Particle counter calibration for cleanroom classification

Cleanroom classification under ISO 14644-1 is a counting measurement, and its credibility rests on the instrument doing the counting. ISO 21501-4 is the standard that says what a light-scattering airborne particle counter has to demonstrate, and it is referenced from the classification standard for that reason.

Counting efficiency, and the 50 % that looks like a fault

The requirement that surprises people is counting efficiency at the smallest channel: 50 % ± 20 % at the smallest size the counter reports, and 100 % (within tolerance) at 1.5 to 2 times that size. A counter that detected everything at its smallest channel would be out of specification.

The reason is that a real counter's response does not switch on sharply at a threshold — it rises over a range of sizes. Specifying 50 % at the nominal size puts the midpoint of that transition at the stated channel, so that particles slightly smaller are under-counted and particles slightly larger are over-counted in a balanced way. The alternative, demanding full efficiency at the smallest channel, would place the whole transition below the stated size and make the counter's nominal channel meaningless.

Flow rate is concentration

A particle counter reports a concentration — particles per cubic metre — obtained by dividing the counts by the volume it believes it sampled. That volume is the flow rate multiplied by the sampling time. A flow rate 5 % low means the counter thinks it sampled 5 % less air than it did, and the reported concentration is 5 % high, with nothing in the count itself to reveal it.

This is why flow verification belongs in routine use rather than only at annual calibration: a partially blocked filter, a tired pump or a kinked sample line changes the flow, and the instrument keeps reporting confident numbers throughout.

Coincidence at high concentrations

When two particles pass through the sensing volume at once, the counter sees one event — and, because the scattered light is greater, may size it larger than either. This coincidence loss means counts fall and sizes shift upwards as concentration rises, which is why counters carry a maximum concentration specification. Using one above that limit in a dirty area or during recovery testing produces results that understate the contamination.

Isokinetic sampling

Where air is moving — unidirectional flow areas in particular — the sample probe should draw air at a velocity matching the airflow it is sampling from. Draw too slowly and larger particles' inertia carries them into the probe disproportionately, over-representing them; draw too quickly and the opposite happens. The probe should also face into the flow. In turbulent or low-velocity areas the requirement relaxes, but the sample line does not: long, coiled or sharply bent tubing loses large particles to the walls before they ever reach the sensor.

What the calibration does not cover

The certificate describes the instrument. It says nothing about whether the sample point was representative, whether the room was at rest or in operation, how many locations were sampled, or how long each sample ran — all of which belong to ISO 14644-1 and to the protocol. A perfectly calibrated counter sampling in the wrong place gives a precise answer to the wrong question.