.6 eV lamp

Standard general-purpose lamp; detects a wide range of common VOCs.

11.7 eV lamp

Detects additional species including methanol and some chlorinated hydrocarbons; shorter lamp life.

9.8 eV lamp

More selective; useful where targeting a narrower compound set.

Compounds with ionisation energies above the lamp energy are invisible to the PID. Methane, for instance, is not detected by any standard PID.

Applications

Where PIDs are used in the field

In workplace hygiene, PIDs are routine for rapid screening — confirming whether a process or task is generating measurable VOC concentrations and where. They are the precursor instrument that decides whether sorbent-tube sampling and laboratory analysis are warranted.

In environmental investigations, PIDs map plume edges around spills, contaminated land and emission sources. The fast response makes walk-through surveys practical in a way that laboratory methods cannot match.

In built-environment work, PIDs identify which spaces and which times show elevated VOC activity, supporting follow-up sampling, source attribution and intervention.

Limits

What PIDs do not tell you

No compound identification

A reading shows that VOCs are present, not which ones — laboratory analysis is the next step.

Humidity effect

High humidity quenches the signal on many PIDs — corrections or selection of humidity-compensated models matter.

Response factor required

Readings are isobutylene-equivalent unless a response factor is applied for a specific target.

Lamp and ionisation limits

Compounds above the lamp energy are not detected at all.

Comparison

PID readings versus laboratory analysis

Method	What you learn	Typical role
PID screening	Total VOC presence and intensity (indicative)	Walk-through, leak surveys, trend
Sorbent-tube sampling + TD-GC-MS	Identified, quantified individual compounds	Exposure assessment, source attribution
Canister sampling + GC-MS	Broad VOC speciation in a snapshot air sample	Investigation of unknowns
Continuous TVOC sensor (MOX)	Indoor trend at low cost	Background trending, not field screening

Suitable for

Who uses PIDs

Occupational hygienists

Rapid screening to scope formal sampling campaigns.

Environmental engineers

Site investigation, leak detection, plume mapping.

IAQ consultants

Building-level VOC investigation, complaint response and intervention validation.

FAQ

PID monitor questions

A photoionisation detector measures the total concentration of volatile organic compounds ionisable by its UV lamp energy — typically expressed as parts per million relative to a calibration gas, most commonly isobutylene. It is a broad indicator, not a compound-specific reading.

Discuss an Air Quality Monitoring Project

PID-based VOC screening, sorbent-tube sampling and source investigation across UK environmental and workplace projects.

Discuss instrument selection