ISO 14644-1

How to choose an airborne particle counter for cleanrooms

Q: Which particle size governs ISO classification?

For most acceptance tests per ISO 14644-1, the 0.5 µm threshold is the reference value. For very high classes up to ISO 5, smaller sizes such as 0.1 or 0.3 µm are also used.

Q: How often must a particle counter be calibrated?

Annual calibration per ISO 21501-4 by an accredited lab is standard. Only with a traceable certificate are the readings valid for cleanroom acceptance.

Q: How many sampling points do I need in the cleanroom?

ISO 14644-1 sets the minimum number of sampling locations based on room area. The larger the area, the more points, so the classification stays representative.

An airborne particle counter measures the concentration of suspended particles and certifies the cleanroom class per ISO 14644-1. This guide covers classification, particle sizes and channels, flow rate and the key criteria for selecting the right instrument.

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ISO 1-9

cleanroom classes 14644-1

0.1-5 µm

typical particle sizes

28.3 l/min

common flow rate (1 cfm)

≥ 2 channels

measured at once

Inhalt

Cleanroom classes
Sizes and channels
Flow and sampling
Frequently asked questions

How are cleanroom classes defined under ISO 14644-1?

ISO 14644‑1 divides cleanrooms into classes ISO 1 through ISO 9. The decisive factor is the maximum permitted particle concentration per cubic metre of air, referenced to defined particle sizes. The lower the ISO number, the cleaner the air.

A particle counter draws in a known volume of air and counts the particles above a size threshold. The class is derived from the concentration and the number of sampling points. For classification, the standard prescribes a minimum number of sampling locations based on the room area.

The limits apply to a nominal condition. The standard also distinguishes the occupancy states at rest and in operation, which must be documented during acceptance testing.

Which particle sizes and channels matter?

A channel is a particle size threshold above which counting begins. Common channels are 0.3, 0.5, 1.0 and 5.0 µm; high-end instruments measure from 0.1 µm. For ISO classification, the 0.5 µm size is the classic reference value.

0.1‑0.3 µm: required for high classes ISO 1 to ISO 5 in semiconductor manufacturing.
0.5 µm: reference size of ISO 14644‑1 for most acceptance tests.
1.0 and 5.0 µm: relevant for pharma and GMP environments (EU-GMP Annex 1).
Multi-channel counters capture up to six sizes at once and save measuring time.

Choose a counter whose smallest channel matches at least the most critical particle size of your application. A 0.3 µm instrument is often enough for ISO 7 and ISO 8; for ISO 5, 0.1 µm should be possible.

Measurement overview

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Why do flow rate and sampling strategy matter?

The flow rate determines how quickly a counter captures a meaningful air volume. Common values are 2.83 l/min (0.1 cfm) for handheld units and 28.3 l/min (1.0 cfm) for portable classification instruments. A higher rate clearly shortens the measuring time per point.

Low ISO classes with very few permitted particles require a large sample volume so the counting statistics stay reliable. For ISO 5 and below, 28.3 l/min is therefore practically mandatory; otherwise a single sampling point takes unnecessarily long.

The counting accuracy of optical particle counters is calibrated per ISO 21501‑4. Ask for a traceable calibration certificate, otherwise the cleanroom acceptance is not recognised.

Frequently asked questions

Which particle size governs ISO classification?

For most acceptance tests per ISO 14644‑1, the 0.5 µm threshold is the reference value. For very high classes up to ISO 5, smaller sizes such as 0.1 or 0.3 µm are also used.

What flow rate should my particle counter have?

For ISO 5 to ISO 7, 28.3 l/min (1 cfm) is recommended because the large sample volume shortens measuring time and improves counting statistics. For simple monitoring, 2.83 l/min is often enough.

How often must a particle counter be calibrated?

Annual calibration per ISO 21501‑4 by an accredited lab is standard. Only with a traceable certificate are the readings valid for cleanroom acceptance.

How many sampling points do I need in the cleanroom?

ISO 14644‑1 sets the minimum number of sampling locations based on room area. The larger the area, the more points, so the classification stays representative.

Looking for the right particle counter?

We supply handheld and classification instruments for cleanrooms - with the right channels, matching flow rate and calibration per ISO 21501-4.

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Standard-compliant

Instruments for acceptance per ISO 14644-1.

Calibrated

Calibration per ISO 21501-4 with certificate.

All channels

From 0.1 µm to 5.0 µm depending on use.

Expert advice

Specialists help with instrument selection.