Multiplexing air samples from various sample points into a single particle counter,such as a Lasair® III, offers the ability to monitor small particle sizes at a reasonable cost. Using a multiplexed system, also called an Aerosol Manifold System, it is possible to monitor 0.1 micron particles with a 1 CFM sample rate for about the same cost per point as using continuous particle counters at 0.3 microns with 0.1 CFM sample rate. The combination of higher sensitivity and higher sample volume provide much better data while the system is sampling from a particular location. Additional cost savings can be realized using the multiplexed system if lower sensitivity is acceptable. The disadvantage is that the duty cycle is less than 100% at any particular location, so it is possible to miss episodic or time dependent events. Two common concerns about manifold systems are particle loss during transport, through the tubing, or significant delays in particles reaching the particle counter.

Successful transport of sample air through tubing necessitates both a thoughtful design and care in installation. To assure successful transport, Particle Measuring Systems uses a non-contaminating valveless design that eliminates the need to purge between samples and improves the sampling duty cycle. Other key parameters that must be controlled are transport velocity, turbulent flow, tubing bend radius, triboelectric charging, and isokinetic sampling. It is generally accepted that a Reynolds number of 2200 is the minimum required for turbulent flow. This corresponds to about 1 CFM flow in tubing of 3/8" internal diameter, which is the standard Particle Measuring Systems ID.

Particle Measuring Systems offers the Aerosol Manifold II as a solution. These manifolds will pull 3 CFM through all tubes simultaneously, with a maximum recommended length of 125 feet. The length of time it takes for a sample to travel from the sensor to the particle counter is dependent on both the tube's material and length. Based on general experience, tubing choice is in this order:

• No. 1 - Stainless Steel
• No. 2 - Bev-a-line
• No. 3 - Polyester (as Polyurethane)
• No. 4 - Polyester Lined Vinyl
• No. 5 - Copper
• No. 6 - High Density Polyethylene
• No. 7 - Glass
• No. 8 - Teflon

It is important to note that the desirability decreases as bulk resistivity increases. Plasticized PVC (Tygon) is not generally recommended because of its slow plasticizer release to the environment. Selection of smaller tubing for the longer lengths is a viable option, however the pressure drop increases with decreased diameter.

The amount of time it takes a sample to reach the particle counter at various lengths and flowrates through basic calculation, using the Particle Measuring Systems standard 3/8" inner diameter tubing, is shown in Figure 1 (download pdf for figure).

Particle Measuring Systems conducted a test using the AM-12 manifold with a Siemens' vacuum pump to determine particle loss in 3/8" ID BEV-A-LINE tubing. Lengths of 10 feet, 25 feet, 75 feet and 125 feet were connected to the manifold. A particle counter was used to sample the air at the tubing input point and at the output of the AM-12 manifold. The results are shown in Figure 2 for four particle sizes.

Note that losses for particle sizes less than 1 micron are negligible. For 5 micron particles, losses are appreciable.

Lasair® is a registered trademark of Particle Measuring Systems, Inc.

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Last Updated 10/97