Direct-reading instruments
Particle counters — counting size, not identity
Particle counters give immediate, size-resolved counts that mass-based monitors cannot. They are powerful field instruments — and they have clear limits about what the numbers mean.
Technology
How particle counters work
Optical particle counter (OPC)
A laser illuminates particles drawn through a sample chamber; scattered light is detected and sized. Standard for 0.3 µm upwards.
Condensation particle counter (CPC)
Particles are grown in a saturated vapour until detectable optically — reaches well below 10 nm. Essential for ultrafine work.
Aerodynamic particle sizer (APS)
Uses time-of-flight measurement for aerodynamic diameter — relevant for inhalation studies.
Mass-based monitors (BAM, TEOM)
Not particle counters as such, but the regulatory comparison point — measure mass rather than number.
Applications
Where particle counters earn their place
In cleanrooms, particle counters are the routine instrument for ISO 14644 classification — sized particle counts at defined locations, frequencies and air volumes. The counter is the measurement; nothing else substitutes.
In indoor air investigations, OPCs surface short-duration events that mass monitors smooth over: a printer cycle, a vacuuming pass, an outdoor episode pushed in by ventilation. Spatial walk-throughs with handheld counters localise the source quickly.
In HVAC and filtration assessment, counts before and after a filter quantify removal efficiency for the size ranges that matter to the application — far more useful than relying on the filter rating alone.
Comparison
When counts and mass disagree
| Question | Use counts | Use mass |
|---|---|---|
| Cleanroom classification | Particle counters per ISO 14644 | Not the regulated metric |
| Regulatory PM2.5 / PM10 | Useful as indicative | Reference method required |
| Filter efficiency by size | Counters either side of the filter | Less informative |
| Long-term exposure | Number trends inform research | Mass aligns with health guidelines |
| Source investigation | Size and spatial counts narrow it down | Mass alone usually too coarse |
Limits
What particle counters do not do
No composition
Counts and sizes only — chemistry needs filter capture and laboratory analysis.
Coincidence limits
Above defined concentrations, optical counters under-count.
Flow and calibration
Sample flow accuracy and zero counts are routine calibration items.
Sub-0.3 µm blind spot
Most OPCs do not resolve ultrafine particles; CPCs do.
Suitable for
Where particle counters fit
Cleanroom operators
ISO 14644 classification, recovery testing and periodic monitoring.
IAQ consultants
Indoor investigations, event characterisation and ventilation/filtration assessments.
Facilities engineers
HVAC commissioning, filter validation and complaint response.
FAQ
Particle counter questions
Discuss an Air Quality Monitoring Project
Particle counter selection, cleanroom classification and indoor particulate investigations for UK clients.
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