Applying ISO methods to counter calibration

Published: 23-Nov-2011

International standards for the monitoring of airborne particles apply to all cleanrooms or clean zones. The international committee, TC209 Working Group 1, has been examining potential revisions to the first two sections of the ISO 14644 document. One key change in the proposed revision is the normative reference to a calibration standard for the instrument to be used for cleanroom classification. Although calibration will probably create some additional cost, there will be noteworthy benefits in both the repeatability and the reproducibility of the measurements obtained

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Dependable results in monitoring airborne particles require well-calibrated particle counters. Joe Gecsey, Met One (Hach), explains how implementing the recent ISO 21501-4 calibration standard can minimise calibration variability.

International standards for the monitoring of airborne particles are used by all industries that employ cleanrooms or clean zones. Some form of airborne sampling for particles is needed for these controlled environments to ensure the quality of the product being manufactured or the success of the process that is being conducted in this controlled or critical environment. Many variables will affect the success of the operations in these areas; the particulate levels in the air are often a significant element in the control of the risk of failure (or, inversely, the likelihood of success) for the controlled operation.

The initial standard for classification of cleanrooms and clean zones from the US, known as “Federal Standard 209E”, had a global impact for many years but was officially replaced in 1999 by a new global standard, ISO 14644-1.

For the past few years, the international committee, TC209 Working Group 1, has been examining potential revisions to the first two sections of the ISO 14644 document. The initial general vote on the proposed revisions by the larger body of nations closed on 2 May 2011; the effective date of the revisions is thought to be imminent.

One key change in the proposed revision of ISO 14644-1 is the normative reference to a calibration standard for the instrument to be used for cleanroom classification. Previously, no mention of a calibration method had been made in ISO 14644-1. But it is well known that the variability of the calibration techniques or methods can have a significant effect on the performance of a particle counting instrument and on the reproducibility between instruments.

A common expectation for customers having multiple particle counting instruments is that each instrument would give the same result as any other when sampling in the same physical space. This is an unrealistic hope, due to the reality that the same aerosol cannot be simultaneously or even sequentially sampled by a pair of instruments.

The particle levels in a room or zone are highly variable over time, depending on activity, number of personnel, speed of activity (even walking), temperature gradients, and changes in air flow rates – to name just a few of the variables. Airborne particle counts in an active area could be compared to counting cars on a road or freeway: each sample might be quite valid for the time it was taken but the numbers one might obtain during the middle of the night could be markedly different from those obtained at the peak of the rush hour. Add into this any concern with the reliability of the instrument being used, and the effort to define the airborne particle levels becomes even more challenging.

“The purpose of this part of ISO 21501 is to provide a calibration procedure and verification method for particle counters, so as to minimise the inaccuracy in the measurement result by a counter, as well as the differences in the results measured by different instruments.”
Translation: What you’ll get if you use ISO 21501-4 methods
1. Better repeatability over the lifetime of the instrument
2. Better agreement between different instruments
And, it will probably cost more because:
1. more tests = more technician time
2. extra tests = extra test equipment

Controlling variability between instruments through ISO 21501-4: The variability of the concentration of airborne particles is a matter of cleanroom design coupled with the control of the process and activities with the clean zone, whereas the variability of the measuring instrument is one of calibration, proper use and maintenance. ISO 21501-4 provides a thorough platform to gain control of the potential variability of instruments.

In many cases, calibrations of an instrument in the years following its initial manufacture could take many forms. Stories abound of 1-point “calibrations” performed by independent or unqualified contractors with little or no traceability to national standards organisations such as NIST, DANAK, BSI, DIN, JIS etc.

Table 1: ISO 21501-4 parameters and limits
Parametertarget limit
Size calibration± 5%
Counting efficiency at first channel50% ± 20%
Counting efficiency at particle size 1.5 to 2 times first channel100% ± 10%
Instrument resolution (at the size specified by manufacturer)≤ 15%
Zero count test≤ 1 count in 5 min
Maximum particle number concentration (specified by manufacturer)≤ 10%
Sampling flow rate (volumetric)± 5%
Sampling time± 1%
Calibration interval≤ 1 year

Methods also vary among different instrument manufacturers. Often calibrations by qualified technicians in the field can consist of only three or four tests. When a calibration is performed following ISO 21501-4, typically at least eight different tests are executed. These tests require both additional time and additional equipment.

Although calibration according to ISO 21501-4 will probably create some additional cost in comparison with what is currently performed, there will be noteworthy benefits in both the repeatability and the reproducibility of the measurements that are obtained.

Repeatability refers to the stability of the values obtained by a given instrument when taking multiple measurements of the same environment with a consistent particulate level.

Reproducibility refers to the ability of two or more instruments to obtain the same results when measuring the same environment at a consistent particulate level.

Proposed wording of Section 2 of 2011 revision of ISO 14644-1
2 Normative reference
The following normative document contains provisions, which, through reference in this text, constitute provisions of this part of ISO 14644. Subsequent amendments to or revisions of this publication do not apply. However, parties to agreements based on this part of ISO 14644 are encouraged to investigate the possibility of applying the most recent editions of the normative document indicated below.
ISO 21501-4:2007 Determination of particle size distribution — Single particle light interaction methods — Part 4: Light scattering airborne particle counter for clean spaces

As previously noted, it is almost impossible to have a consistent level of airborne particulates in an environment in the real world so the variation of readings from one instrument taking consecutive readings, or between two instruments taking parallel readings, will always exist.

What ISO 21501-4 provides is verification that in controlled circumstances and in testing a controlled and consistent aerosol, the instrument can provide predictable and consistent readings. This process thus creates the basis for confidence that the readings obtained “in the real world” – with its inherent variability – will be a true reflection of the contamination level at a given moment.

This article is based on a white paper that first appeared on Pharmaceutical Online, on 19 May 2011.

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