Additional cost savings can be realised using the multiplexed system if lower sensitivity is acceptable. The disadvantage is 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, PMS uses a noncontaminating 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 PMS standard ID.
Figure 1: Calculated time through 3/8 inch tubing
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
The amount of time it takes a sample to reach the particle counter at various lengths and flow rates through basic calculation, using the PMS standard 3/8” inner diameter tubing, is shown in Figure 1. Note that losses for particle sizes less than 1µm are negligible. For 5µm particles, losses are appreciable.
Figure 2: Particle loss rates in Bev-a-line XX tubing, 3/8 I.D., using PMS 3 CFM aerosol manifold flow rate