The Effects of Tubing Length on Airborne Particulate Sampling
Published: 7月 29, 2026
Summary
- Tubing length and configuration can impact air particulate sampling results.
- Tubing diameter and particle counter selection can improve sampling performance and reduce particle loss.
- Consult Beckman Coulter air particle counting specialists to help ensure compliance with current cleanroom monitoring regulations.
Abstract
Tubing length, tubing diameter, bends, and flow rate can affect particle transport between the sampling point and the counter.
In this study with Pharmagraph, we tested MET ONE particle counters with Hytrel tubing configurations from 0.5 m to 3 m, including straight runs, bends, and a vertical drop. The tested configurations showed little to no effect on 0.5 µm particle counting. For 5.0 µm particles, the MET ONE 6000 Remote Air Particle Counter series 1 CFM configuration with ¼ in. tubing showed substantial particle loss as tubing length and bends increased.
The MET ONE 3400+ Air Particle Counter series configurations with 3/8 in. tubing at 28.3 LPM and 100 LPM showed negligible average loss across the tested configurations, with wider confidence intervals at 5.0 µm. When 5.0 µm and larger particles are critical to continuous monitoring, users should assess the installed tubing configuration and reduce tubing length, bends, and vertical drops where practical.
Design of Experiment
Air particle counters are qualified using the requirements described in ISO 21501-4:2018 Determination of particle size distribution — Single particle light interaction methods — Part 4: Light scattering airborne particle counter for clean spaces. Section 6.2, Counting efficiency, describes the requirements for airborne particle counters to count particles within acceptable limits compared to a reference particle counter and was used for this test.
When running this test to calibrate particle counters, the unit under test and the reference counter are connected to a small chamber infused via short 0.5-meter-long tubing lengths with a measured concentration of NIST traceable reference particles near the sensitivity limit of the counter (0.5 µm for particle counters for pharmaceutical applications) and a size 50% to 100% larger than the sensitivity limit.
For this experiment, each unit under test was connected to the chamber through a defined tubing configuration. Counting efficiency measured through the various tubing configurations was compared to the control counting efficiency measured using the 0.5 m reference tubing configuration.
Results are presented as the percent performance against the reference standard, where 100% would indicate the tubing configuration measured the same counting efficiency as the 0.5 m reference tubing configuration.
Due to natural variations in these measurements, thirty one-minute-long runs were taken, with the average and standard deviation in results being used to determine the 95% confidence interval in the result.
Five-micron particles were tested along with 0.5 µm particles, as these are the sizes of interest for pharmaceutical applications.
When bends were tested, these were fixed to the minimum 4-inch (100 mm) bend radius recommended by Beckman Coulter Life Sciences.
MET ONE 6000 Air Particle Counter Series Results
The MET ONE 6000 instruments have a ¼-inch (6 mm) barb inlet for 1 CFM (28.3 LPM) operation required for pharmaceutical applications. The lower flow 0.1 CFM (2.83 LPM) versions of the MET ONE 6000 Air Particle Counter series were not evaluated as they are not appropriate for pharmaceutical applications and are typically used in less clean industrial environments where particle loss in tubing isn't as critical. Industry standard Hytrel lined tubing with an inside diameter of ¼ inch (6 mm) was used. The following results were observed along with the 95% confidence interval of the measurements (Table 1).
| Size | 2 m Straight Tubing | 3 m Straight Tubing | 3 m with 2 Bends | 3 m with 4 Bends | 3 m with 1 m Drop |
|---|---|---|---|---|---|
| 0.5 µm | 92% ± 10% | 98% ± 8% | 98% ± 20% | 93% ± 14% | 98% ± 5% |
| 5.0 µm | 76% ± 20% | 37% ± 12% | 21% ± 13% | 16% ± 7% | 15% ± 5% |
Table 1. Relative Counting Efficiency by Tubing Configuration with a MET ONE 6000 Air Particle Counter
Values represent counting efficiency relative to the 0.5 m reference configuration. A value of 100% indicates equivalent performance to the reference configuration.
For 0.5 µm particles, average relative counting efficiencies ranged from 92% to 98% across the tested tubing arrangements.
For 5.0 µm particles, substantial particle loss was observed as tubing length and bends increased. Average counting efficiency decreased from 76% with 2 m straight tubing to 37% with 3 m straight tubing, and to 16%–21% for the 3 m configurations containing bends. These results indicate that tubing length and bends contributed to particle loss in the MET ONE 6000 Air Particle Counter 1 CFM configuration using ¼ in. tubing.
