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ISO Technical Committee 209 has been working on the revision of the basic airborne cleanliness classification standard for the last 4 years. The ISO community voted in favour of revision to update and improve the standard specifically to address the following: - Simplify the classification process, and if possible remove the need to evaluate the 95% UCL for 2-9 sample locations
- Review the classification procedure and make it more applicable to rooms in operation. In this situation, the contamination isn't expected to be evenly distributed, an assumption the current statistical approach makes
- Generally update the standard as required to current thinking and industry requirements
- Avoid any radical change to the principles of the current ISO cleanliness classes 1-9
So, this was the challenge, and now we have the first public airing of the proposals. The DIS (draft International Standard) should be published for public comment and national vote in December 2010. The revised standard has some important new and revised requirements. These are summarised below: - The classification would be based on a table (see fig.1.), with the well known formula used for the intermediate decimal classes (see fig.2.). In the current standard, the table is provided in an informative annex (see fig.3.). By using a look-up table as the basis for classification, it is easier to constrain the reader to appropriate particle sizes for specific classes
Fig.1 The basic classification table proposed in ISO DIS 14644-1:2010 | Table 1 Selected airborne particulate cleanliness classes | | ISO Classification Number (N) | Maximum concentration limits (particles/m3) | | 0.1 mm | 0.2 mm | 0.3 mm | 0.5 mm | 1.0 mm | 5.0 mm | | ISO Class 1 | 10 | | | | | | | ISO Class 2 | 100 | 24 | 10 | | | | | ISO Class 3 | 1 000 | 237 | 102 | 35 | | | | ISO Class 4 | 10 000 | 2 370 | 1 020 | 352 | 83 | | | ISO Class 5 | 100 000 | 23 700 | 10 200 | 3 520 | 832 | | | ISO Class 6 | 1 000 000 | 237 000 | 102 000 | 35 200 | 8 320 | 298 | | ISO Class 7 | | | | 352 000 | 83 200 | 2 930 | | ISO Class 8 | | | | 3 520 000 | 832 000 | 29 300 | | ISO Class 9 | | | | 35 200 000 | 8 320 000 | 293 000 | Fig.2 The formula proposed for intermediate decimal classes in ISO DIS 14644-1:2010
Fig.3 The informative classification table provided in ISO 14644-1:1999 | Table 1 Selected airborne particulate cleanliness classes | | ISO Classification Number (N) | Maximum concentration limits (particles/m3) | | 0.1 mm | 0.2 mm | 0.3 mm | 0.5 mm | 1.0 mm | 5.0 mm | | ISO Class 1 | 10 | 2 | | | | | | ISO Class 2 | 100 | 24 | 10 | 4 | | | | ISO Class 3 | 1 000 | 237 | 102 | 35 | 8 | | | ISO Class 4 | 10 000 | 2 370 | 1 020 | 352 | 83 | | | ISO Class 5 | 100 000 | 23 700 | 10 200 | 3 520 | 832 | 29 | | ISO Class 6 | 1 000 000 | 237 000 | 102 000 | 35 200 | 8 320 | 298 | | ISO Class 7 | | | | 352 000 | 83 200 | 2 930 | | ISO Class 8 | | | | 3 520 000 | 832 000 | 29 300 | | ISO Class 9 | | | | 35 200 000 | 8 320 000 | 293 000 |
The selection of number of sample locations has been based on a more rational basis intended to confirm with 95% confidence that 90% of the cleanroom will meet the intended classification (see fig.4.) . Also shown is the difference between the number of old and new sample locations
Fig.4 The table proposed for determining the number of sample locations in ISO DIS 14644-1:2010
| Area of zone [m2] | Old number of sample locations | min number new sample locations | | 2 | 2 | 1 | | 4 | 2 | 2 | | 6 | 3 | 3 | | 8 | 3 | 4 | | 10 | 4 | 5 | | 24 | 5 | 6 | | 28 | 6 | 7 | | 32 | 6 | 8 | | 36 | 6 | 9 | | 52 | 8 | 10 | | 56 | 8 | 11 | | 64 | 8 | 12 | | 68 | 9 | 13 | | 72 | 9 | 14 | | 76 | 9 | 15 | | 104 | 11 | 16 | | 108 | 11 | 17 | | 116 | 11 | 18 | | 148 | 13 | 19 | | 156 | 13 | 20 | | 192 | 14 | 21 | | 232 | 16 | 22 | | 276 | 17 | 23 | | 352 | 19 | 24 | | 436 | 21 | 25 | | 500 | 24 | 26 |
- A semi-random sampling technique is proposed based on a "hypergeometric" distribution, which is the statistical model for sampling without replacement. This is a significant change from current practice, and means that each time a zone is classified, the sample locations may be different. If a firm has determined through a risk assessment, that certain locations need to be examined specifically, then these should be designated in addition to the randomly selected locations
- Recognising that the <=5.0 micron class limit for ISO 5 has been removed in the revised standard, parties wishing to use the standard for classifying the environments EU GMP Grade A, and B "at rest" will have to use the macro-particle descriptor retained in the standard
Author:
Gordon Farquharson
Convenor ISO TC209 WG1
Critical Systems Ltd.,
Guildford, Surrey, UK, GU1 2SY
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