On the surface, a particle counter is a simple device. You press a button and results tumble out. But whenever you’re measuring anything small, it’s important to educate yourself on the technology employed and understand the benefits and limitations. No measurement is absolute―all are relative to the measuring technique. For example, if we measure particles with a scanning electron microscope, it would not be a surprise to get slightly different results. We might even get vastly different results at certain sizes, because the produced response differs from the particle counter’s. In this blog two-part blog series, we look at the ingredients of how particle counters work.
Particles exist in a tremendous range of sizes, shapes and compositions; for example, inside a cleanroom we could measure flakes of skin, small pieces of silicon or metal, or fungal spores. The sources can be very broad.
Particle counters often measure in micrometers (i.e., microns), which is a millionth of a meter or a thousandth of a millimeter. State of the art semiconductor facilities measure in nanometers, a millionth of a millimeter. In contrast, pharmaceuticals, healthcare, and medical device industries are concerned more with 0.5 and 5 micron particles. To put size into perspective:
- Visible particles are around 50 microns (e.g., a human hair would be 50 to 150 microns).
- Non-visible particles such as bacteria are between sub 1 and 15 microns.
- Plant spores and pollens fall between the visible and non-visible at around 10 to 100 microns.
Most particles are non-uniform in structure, and this poses a question: How is the size qualified? If you ask those working in industry, some would say it’s based on the longest length, the volume, or an equivalent hole size through which a particle could pass. There would be many different answers and none of them would be wrong, provided they are qualified.
Particle counters size particles by matching a signal response generated by the contaminant particle to an equivalent size of latex sphere. Users often look at the size and number distribution reported by an instrument and treat this data as absolute without recognizing that there are a number of operating variables, including but not limited to:
- Physical properties
- Refractive index
These subtle details play a part in the size indicated, and therefore the size channel in which the particle is counted.
Calibration Reference Standard using Latex Spheres
Technicians calibrate particle counters for size by sampling monodispersed (i.e., single size) polystyrene latex spheres (PSLs). These PSLs are nebulized into HEPA/ULPA grade filtered airflow in a controlled setting. They adjust the instrument for each test particle size in order to generate a calibration curve within the instrument. The instrument references the sizing response from environmental particles as an equivalent to the ideal shape of a latex sphere and counted in one particular size range or channel. As a result, any false sizing could affect not only the stated size, but the size channel the particle is assigned to and the total number distribution.
What’s Next in How Particle Counters Work?
Now you know what particle counters need to count, the physical properties of particles that need to be accounted for, and how their calibration determines accuracy. It’s time to learn about the technology behind those generated counts! Join us next time for a high-level overview of lasers, photodetectors, and signals.