Most industries follow international standards to monitor airborne particles in clean zones or cleanrooms. In these controlled or critical environments, some form of airborne sampling of particles is required so that the quality of the product being produced or the effectiveness of the process being performed is ensured.
However, there are a number of variables, which will influence the success of the operations performed in these areas; the level of airborne particulates in the environment is often a major element in managing the risk of failure in the controlled operation.
The preliminary standard for classification of clean zones and cleanrooms, called as Federal Standard 209E, had a universal impact for a number of years; however, this was substituted with a new global standard ISO 14644-1 in 1999.
Revision of ISO 14644-1 and -2 (TC209 Working Group 1)
Over the past few years, an international group called TC209 Working Group 1 (WG1) was reviewing possible revisions to the first two sections of the ISO 14644 document. The general vote on the proposed revisions made by the larger body of nations ended on 2 May 2011; however, there was a possibility that the date of the revisions could be placed six months later.
The proposed revision of the ISO 14644-1 document specified the normative reference to a calibration standard for the instrument to be utilized for classification of cleanrooms. Earlier, calibration method was not mentioned in ISO 14644-1.
However, it is a well known fact that the variability of the calibration methods can have a considerable effect not only on the performance of a particle counting instrument, but also on the reproducibility between instruments.
The Ultimate Quest
Customers using multiple particle counting instruments often believe that each instrument would deliver the same result when sampling in the same physical space. This is not possible because the same aerosol cannot be sampled sequentially or concurrently by a couple of instruments.
The levels of particles in a zone or room can change considerably over time, depending on various variables, such as activity, speed of activity, changes in air flow rates, temperature gradients, and number of personnel, amongst others. In addition, concern over the reliability of the instrument being utilized makes it more complicated to define the levels of airborne particles.
Controlling Variability between Instruments through ISO 21501-4
The inconsistency of the concentration of airborne particles is a subject of cleanroom design along with the control of the activities and process with the clean zone, while the inconsistency of the measuring instrument is one of proper calibration, use, and maintenance. Figure 1 shows factors that contribute to the variability in readings of airborne particles in cleanrooms.
ISO 21501-4 offers a complete platform to control the possible discrepancy of the instruments. In the majority of cases, instrument calibration performed over the years following its original manufacture could take a number of forms. There have been plenty of occasions where 1-point "calibrations" carried out by unqualified or independent contractors with little or no traceability to NIST, BSI, DANAK, JIS, DIN, and other national standards organizations.
Techniques also differ between different manufacturers of instruments. Most calibrations performed by qualified technicians in the field can usually involve just three or four tests. When calibrations are carried out subsequent to ISO 21501-4, a minimum of eight different tests are normally performed.
These tests demand additional instruments and also additional time. While calibration performed in accordance to ISO 21501-4 may result in additional cost when compared to the ISO 21501-4:2007, Determination of particle size distribution — Single particle light interaction methods — Part 4: Light scattering airborne particle counter for clean spaces presently performed, there will be significant benefits in terms of reproducibility and repeatability of measurements obtained.
Reproducibility refers to the capability of two or more instruments to achieve the same results when determining the same environment with a reliable particulate level, while repeatability applies to the stability of the values accomplished by a specified instrument when taking various measurements of the same environment with a reliable particulate level.
As noted above, it is not easy to have a reliable level of airborne particulates in an environment in the actual world. As a result, the difference of readings from one instrument taking successive readings, or between two instruments taking corresponding readings, will exist at all times. ISO 21501-4 parameters are summarized in Table 1.
Table 1. ISO 21501-4 parameters.
|ISO 21501-4 Parameters
|Counting Efficiency at First Channel
||50% ± 20%
|Counting Efficiency at particle Size 1.5 to 2 times First Channel
||100% ± 10%
|Instrument Resolution (at the size specified by manufacturer)
|Zero Count test
||≤ 1 count in 5 mins
|Maximum particle number concentration (specified by manufacturer)
|Sampling Flow Rate (volumetric)
||≤ 1 year
ISO 21501-4 helps in substantiating that under controlled conditions and while testing a reliable and controlled aerosol, the equipment delivers expected and reliable readings. This method provides improved confidence that the readings this achieved in the real world will actually reflect the level of contamination at a given moment.
This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.
For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.