"Measuring dust" can mean three quite different things: counting individual particles in a cleanroom, estimating how many milligrams of dust per cubic metre a worker is breathing, or collecting particulate matter from outdoor air on a filter for laboratory weighing. Each needs a different instrument, and using the wrong one gives numbers that cannot be compared with the relevant limit.
Optical Particle Counters: Counts by Size
A particle counter draws air at a fixed flow rate through a laser beam. Each particle scatters a pulse of light whose height depends on its size, and the instrument counts the pulses into size channels. The result is a number concentration — particles per cubic metre (or per cubic foot) at or above each size.
This is what cleanroom classification uses. ISO 14644-1 defines cleanliness classes by the maximum allowed concentration of particles at or above specified sizes, from 0.1 µm upward, and sets out how many locations and what sample volumes are needed to classify a room.
MEWOI's handheld particle counters measure 0.3–10 µm with user-selectable channel sizes (0.3, 0.5, 0.7, 1.0, 2.0, 2.5, 5.0, 10 µm). The HPC300 samples at 2.83 L/min (0.1 cfm), and the supplier quotes its count-limit warnings for ISO 14644-1 Classes 5 to 9. The HPC600 adds six channels and a printer. Counters are also used to check filter efficiency (upstream versus downstream counts) and to trace contamination sources.
Dust Meters: Mass Concentration
Occupational exposure limits and many environmental guidelines are written as mass concentrations in mg/m³ or µg/m³. A handheld dust meter or aerosol monitor estimates this optically, using light scattering or attenuation by the dust cloud, and gives a real-time reading.
The relationship between scattered light and mass depends on particle size, shape, density and colour, so optical dust meters are calibrated against a reference dust and are best used for relative measurements: comparing locations, finding sources, showing trends, and checking that controls work. For a result to compare against a statutory limit, a gravimetric reference sample is normally required, or a site-specific correction factor derived from one.
MEWOI's dust meters include the JY1000, which the supplier specifies for 0.1–1000 mg/m³, with a calibration mode for pulverised coal dust, and the GT1000FC, which combines particle counting and concentration.
High-Volume Samplers: Gravimetric Reference
For ambient air monitoring, the reference approach is gravimetric: draw a large, accurately controlled volume of air through a pre-weighed filter, then weigh the filter again under controlled conditions. A size-selective inlet determines what is collected — total suspended particulates (TSP), PM10 or PM2.5.
The MEWOI-UH1000H is an automatic high-volume sampler for TSP, with optional PM10 and PM2.5 separators. The supplier specifies a flow range of 0.8–2.0 m³/min, ±2.5% flow accuracy, constant-flow control with a response time under 5 s, and an all-weather enclosure.
Don't Forget the Airflow
Particle levels in cleanrooms and ventilated workplaces depend on airflow. Supply velocity, air change rate and extraction performance all matter. A vane anemometer such as the MEWOI air speed anemometers measures velocity and, with the duct or grille area entered, volume flow, so ventilation and particle results can be read together.
Quick Selection Guide
| Need | Instrument | Output |
|---|---|---|
| Cleanroom classification, filter testing | Optical particle counter | Particles per m³ by size |
| Workplace dust surveys, source finding | Optical dust meter | Real-time mg/m³ (estimated) |
| Ambient TSP/PM reference sampling | High-volume gravimetric sampler | Filter mass, µg/m³ |
| Ventilation checks | Anemometer | m/s, m³/h |
Frequently Asked Questions
Can a particle counter give PM2.5 in µg/m³? Some instruments estimate it from counts by assuming particle density and shape, but the result is an estimate and is not equivalent to a gravimetric PM2.5 measurement.
Why does sample flow rate matter for cleanrooms? Clean classes allow very few particles, so larger sample volumes are needed for statistically meaningful counts. A 2.83 L/min counter needs longer at each location than a 28.3 L/min counter.