MET ONE 3400+ Air Particle Counter Series Results
The MET ONE 3400+ series uses 3/8-inch (9.5 mm) tubing at flow rates ranging from 28.3 LPM to 100 LPM. Testing was performed at 28.3 LPM and 100 LPM to characterize the tubing loss performance. Industry standard Hytrel tubing with an inside diameter of 3/8-inch (9.5 mm) was used.
MET ONE 3415+ Air Particle Counter (28.3 LPM)
| Size | 2 m Straight Tubing | 2 m with 2 Bends | 3 m Straight Tubing | 3 m with 2 Bends |
|---|---|---|---|---|
| 0.5 µm | 97% ± 3% | 93% ± 3% | 95% ± 3% | 94% ± 3% |
| 5.0 µm | 98% ± 16% | 92% ± 17% | 93% ± 16% | 96% ± 20% |
Table 2. Relative Counting Efficiency by Tubing Configuration with a MET ONE 3415+ Air Particle Counter
Values represent counting efficiency relative to the 0.5 m reference configuration. A value of 100% indicates equivalent performance to the reference configuration.
Regardless of the particle size, tubing length had negligible impact on the counting efficiency observed. There was greater variation in the results at 5.0 µm, but the average was consistent with the reference results.
MET ONE 3445+ Air Particle Counter (100 LPM)
| Size | 2 m Straight Tubing | 2 m with 2 Bends | 3 m Straight Tubing | 3 m with 2 Bends |
|---|---|---|---|---|
| 0.5 µm | 100% ± 3% | 100% ± 4% | 96% ± 3% | 100% ± 6% |
| 5.0 µm | 100% ± 32% | 100% ± 36% | 100% ± 33% | 100% ± 32% |
Table 3. Relative Counting Efficiency by Tubing Configuration with a MET ONE 3445+ Air Particle Counter
Values represent counting efficiency relative to the 0.5 m reference configuration. A value of 100% indicates equivalent performance to the reference configuration.
Again, regardless of the particle size, tubing length had negligible impact on the counting efficiency observed. As with the 28.3 LPM results, greater variation is observed with 5.0 µm particles in all configurations, likely due to the experimental setup and the flow ratio between the reference unit and the tubing configuration instrument.
Conclusion
Tubing configuration should be assessed during system design and qualification.
The effect depends on particle size, tubing diameter, flow rate, tubing length, bends, and vertical drops and will impact any air particle counter.
In these tests, 0.5 µm particle recovery was largely unaffected by tubing configurations up to 3 m.
For 5.0 µm particles, the MET ONE 6000 air particle counter series 1 CFM configuration with ¼-inch tubing showed increased loss as tubing length and bends increased.
The MET ONE 3400+ Air Particle Counter configurations using 3/8-inch tubing at 28.3 LPM and 100 LPM showed negligible average loss across the tested configurations.
When assessing overall airborne particle concentrations, 0.5 µm particles are typically present at higher concentrations than larger particles and therefore provide greater statistical sensitivity to changes in cleanroom performance. ISO 14644-1:2015 Appendix E.2 notes that the concentration limits at larger sizes go down exponentially; conversely, it is expected in a cleanroom of a certain class that exponentially more particles will be expected at smaller sizes.
Based on these results, minimizing tubing length is recommended when monitoring 5.0 µm particles. ISO 14644-21:2023 identifies sampling runs greater than 1 m as configurations that warrant review for particle losses.
Larger particles are more susceptible to sampling losses caused by settling, impaction, and tubing geometry, which is reflected in ISO guidance on remote sampling. When considering counting efficiency ISO 21501-4:2018+1:2023 specifically states:
… depending on the application requirements, a tolerance of ±10 to ±30% is recommended at a nominal particle diameter of 5 µm.
ISO 14644-21:2023 Figure 5 shows similar results of particle loss at 3 meters for 5.0 µm particles due to the length of tubing. As a result, it contains these recommendations when measuring particles through tubing lengths greater than 1.0 meter:
Particle losses in these applications [≥5.0 µm in tubing lengths ≥1 meter] can affect the quality of measurement. A review of the system installation is performed to fully evaluate the impact of particle losses.
This review and assessment of the system installation for longer tubing lengths should contemplate ways to reduce the length of the tubing, use of stainless-steel tubing for as much of the run as possible, apparatus to fix bend radii, and quantitative reduction of the applicable limits to account for the possible particle loss.
Figure 1. Control configuration with 50 cm of tubing from dispersion chamber to test instrument.
Figure 2. 3 meter straight tubing configuration under test.
