Introduction

This paper describes fundamental relationships between volume, pressure, and flow, and how these relationships effect particle counting. Also, a simple plumbing technique is offered as an alternative method for occasionally sampling high-pressure inert gases.

Sample Pressure, Volume, and Flowrate

Accurate particle counting must account for the effects of changes in atmospheric pressure and air flow increases as sample moves from a high-pressure zone to a lower-pressure zone. When air flow increases, the air volume passing through the system also increases.

Volume, pressure, and flow have different units of measure and are, therefore, distinct. Volume is the amount of space occupied by a three-dimensional object as measured in cubic units. Pressure is a measure of force per unit area, and flow is the movement of a volume of material during a period of time, measured, in this case, in cubic feet per minute (CFM) or cubic liters per minute (LPM).

The casual user may not understand that pressure has a significant effect on the sample's flow rate and, therefore, the volume of sample the particle counter "sees."

Boyle's Law states that as pressure increases, volume decreases.

P1V1 = P2V2, (where P = pressure and V = volume)

Therefore, a cubic foot of air at sea level is not equal to a cubic foot of air at 5000 feet above sea level, where the lower atmospheric pressure allows the volume of air to expand compared to the same volume of air at sea level.

The atmospheric pressure at 5000 feet is about 18% less than that at sea level. As a result, a volume of air at sea level containing 10 particles will contain about 6 particles at 5000 feet. This phenomenon is due to particle dispersion through the expanded volume, which has increased about 189% at the higher altitude.

If a particle counter's pump is adjusted to move one CFM of air at sea level, inacurate particle counts will result if the particle counter is used at a much higher altitude. The lower ambient pressure will result in a proportionally less volume.

Flowrate Control

Because volumetric corrections for ambient air pressure is necessary, particle counters usually have flow-rate controls that change pump speed or allow some airflow to bypass the sampling chamber.

These controls are adjustable through software settings, mechanical adjustments, or automatic regulation based upon ambient pressure. Each method is only a reasonable approximation of accurate flow measurements, that respond to ambient pressure changes (equal pressure on either flow side of the pump) due to altitude changes.

So, moving a particle counter adjusted for use at sea level (14.69 psi) to an altitude of 5000 (12.23 psi) requires that its altitude control be adjusted.

NOTE: Particle Measuring Systems' LASAIR® II is the only particle counter that recognizes an over-pressure condition and then rejects the sample. Other particle counters ignore an over-pressure environment and allow the sample to proceed, even though the reported particle counts and particle sizing will be inaccurate.

LASAIR^® II Particle Counter (Download this paper for all tables and figures) (587.0 KB)

Sampling gases at pressures greater than sea level, such as 40-100 psi, are more challenging. Connecting pressurized air or gas to a particle counter can defeat its flow-metering system. High Pressure Diffusers (HPDs) were developed to correct this problem. HPDs reduce the pressure of the gas to about that of sea level by allowing some of the air/gas to diffuse. The particle counter's flow-metering system can then function as designed.

Although the HPD is the best solution when sampling high-pressure gas, some customers can use a less-costly pressure-release valve.

This pressure-release "T" fitting easily connects to most high-pressure air/gas lines and provides a solution for customers who sample their air/gas line less frequently. Because the valve discharges a slightly higher volume of air than normally required for monitoring, this option is more costly in terms of air/ gas loss. If high-pressure air/gas sampling is more frequent, a HPD may offset the costs of air/gas loss.

Conclusion

Altitude has a significant effect on particle counters and must be considered when monitoring with sensitive equipment. Also, if gas at high-pressure is being sampled, the user should use either a High Pressure Diffuser (accurate, but too expensive for short-term use-best for long term use) or a pressure-release "T" valve (accurate and inexpensive for short-term use).

(Download this paper for all tables and figures) (587.0 KB)

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

Author, Steven Kochevar, Particle Measuring Systems

Published: Sept 2006