How to choose particle counter sampling flow rate?

The particle counter sampling flow rate is a parameter that needs to be determined in advance in the detection plan. The flow size affects the sampling time, sample volume and disturbance to the measured space. If the flow rate is too large, it may disturb the air flow in the small cavity; if the flow rate is too small, it will take too long to reach the required sampling volume.

When choosing traffic, you can't just look at the factor 'larger traffic is faster'. A comprehensive judgment should be made based on the required sample size, available testing time, and evaluation space conditions.

First confirm the required sample volume and particle size channel

The selection of sampling flow rate must first meet the sample size requirements of the classification standards. After the required sampling volume is determined, the relationship between flow rate and time is also determined.

Things that need to be confirmed include:

• Standard and minimum sampling volumes used;

• Target particle size channel;

• Sampling time of each sampling point;

• Instrument flow range.

Particle size channels and flow rates need to match. If the instrument cannot cover the target particle size, no matter how appropriate the flow rate is, it cannot meet the detection requirements. To determine the specific sampling volume, please refer to How does the particle sampling volume meet classification needs?。

The relationship between flow rate and sampling time

The sample volume is equal to the flow rate multiplied by the sampling time. The greater the flow rate, the shorter the time required to reach the same volume.

If there are many sampling points and each point requires a larger sampling volume, high-flow instruments can shorten the total test time. If the instrument flow rate is smaller, longer sampling time is required, which may affect acceptance arrangements.

Pay attention to the consistent units when calculating. The flow rate is often expressed in L/min, and the sampling volume is often expressed in L or m³. Please refer to the conversion method How to convert face wind speed and air volume?。

Sampling from small chambers may disturb airflow

In enclosed equipment cavities or local protected areas, sampling involves extraction of air. When the extraction flow rate is large, the chamber pressure or air flow path may be changed, affecting the measured state.

Things to note when sampling small cavities:

• After the air is extracted, where does the supplemental air come from?

• Whether the position of the sampling probe affects the critical area;

• Whether sampling changes the cavity pressure difference;

• Whether the flow rate needs to be reduced to reduce disturbance.

If the sampling itself changes the protection conditions, the measurement results may not represent normal operating conditions. For airflow planning in closed equipment cavities, please refer to How to plan clean air supply for closed equipment cavities?。

Applicable conditions for large flow instruments

Large flow instruments are suitable for occasions with large sampling volume requirements, limited test time, and large measured space. Please note when using:

• Whether the instrument range covers the target concentration range;

• Whether the length and diameter of the sampling pipe match;

• Whether the sampling probe fits the site space;

• Whether the instrument calibration is within the validity period.

Larger flow does not mean more suitable for all occasions. In space-constrained or airflow-sensitive locations, there may be a trade-off between flow and turbulence.

Instrument calibration and flow accuracy

The sampling flow rate needs to be accurate, otherwise the sampling volume calculation will be biased. The instrument should be calibrated regularly, and flow calibration records should be kept with the acceptance data.

When using, you should also confirm:

• The flow setting is consistent with the instrument display;

• There is no obvious leakage in the sampling pipeline;

• The sampling time is recorded accurately;

• The instrument status is normal.

If the flow rate is inaccurate, both the sample volume and particle count may deviate from reality.

Effect of sampling line on readings

Sampling tube length, bends, and materials can cause particle loss and affect readings. The longer the pipeline, the more significant the losses may be, and larger particles are of particular concern.

For specific impacts, please refer to Will sampling tubes that are too long affect particle readings? , to understand the impact of pipelines on particle transport. When selecting the flow rate, you should also consider whether the pipeline resistance is within the allowable range of the instrument.

Coordination with the number of sampling points and testing time

The sampling flow rate, number of sampling points and sampling volume together determine the total test time. When formulating a plan, consideration should be given to:

• Number of sampling points;

• Sampling volume at each point;

• Instrument flow;

• available testing time;

• How long can the system maintain a stable state?

If the test time is too long, the system status may change, affecting the representativeness of the results. To determine the number of sampling points, please refer to How to determine the number of cleanliness sampling points?。

Project judgment example

A certain project uses small flow instruments in a high cleanliness area, which requires a long sampling time. When arranging the test, it was found that the system status of some sampling points fluctuated during long-term sampling. Adjustment method: Arrange sampling during the stable period of the system, and reasonably allocate the order of each point. Improved data repeatability after adjustment.

Another project uses high-flow sampling in a small cavity of the equipment, and the cavity pressure changes, affecting the protective air flow. Adjustment method: reduce the sampling flow rate, increase the sampling time, and reduce the disturbance to the cavity air flow. After adjustment, the sampling conditions are closer to normal operating conditions.

These two situations illustrate that the sampling flow rate must be selected based on the sample volume requirements, test time and measured space conditions, and cannot just look at the flow rate.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

Sampling flow selection will affect test conditions and equipment operating status. Aili Industrial has collaborative capabilities in fans, motors, filters, control systems and complete machine assembly. It can cooperate to check air volume, pressure difference and air flow stability during equipment debugging and operation stages, helping projects arrange appropriate sampling methods in small cavities or sensitive areas to reduce sampling interference on protective air flow. Specific models can be viewed FFU clean air supply products , the supporting equipment can be viewed EFU equipment end fan filter unit , please refer to the filter package. HEPA and ULPA filters。