Reading a particle counter calibration certificate (ISO 21501-4)

Every particle counter used for cleanroom classification or monitoring needs a calibration certificate, and most of those certificates get filed without anyone reading past the first page. That is a shame, because the certificate tells you whether the counter can actually be trusted to do the job. ISO 14644-1 requires counters used for classification to be calibrated to ISO 21501-4, and that standard defines what a proper calibration has to check.

Here are the items worth reading, and what a good result looks like.

Size setting

The counter sorts particles into size channels by how much light they scatter. Calibration checks that the channel thresholds are set correctly, using reference particles of known size, usually polystyrene latex spheres. If the 0.5 µm channel is set too high, the counter under-counts and a room can look cleaner than it is.

Counting efficiency

This is the ratio of particles the counter reports to the particles actually present, measured against a reference instrument. ISO 21501-4 sets two checks:

• At the smallest size the counter claims to detect: 50 % ± 20 %.
• At a size of 1.5 to 2 times the smallest size: 100 % ± 10 %.

The first figure surprises people. It reflects how the minimum detectable size is defined: it is the size at which the counter sees about half of the particles. That is why a counter rated at 0.3 µm is a good choice for counting at 0.5 µm, where it should be close to fully efficient.

Size resolution

Size resolution describes how cleanly the counter separates particles of nearby sizes. The requirement is 15 % or better at the specified size. A counter with poor resolution smears counts across channels, which matters when your limits are set on one channel.

False count rate

With a filter on the inlet, the counter should report almost nothing. Counts in clean air come from electronic noise or internal contamination. This matters most in Grade A and ISO 5 areas, where real counts are low and a few false counts per cubic metre can push a result over an alert level.

Flow rate and sampling time

The counter converts counts into a concentration using its flow rate. If the flow is off by 10 %, the concentration is off by 10 %. ISO 21501-4 allows a tolerance of ± 5 % on the sampling flow rate, and the certificate should show the as-found and as-left values.

Maximum concentration

At high concentrations, two particles can pass through the beam together and be counted as one. The certificate states the concentration at which this coincidence loss reaches 10 %. For Grade C and D rooms in operation, check that your expected counts are comfortably below it.

As found, as left

A good certificate shows results before any adjustment as well as after. If the as-found values were out of tolerance, every measurement made since the last calibration may be affected, and that needs to be assessed. A certificate that only shows the as-left values hides exactly the information you need.

How often

ISO 21501-4 recommends calibration at least once a year. Counters in heavy use, or those moved often between rooms, may justify a shorter interval, especially if the as-found results show drift.