How to determine the number of cleanliness sampling points?

The number of cleanliness sampling points is something that must be specified in the acceptance plan. If the quantity is too small, local contamination may be missed; if the quantity is too large, the test time and cost will be increased. The number of sampling points is not set arbitrarily based on experience, but should be determined based on the standards adopted, the area of ​​the evaluation area and the applicable rules.

Different standard versions have different regulations on the number of sampling points. If you continue to use the old formula without checking the current standards, there may be insufficient points or inconsistency with the actual requirements. The following will start from the standard basis, area calculation, location distribution, and risk supplement.

First confirm the adopted standard and version

To determine the number of sampling points, the first step is to confirm the standards and versions adopted by the project. Different standards have different provisions on the number, location and calculation methods of sampling points.

Things that need to be confirmed include:

• The name and version of the adopted standard;

• Calculation rules for the number of sampling points;

• Distribution requirements for sampling locations;

• Minimum sampling volume and sampling time requirements.

If the project adopts ISO 14644-1:2015, the number of sampling points should be determined according to the area table rules of that version. The old empirical formula cannot be used mechanically. For specific testing requirements, please refer to What preparations need to be completed before clean room acceptance?。

Confirm the evaluation area and area

The number of sampling points is directly related to the area of ​​the evaluation area. Before determining the quantity, the boundaries and area of ​​the evaluation area should be clearly defined.

Need to pay attention to:

• Whether the evaluation area is the entire room or a local clean area;

• Whether area boundaries are demarcated by actual building envelope;

• Whether the area calculation includes non-working areas;

• Unconfirmed area input cannot be shared between local protection areas and entire rooms.

If the boundaries are unclear, the area calculation will be biased and the number of sampling points will be distorted. For the effective protection range of the work area, please refer to Can a clean workbench replace an entire clean room? , understand the difference between local area and whole room range.

Calculate minimum sampling points according to standard rules

ISO 14644-1:2015 uses area table rules to determine the minimum number of sampling points. The larger the area, the minimum number of sampling points increases accordingly. When calculating, the corresponding table given in the standard should be used instead of applying other formulas by yourself.

Please note when calculating:

• The area unit is consistent with the standard requirements;

• Rounding rules are implemented according to standards;

• During partition evaluation, the points of each area are calculated separately;

• Record the calculation process and basis.

The specific correspondence between area and points should be checked according to the standard form. If the calculation of sampling volume and flow rate is not clear, please refer to How does the particle sampling volume meet classification needs?。

Sampling locations should be reasonably distributed

After the number of sampling points is determined, the location distribution must also be determined. The number is sufficient but the location is concentrated, and local contamination may still be missed.

Sampling locations should cover:

• various locations within the region rather than being concentrated in the centre;

• Key workstations and product exposure locations;

• Areas where airflow may become stagnant or short-circuited;

• Near the air supply outlet and return air outlet (according to standard requirements).

The location of the sampling point cannot be just below the air supply outlet. For related reasons, please refer to Why can’t the sampling point be located just below the air supply outlet?。

Sampling locations can be added at risk points as needed

The standard specifies minimum sampling points. For special process points, key workstations and suspected weak locations, additional sampling points can be added according to the risk.

Supplementary sampling points include:

• The actual location where the product is exposed;

• Areas with frequent human activities;

• Near the location where the equipment produces dust or generates heat;

• Door opening and material entry and exit locations.

Supplementary sampling points are used for risk monitoring and cannot replace the minimum sampling locations specified in the standard. The results and uses of both should be recorded separately.

Relationship between working height and sampling position

Sampling height will affect test results. Different working heights correspond to different airflow and particle conditions. The sampling location should be determined based on the actual exposure height of the product.

If the workstation is changed from a low table to a higher assembly rack, the original sampling results may no longer be applicable. For the impact of changes in working height on the detection position, please refer to How does a change in working height affect the detection position?。

Partition evaluation and multi-point computing

If the clean area is divided into multiple evaluation areas, the number of sampling points in each area should be calculated separately. Different areas may have different sizes, grades and uses, and one total cannot cover all areas.

Pay attention to the following when evaluating partitions:

• Are the boundaries of each region clear?

• Whether the area of ​​each area is calculated separately;

• Whether sampling points in each area are recorded separately;

• Whether the results correspond to the respective evaluation ranges.

If the project is constructed in phases, please refer to What are the expansion conditions for using independent modules in clean systems? , unified evaluation area division and sampling rules in advance.

Equipment selection and terminal matching

After the number of sampling points is determined, the equipment and filtering configuration involved in the test can be referred to FFU clean air supply products and EFU equipment side fan filter unit . Filter installation, sealing and leak detection requirements can be learned HEPA and ULPA filters . If you are still unclear about the difference between space rating and filter efficiency, you can read What is the difference between cleanliness level and filtration efficiency?。

Project judgment example

A certain project determined the number of sampling points based on the old version of the empirical formula. During the acceptance inspection, it was found that the number of points was less than the current standard requirements. Adjustment method: Recalculate according to standard area table rules, add sampling points and retest. After adjustment, the test plan meets the standard requirements.

Another project had a sufficient number of sampling points, but the locations were concentrated in the center of the room, leaving edges and key workstations uncovered. Adjustment method: Re-distribute points according to standard location distribution requirements, and add key workstation supplementary points. After adjustment, the test results can better reflect the actual status.

These two situations illustrate that: the number of sampling points must be calculated according to standards and areas, and the location distribution must cover areas and key workstations, and cannot only meet the quantity requirements.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

The number of sampling points is related to the area of ​​the area, standard rules and evaluation boundaries. Aili Industrial has collaborative capabilities in fans, motors, filters, control systems and complete machine assembly links. It can cooperate to check the air supply distribution, filter sealing and operating status during the equipment debugging phase, helping the project establish stable test conditions so that the sampling results can reflect the actual performance of the equipment. Specific models can be viewed FFU clean air supply products , supporting equipment can be viewed EFU equipment end fan filter unit , please refer to the filter package. HEPA and ULPA filters。