Will too long a sampling tube affect particle readings?

The sampling tube is the connection channel between the particle counter and the sampling point. The air sample enters the sampling tube from the sampling point and reaches the instrument after traveling a certain distance. If the pipeline is too long, has too many bends, or is made of improper material, the particles may be deposited, collided, or retained during the transportation process, and the number will be reduced when they reach the instrument, resulting in a low reading.

Low readings can easily be misjudged as environmental improvements. When judging the sampling results, the impact of pipelines on particle transport needs to be considered at the same time.

What changes in the pipeline is the sample reaching the instrument

During the transportation process of air samples in the sampling tube, particles may be lost due to the following reasons:

• Gravity settling: larger particles settle in horizontal pipe sections;

• Inertial collision: When the airflow passes through the elbow, the particles hit the pipe wall;

• Electrostatic adsorption: charged particles are adsorbed on the pipe wall;

• Diffusion deposition: tiny particles diffuse and deposit near the pipe wall;

• Residual contamination: Original particles in the tube are released, interfering with subsequent sampling.

These losses cause the number of particles reaching the instrument to be lower than the actual concentration at the sampling point. Low readings do not necessarily indicate an improvement in the environment and may be caused by pipe damage.

The larger the particles, the more obvious the pipeline loss will be.

Particles of different sizes behave differently in pipelines. Larger particles have greater inertia and are more likely to collide and deposit at elbows and pipe walls; smaller particles may be deposited due to diffusion and electrostatic effects.

Therefore, the effect of sampling tubes on larger particles is usually more pronounced. If the detection target contains larger particle sizes, tubing length and bends require special attention. To determine the sampling particle size channel, please refer to How does the particle sampling volume meet classification needs?。

Effect of pipe length and number of bends

The longer the pipe, the longer the particles stay in the pipe and the greater the chance of deposition. As the number of bends increases, inertial collisions and local turbulence also increase losses.

The layout of sampling pipes should follow the following principles:

• Keep the pipeline length as short as possible;

• Reduce bends and unnecessary turns;

• Avoid long horizontal straight pipe sections;

• Keep the pipe diameter consistent and avoid diameter changes.

If site conditions limit the ability to shorten the pipeline, check with the requirements of the instrument and test method to evaluate whether the sampling plan needs to be adjusted. The selection of sampling flow rate will also affect pipeline loss. Please refer to How to choose particle counter sampling flow rate?。

Pipe diameter and flow rate need to match

If the pipe diameter is too small, the flow rate and resistance in the pipe will increase, which may lead to increased particle collision; if the pipe diameter is too large, the flow rate will decrease and the particles will settle more easily. The pipe diameter should match the flow rate of the instrument so that the flow rate in the pipe is within an appropriate range.

For specific pipe diameter and flow requirements, please refer to the instrument instructions and test methods. The sampling flow rate and recommended pipe diameter of different instruments may be different and cannot be applied uniformly.

Residual contamination in the tube may cause interference

After the sampling tube is used, particles may remain in the tube. If the lines are not cleaned thoroughly, residual particles may be released during subsequent sampling, causing high or fluctuating readings.

Pay attention to the following when cleaning and maintaining pipelines:

• Clean or replace pipelines as required by the instrument;

• Check the pipeline status before and after sampling;

• Avoid long-term exposure of pipelines to polluted environments;

• Record pipeline usage and maintenance.

Prioritize unnecessary conveying paths

The most direct way to improve the impact of pipelines is to shorten the conveying path. Can:

• Place the instrument as close to the sampling location as possible;

• Reduce unnecessary extension tubes;

• Use appropriate sampling probes to reduce pipeline length;

• Evaluate piping impacts when fixing online systems.

If it cannot be shortened, check whether the transmission loss is within the acceptable range rather than adding unverified correction values ​​arbitrarily. The correction value needs to be based on evidence and cannot be estimated based on experience.

Other effects of pipelines on sampling conditions

In addition to particle loss, piping can also affect:

• Sampling response time;

• Stability of sampling flow;

• Representativeness of sampling points;

• Repeatability of instrument readings.

These effects may accumulate when sampling over long periods of time or at multiple points. The test plan should be considered comprehensively.

Relationship to sampling location and test plan

Sampling tube layout is related to sampling location selection. If the sampling points are spread over a larger area, the pipeline may be extended accordingly. When developing a plan, balance sampling point distribution and pipeline length.

When determining the number of sampling points, the impact of pipelines on the test should also be considered. Related methods can be found in How to determine the number of cleanliness sampling points?。

Project judgment example

After a longer sampling tube was replaced at the same location in a certain project, the readings dropped significantly. The investigation found that the particle transmission loss increased after the pipeline was extended. Adjustment method: shorten the pipeline, reduce elbows, and restore the reading after resampling. The differences in the data indicate that pipeline length has an impact on the results.

Another project used a fixed in-line sampling system with longer lines. After the assessment, it is confirmed that the transmission loss is within the acceptable range, and the system status is confirmed through regular comparison. Adjustment methods: Establish pipeline maintenance and comparison plans to ensure long-term data comparability.

These two situations illustrate that if the sampling pipe is too long, it will affect the particle readings. Priority should be given to shortening the pipeline or checking the transmission loss. The reduction in readings cannot be directly used to determine environmental improvement.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

Sampling tube layout can affect the accuracy of particle readings. Aili Industrial has collaborative capabilities in fans, motors, filters, control systems and complete machine assembly. It can cooperate to check the sampling location, pipeline layout and air flow stability during the equipment debugging and operation stages, helping the project reduce the impact of pipelines on measurement results and making the data closer to the actual state. 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。