Why must the particle size be indicated in the cleanliness grade?

Direct answer: Because the definition of ISO grade itself includes particle size. Grade numbers are meaningless apart from particle size.

The classification logic of ISO 14644 is to select a particle size threshold, count the number of particles larger than or equal to this size in a unit volume of air, and then compare it with the limit value given by the standard. The grade number is the result, and the particle size is the premise. If the premise is not written, the result will be left hanging in the air.

There are more problems than expected in the project because the particle size is not clearly stated.

Particle size determines the statistical particle range

Not less than 0.5μm, which means adding up all particles of 0.5μm and above. It is not a narrow range, but starts from 0.5μm and goes up.

Instruments usually have accumulation mode and binning mode. The accumulation mode is to accumulate according to the threshold, and the binning mode is to count separately according to the particle size interval. Data from the two modes cannot be mixed. The report says '0.5μm'. If it is not indicated whether it is cumulative or divided, the interpretation may be biased.

For the same air sample, the number of particles differs greatly between statistics based on ≥0.5μm and statistics based on ≥5μm. Putting these two numbers together, there is no comparison.

Process sensitive particle size must be agreed separately

Different processes have different sensitive particle sizes.

Some products are sensitive to sub-micron particles, while others are more concerned about particles of a few microns. If the project only writes an ISO level and does not indicate the sensitive particle size, equipment selection and acceptance may deviate from actual requirements.

For example, semiconductor photolithography processes may focus on smaller particle sizes, while general mechanical assembly may be more sensitive to larger particles. Different target particle sizes will lead to differences in filter selection, channel configuration of detection instruments, and sampling methods.

In ISO standards, there are applicable restrictions for certain combinations of grades and particle sizes. You cannot fill the blank spaces in the table with zeros by yourself, nor can you extend them with unfounded formulas. When selecting, first confirm the target particle size, and then check whether the corresponding grade is suitable.

When interpreting the report, you must first check the particle size.

If you get two reports that both write ISO 5, you cannot directly say that they are the same.

The first part may measure ≥0.5μm, and the second part may measure ≥5μm. In the same room, the particle concentration varies greatly according to different particle sizes, and the corresponding limits are also different. The conclusions of both reports are called ISO 5, but the actual control level may not be at the same level.

So when reading the report, first look for the particle size conditions. If you can't find it, check with the testing party or supplier. As mentioned in What is the Difference Between Cleanliness Grade and Filtration Efficiency ? The space grade evaluates the particle concentration, while the filter efficiency evaluates the product performance. Both depend on particle size.

The contract and acceptance must be fully written

Write the ISO grade in the technical agreement, followed by the particle size.

The complete statement includes at least: ISO grade 、particle size threshold 、operating status 、area boundary . For example, 'ISO 7, ≥0.5μm, dynamic, assembly area'. Only in this way will there be a clear basis for judgment.

Just write 'ISO 7' or 'Class 10,000', and the acceptance personnel will not know which particle size to measure. If the two parties have inconsistent understandings, disputes may easily arise in the future. How Class 100, Class 1,000, and Class 10,000 correspond to ISO grades illustrates the difference between traditional names and ISO expression calibers. Particle size is one of the key conditions. It is also mentioned in how to update the grade description of the 100,000-grade workshop . When updating the grade description, the particle size cannot be leaked.

Equipment configuration should also be based on particle size

When selecting equipment, particle size is one of the benchmarks for filter efficiency evaluation.

Efficiency reports for HEPA and ULPA usually give data for specific particle sizes. The HEPA and ULPA filter pages have parameter descriptions for different levels of products. When selecting equipment, check the filtration efficiency at the target particle size, rather than just looking at the grade name.

The operating point of the air supply equipment is also related to the filter resistance. FFU clean air supply products include standard products and customized solutions. For partial purification at the equipment end, please refer to EFU equipment side fan filter unit . If the project requires zoning control, The FFU group control system can configure operating strategies by region.

Aili Industrial has manufacturing capabilities in motors, fans, control systems, sheet metal, air filters and complete machine assembly. When doing a clean level project, you can check the target particle size, filter efficiency, fan working point and structural interface under the same project conditions to make the equipment configuration and acceptance conditions match.

Scene judgment

Two cleanliness reports for a certain project both stated ISO 5. One part measured ≥0.5μm and the other part measured ≥5μm. The project team initially believed that the two results could be directly compared, but later discovered that the particle concentration values ​​were very different. After checking, I found out that the particle size and caliber were different and the data were not comparable.

Another project only wrote ISO 7 in the technical agreement, without specifying the particle size and status. During the acceptance inspection, the two parties had inconsistent understandings of the test conditions, and the technical agreement was later supplemented to resolve the issue.

These two situations illustrate that particle size is not optional, it is part of the grade expression.

Key points for selection and acceptance inspection