Straightening out the cost of inaccurate particle counting: Tubing matters

There are different factors that influence the accuracy of light-scattering particle counting within the cleanroom. There are variations between different instruments in terms of their collection efficiency (such as the effect of coincidence losses within the sensor), ensuring that representative areas are sampled, and capture areas (with capture area being dependent upon the rate of sampling – the lower the sampling rate then the smaller the capture area becomes). Another important consideration is with the tubing.

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The tube used to transport airborne particles to be counted (through a laser in the sensing zone of the counter in order to generate a detectable pulse) represents a weak point in the design of any particle counter in terms of the accuracy of what was actually drawn in by the counter into the tube and what is actually counted and recorded by the counter software.

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There are several factors that influence the loss of particles in the tube. These are:

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Brownian diffusion

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Brownian diffusion is the characteristic random wiggling motion of small airborne particles in still air, resulting from constant bombardment by surrounding gas molecules. Brownian diffusion is the product of Brownian motion, which is the random motion of particles suspended in a fluid (a liquid or a gas) resulting from their collision with the fast-moving atoms or molecules in the gas or liquid. In terms of the differences between the concepts, Brownian motion is the movement of the particle not in a fixed direction while in diffusion the particle moves in the direction of high concentration to low concentration. This factor affects particles smaller than 0.1 μm (1).

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Sedimentation

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Particles of a larger size are more likely to be removed from the airstream and become affixed to tubing, as are particles that become larger through coagulation.

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Inertial deposition

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Inertial impaction, deposition and gravity are the main deposition processes for large aerosol particles. The particle motion and deposition rate is also influenced by flow velocity. This factor affects particles lager than 1.0 μm (2). To impact a surface, large particles must have enough inertial momentum to overcome the drag force of the gas flow on the particles. Smaller particles will tend to succumb to the drag effect and follow the air streamlines. This factor becomes more commonplace when there are bends in the tubing (see below).

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Electrostatic attraction

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Electrostatic attraction is an applied external force upon the streamline. This occurs when particles are drawn to the wall of the tube and hence the tube material is of importance. Evaluations conducted on tube materials for particles in the range 0.3 to 1.0 μm, targeting no more than a 5% loss, have evaluated the transportation efficiencies of different materials. By standardizing the bore (10mm smooth bore) and controlling the air velocity and counting rate, the following outcomes were obtained (3):

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1.?????Stainless steel is the best performing material.

2.?????Conductive plastic lined PVC was the second best performer.

3.?????Polyester lined PVC was the third best performer.

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Tubing length and orientation

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The longer the tubing length, then the greater the possibility of falls in the line based on one or more of the above listed factors occurring is.

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With the orientation of the tubing, the important operational aspect is with maintaining a Reynolds number of 5,000 to 10,000. The Reynolds number is a dimensionless number used to assess fluid flow. Laminarity tends to occur below 2,000 and turbidity above 4,000 (and a state of uncertainty and variability in-between these values). The Reynolds number is a product of velocity, tube diameter, air density and air viscosity. Generally, the large the tube diameter, the lower the Reynolds number becomes (4). Excessive curvature increases the possibility of inertial impaction and diffusion occurring.

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Any radius curvature will affect the airflow through the tube and the general advice is for the radius curvature not to be below 100mm (10cm) (5).

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Residence time

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The shorter the time period that a particle spends in the tube then the more likely it is to be counted. The length of time will be partly affected by the tube length and orientation. Targeting 5 seconds or less is optimal.

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Pressure loss

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A loss of pressure within the sample tubing will affect the flow (many counters are based on one cubic foot per minute / 0.03 cubic meters per minute). The degree of pressure loss partly relates to the ability of the particle counter pump to compensate for any variations. With flow rate, 1 cfm of above is required if accurate particle size distributions are to be obtained for larger size particles.

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The above factors cannot be considered in isolation and a cumulative effect leading to greater accuracy or inaccuracy can occur. The accuracy of the count is also influenced by the time period (with longer periods tending to be more accurate) and the by increasing the sample flow rate.

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Other factors

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Other influencing factors include thermal precipitation, eddy impaction, and agglomeration. It is additionally important that particle counter tubing is changed at regular intervals (such as annually). All particle counter tubing will, over time, accumulate particles, particularly where particle counters are used for continuous monitoring. A phenomenon which can arise is the sudden release of particles (previously suspended on tubing walls) which may lead to an unusually high count or series of counts. Where issues with particle tubing are suspected, tubing should be changed more frequently.

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Summary

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Any or all of the above described factors can cause the particles to reach the wall of the tube where they may become trapped (6).

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How much of this matters? To improve the accuracy of particle counting we should ensure that the materials we used for tubing are made of suitable material in order to minimize particle loss. We should place limitations on the length of the tubing (EU GMP Annex 1 recommends 1 meter maximum) and avoid excessive curvature - bends and contractions of tubing (such as ensuring that if curvature needs to occur it is wider than 10 centimeters).

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References

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1.?????Douglas W. Cooper (1986) Particulate Contamination and Microelectronics Manufacturing: An Introduction, Aerosol Science and Technology, 5:3, 287-299, DOI: 10.1080/02786828608959094

2.?????Dunnett, S. J., and Clement, C. F. (2012). Numerical Investigation into the Loading Behavior of Filters Operating in the Diffusional and Interception Deposition Regimes. J. Aerosol Sci., 53:85–99

3.?????Hinds W C 1999 Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles (New York: Wiley)

4.?????Yung-sung Cheng, Chiu-sen Wang. (1975) Inertial deposition of particles in a bend, Journal of Aerosol Science, 6 (2): 139-145, https://doi.org/10.1016/0021-8502(75)90007-5

5.?????Von der Weiden, S.-L., Drewnick, F., and Borrmann, S.: Particle Loss Calculator – a new software tool for the assessment of the performance of aerosol inlet systems, Atmos. Meas. Tech., 2, 479–494

J. Sramek, J. Sperka and R. Jankovych, The measurement system for the calibration of particle counters, 2016 17th International Conference on Mechatronics - Mechatronika (ME), Prague, Czech Republic, 2016, pp. 1-5