Particle Counter Overview
Particle counters allow detection of micro-particles moving through the air. The most common applications include cleanroom contamination tests, industrial aerosols detection, and industrial filter testing. For a general overview, about how optical particle counters work here.
Why Do Particle Counters Need Calibration?
Prior to 2007, there were no standards defining calibration and performance of optical particle counters within ISO 14644-1. However, comprehensive classification via non-ISO standards did exist and had been employed by most major manufacturers.
- IEST-RP-CC-014 provides actual methods to perform the calibration.
- JIS B 9921: 1997 is a Japanese standard which comprehensively deals with OPC design performance, specifically for counting efficiency.
Optical Particle Counters (OPCs) allow real-time characterization of airborne particles (aerosols) between 0.3 – 20 microns. The calibration procedure identifies inaccurate measurement and reduces variation between different instruments. ISO 21501 ensures that optical particle counter instruments will size and count particles correctly by providing specific calibration and verification techniques.
Light scattering particle counters (LPC) detect particles suspended in air through pulse height analysis (PHA) which categorizes photons witnessed by solid-state photodetectors. Light energy is then converted into electrical signals.
How Often Does a Particle Counter Require Recalibration?
An optical particle counter is tested by comparing against a standard aerosol electrometer and often cross-referenced using a previously calibrated particle meter with a higher sensitivity. All calibration of airborne particle counters is specified under ISO 21501-4: 2007. For ISO 14644 compliance, all particle counting equipment requires a calibration certificate. The frequency of testing is often 6 months for pharmaceutical applications and 12 months for general manufacturing.
Test results with unexpectedly, wavering results indicate a need for retesting and establishment of a long-term control plan. Without clear causes culprits or increases in activity, a shorter duration of time between retests is justified.
The chart below indicates how often a cleanroom should be retested for compliance within ISO Class particle count ranges.
For the most critical environments, handheld particle counters are generally not sufficient. Therefore, monitoring equipment is often built-in to the HVAC system or hard-wired at a static position.
Many facilities deploy integrated air quality systems for hardwired, multi-room configuration. Dedicated systems incorporate alarms and merge with computer networks for real-time data analysis and data logging.
Particle Counter Test Methods
Particle Counter Performance
Key Features and Characteristics
Utility and Features / Extra FeaturesRelative humidity (RH) and temperature readings provide key insights for cleanroom air quality management, but also impact the performance of light scattering devices. As RH decreases, temperature increases. How these factors differentiate from sample to sample, or cleanroom to cleanroom, provides extra clues for highlighting problematic areas from a localized device.
Plug and Play
Plug and play particle counters provide both onboard and exportable databases for reference, data logging, and comparison. Units capable of storing tens-of-thousands of sample records provide the highest degree of testing freedom without overwriting, offloading, or deleting valuable data.
The ideal counting efficiency is an accurate detection and counting of 100% of particles. Each particle input registers an output. As particle size increases, deviations are reduced. A larger air volume is generally associated with decreased counting efficiency on a sliding scale. Therefore commercial particle counters for multi-room testing must not only be accurate, but also accurate when sampling large volumes of air. The particle size in question, the amount of air sample volume, and particle characterization all influence accuracy. Most facilities consult with a cleanroom engineer to determine which OPC provides the most accurate results with respect to cost and long-term value.
Response time is a particle counter’s ability to recover and reset after a test, in preparation for another test.
Laser Design Quality
A laser beam directs the most energy at the center of the focus point and loses intensity as it radiates outward from the center. Integrated features like lenses and shaping masks help generate more consistent particle sizing with repeatability and accuracy.
Accidents happen. Because handheld particle counters move under human power, the likeliness of them being dropped and potentially damaged isn’t uncommon. Manufacturers of quality instruments use lightweight and rugged injection molded plastics to mitigate the risk of impacts or drops.
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