Particle counting evolves into a science
by Tammy Wright
In spite of what many think, there`s more to counting particles than just figuring numbers. Over the last 30 years, industry sources claim it`s been evolving into a science that grounds instrumentation in processes and applications.
Particle counters allow process engineers in multiple industries to analyze particles by size and quantity, thereby maintaining the cleanliness levels required for productive manufacturing environments.
“Anybody can count, but counting particles involves advanced physics and application knowledge. If you are aware of that, reliable instruments with trustworthy results can be built,” says Dr. Holger Sommer, president of Art Instruments, a particle counter manufacturer in Grants Pass, OR, and a 25-year industry veteran who holds 14 particle counting instrumentation patents.
According to particle counter manufacturer Hiac-Royco (Silver Spring, MD), some uses for the instrument include:
documenting the effectiveness of cleaning operations;
checking critical products;
replacing the subjective statement of “no visible particles” with a quantitative number;
settling disputes.
Dr. Sommer says particle counters have been used mainly for trending of contamination levels. “Their value is in giving process engineers a quick indication of whether or not they are within contamination limits,” he explains, noting that comparisons of absolute particle counting results between similar environments are still difficult with current technology because of a lack of instrument standardization.
Today, optical particle counters that incorporate light scattering methodology are reportedly the primary tools used in cleanrooms to count and size particles in air, gases and liquids. This technology detects particles by measuring the light scattered by a single particle as it passes through a light beam.
“For the moment, sensitivity is pushed to its limit,” says Dr. Sommer. “The most sensitive instruments currently size and count 0.05-micron or 50-nanometer particles in air or deionized water.”
Particle counting technology has been stable for some time, according to manufacturers, and could use a fresh approach — one that goes beyond classic light scattering. The solution, however, seems to be a matter of opinion.
For example, Dr. Sommer believes accurate contamination representation in cleanrooms may not be (related to) sensitivity. He says, “more representative measurement statistics can only be obtained if higher number counts are associated with the detection of smaller particles.”
“At 0.01 micron,” Dr. Sommer explains, “the number of particles counted is reduced because other filtration mechanisms [of HEPA filters] take over, removing smaller particles with greater efficiency than larger particles.”
Others disagree with that logic, saying current technology could be improved by further refining it to detect the smallest sized particles possible.
“Particle counters monitor the condition of room air to make sure filters are doing their job and to determine in-room contamination,” counters Joel Sorem, technical marketing manager at Specific Scientific Instruments, a particle counter manufacturer in Grants Pass, OR.
Whether users are manufacturing wafers or doing pharmaceutical filling, Sorem says they “still want to know the room contamination level after the air has been filtered because they still have to determine what`s being generated by the machinery, processes and people in the room.”
Both he and Dr. Sommer agree, though, that innovation in particle counting will focus on molecular contamination, which involves particles and vapors. Other advancements include a move toward particle characterization.
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Above: Particle counter size standards. (Duke Scientific)
Right: APC Plus. (Biotest Diagnostics)
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Abacus 301. (Particle Measuring Systems)