Sampling High-Pressure Gases
This paper describes fundamental relationships between air or gas volume, pressure, and flow, and how these relationships affect particle counting. Also, a simple plumbing technique is offered as an alternative method for occasionally sampling high pressure inert gases.
When a sample volume of media (air or gas) moves from a high-pressure zone to a low-pressure zone, the movement causes changes in pressure and flow. In order to preserve accuracy, particle counters must passively measure these rapid changes with no indication. Mass flow controllers or pressure sensors inside the particle counter compensate for fluctuations in volume and pressure while ensuring a constant flowrate.
Volume, pressure, and flow have different units of measure, presenting a unique challenge for particle counters. Volume is the amount of space occupied by a three-dimensional object and is measured in cubic units (e.g., cubic feet). Pressure is the amount of force per unit area and is typically measured in pounds-per-square-inch (e.g., psi). Flow is the movement of media, such as a gas or liquid, during a specific time period commonly measured in cubic-feet-per-minute (cfm) or liters-per-minute (lpm).
Arguably, pressure has the most significant effect on the particle counter’s flowrate and the sample volume available for analysis. Particle counter design is dependent on the following principle: as pressure increases, volume decreases. Below is the equation summarizing this principle (Boyle’s Gas Law).
Applying Boyle’s Gas Law, we can predict that a cubic foot of air at sea level is not equal in composition to a cubic foot of air at 5000 feet. When compared to sea level pressure, atmospheric pressure at 5000 feet is about 18% less. To continue this example, if a volume of air at sea level contains 10 particles, the same volume at 5000 feet will contain about 6 particles. This result occurs because the particles disperse through the expanded volume that has increased about 189% at the higher altitude. If a flowrate control system is not in place, a particle counter adjusted to provide 1 cfm of air at sea level will provide significantly inaccurate particle counting data …
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