Liquid particle Counter Calibration

Liquid electronic particle counters are used to monitor contamination levels in drinking water and hydraulic oils. They are also used to establish component and assembly cleanliness standard specifications. And lastly, they are used to determine filter efficiencies and size ratings. In the late 1990’s, particle counter calibration for hydraulic purposes used ISO 4402 and AC Fine Test Dust. There were no better methods even if there are shortcomings of these methods until recently. The approval of ISO 11171 which is a National Institute of Standards and Technology (NIST) traceable method of calibration changed the previous method. The quality and reliability of liquid particle counter calibration and filter data is expected to improve, increasing their usefulness to the hydraulics, automotive and aerospace industries.

With the requirement in many manufacturing industries for cleaner product processing atmospheres, optical and electronic particle counters have become progressively more strategic in environmental monitoring of cleanrooms. With new GMP procedures and traceability, particle counter accuracy is vital in preserving a quality environmental monitoring system. Therefore with such a big role to play in product quality and ensuring overall product safety, especially in the pharmaceutical, biotech and medical device industries, particle counter calibration is critical.

Calibration is the process of establishing the relationship between a measuring device and the units of measure which is achieved by comparing a device or the output of a device to a standard having known measurement characteristics. To improve the quality of the calibration and have the results accepted by outside organizations, it is desirable for the calibration and subsequent measurements to be “traceable” to internationally defined measurement units. Establishing traceability is accomplished by a formal comparison to a standard that is directly or indirectly related to national standards, international standards, or certified reference materials.

Prior to calibration, it is worth developing an understanding of how particle counters work. The counter takes a sample and the particles in that sample pass through a laser beam thus scattering the light energy, and the electronics inside the sensor convert this light energy into a voltage. The voltage is proportionate to the size of the particle or the amount of light the particle scatters. The electronic circuitry picks up the voltage signal from the photo detector (which converted the light energy into the voltage signal). Digital threshold circuitry then sizes and counts the particles.

In comparison to liquid particle counters, the calibration of air particle counters presents greater challenges due to the need to generate air samples containing sub-microscopic particles of homogenous size and distribution. Although the technology of air particle counting is well understood, the ability to calibrate any two air particle counters so that they produce the same results when sampling the same air sample has proven to be challenging, bringing into question the accuracy of these instruments. ISO 21501 now delivers a calibration method that can significantly improve the repeatability and reproducibility of air particle counters.

ISO 21501 also applies to liquid particle counters used for determination of particulate contamination in infusions and injections. Until recently, the calibration requirements for liquid particle counters used to test infusions and injections were described in detail in the United States Pharmacopoeia (USP). However, in the interest of international harmonization of the pharmacopoeias, the details of these IST calibration methods have been removed in order to simplify the text of USP. ISO 21501 now offers an alternative to these IST tests and establishes calibration methods to ensure accurate and repeatable performance of liquid particle counters.