How does the particle sampling volume meet classification needs?

Particle sampling volume is a key parameter in cleanliness classification testing. If the sampling volume is insufficient, the number of particles measured is too few, and the results lack statistical representativeness; if the sampling volume is too large, the test time will be extended, which may affect production arrangements. The sampling volume is not set arbitrarily and should be determined based on the adopted classification standard, target particle size and instrument flow rate.

The particle concentration in high cleanliness areas is very low, and only a few or even zero particles may be measured in the same sampling volume. In this case, whether the sampling volume is sufficient directly affects the result judgment. The following will start with the classification requirements, instrument flow, sampling time, and low concentration statistics.

Sampling volume is determined by grading standards

Different classification standards have different requirements for sampling volume. The particle sampling volume usually includes the minimum sampling volume and the minimum sampling time. Before determining the sampling volume, you should confirm:

• The name and version of the adopted standard;

• Target cleanliness level and particle size;

• Minimum sampling requirements specified in the standard;

• Sampling time and volume for each sampling point.

If the standard specifies a minimum sampling volume, the sampling volume should meet that requirement. The sampling time or volume cannot be set arbitrarily based on experience. Standard basis and evaluation scope can be referred to How to determine the number of cleanliness sampling points? , first clarify the overall framework of the test plan.

Sampling volume is related to particle size limits

Sampling volume requirements are related to target particle size and concentration limits. Different grades and particle sizes have different limits, and the corresponding minimum sampling volumes may also be different.

High cleanliness classes have lower concentration limits and require larger sampling volumes to obtain sufficient statistical samples. If the target particle size is smaller, the sampling volume may also need to be adjusted. When making judgments, calculations should be made according to standard regulations and a single volume cannot be applied uniformly.

Instrument flow determines the sampling time

The sample volume is equal to the instrument flow rate multiplied by the sampling time. After the instrument flow rate is determined, the sampling time is determined by the required sampling volume.

For example, with an instrument flow rate of 28.3 L/min, it takes approximately 35 minutes to obtain a sample volume of 1 m³ (1000 L). If the flow rate of the instrument is larger, the time required to reach the same volume is shorter. Please refer to the specific conversion How to convert face wind speed and air volume? , first confirm that the units of flow rate and volume are consistent.

When selecting the instrument flow rate, the sampling volume requirements and test schedule should be taken into consideration. Small flow instruments require longer sampling times, which may affect the acceptance progress. For the selection of particle counter sampling flow rate, please refer to How to choose particle counter sampling flow rate?。

Statistical representativeness at low concentrations

When the particle concentration in high cleanliness areas is low and the sampling volume is insufficient, the number of particles measured may be very small. At this time, the results are greatly affected by random fluctuations, and the statistical representativeness is insufficient.

To determine whether the sampling volume is sufficient, attention should be paid to:

• Whether the number of particles measured is sufficient to support classification judgment;

• Whether the minimum sampling volume specified in the standard is met;

• Are the results reproducible?

• Whether the system status is stable during sampling.

If the reading is zero, it cannot be directly interpreted that the area is permanently free of contamination. It can only mean that no particles were measured under the sampling conditions.

Sampling time and system stability

When the sampling time is long, the system state needs to remain stable during the sampling period. If people come in and out, equipment starts and stops, or air volume changes during sampling, the results may be affected.

When arranging a sampling plan, you should confirm:

• The system state remains stable during the sampling period;

• There were no abnormal operations during the sampling period;

• Coordinate sampling time with production schedule;

• Sampling records contain run status.

To determine the operating status of the equipment during acceptance, please refer to Which equipment should be in operation during acceptance?。

The relationship between sampling volume and number of sampling points

The sampling volume and the number of sampling points jointly affect the total test time. If there are many sampling points and large sampling volume, the test time will increase accordingly.

When developing a test plan, balance the number of sampling points, sample volume, instrument flow, and available time. If the instrument flow rate is limited, the sampling sequence and timing may need to be rationally arranged. To determine the number of sampling points, please refer to How to determine the number of cleanliness sampling points?。

Record sampling volume and raw data

Sampling records should contain:

• Instrument flow and sampling time;

• Sampling volume at each point;

• Raw particle count;

• particle size channel;

• The running status during sampling.

These records are used to determine whether the results meet the standard requirements. If the original data is missing, it is impossible to subsequently check whether the sampling volume is sufficient.

Project judgment example

In a certain project, a small flow instrument was used in the high cleanliness area, the sampling time was short, and the number of particles measured was zero. The review found that the sampling volume did not meet the standard requirements and the results were not representative enough. Adjustment method: Extend the sampling time and retest after meeting the minimum sampling volume requirements. The adjusted data meets the needs of classification judgment.

Another project calculated the sample volume as per standard, but the system status fluctuated during the test. Adjustment method: Rearrange sampling after the system is stable and record the operating status. Improved data repeatability after adjustment.

These two situations illustrate that the sampling volume must be determined according to standards and particle size limits, and the system must remain stable during the sampling period so that the results can be used for classification judgment.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

Whether the sampling volume is sufficient directly affects the validity of the classification test results. Aili Industrial has collaborative capabilities in fans, motors, filters, control systems and complete machine assembly. It can cooperate to check air volume stability, filter status and operation settings during the equipment operation and debugging stages, helping the project maintain stable test conditions during the sampling period and reducing repeated tests due to system fluctuations. 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。