With new cGMP guidelines and traceability, particle counter accuracy is essential in maintaining 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, medical device and biotechnology industries) particle counter accuracy is critical.
How accurate are particle counters once they are calibrated outside of the factory? The only real way to find out is to make some kind of comparison against references following a well documented and validated calibration procedure. Hence the arrival of ISO 21501.
So what happened before ISO 21501? First of all, let's define what calibration actually means.
What is calibration?
"Calibration" is the process of establishing the relationship between a measuring device and the units of measure. This is done by comparing a device or the output of an instrument 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 which is directly or indirectly related to national standards, international standards, or certified reference materials.
"Quality management systems call for an effective metrology system which includes formal, periodic, and documented calibration of all measuring instruments. ISO 9000 and ISO 17025 require effective calibration systems."
How do particle counters work?
Particle Counting Technology has its roots in the early 1900s when John Tyndall an Irish physicist scientifically characterised the scattering of light by dust and large molecules in the air, now known as the Tyndall Effect, (similar to Rayleigh scattering).
In studying the interactions between light and the constituents of air, he developed the nephelometer (an instrument for measuring suspended particulates in a liquid or gas colloid). The technology has since evolved and become more accurate due to laser technology and microelectronics. Basically a particle 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. This voltage is proportionate to the size of the particle or the amount of light the particle scatters or reflects.
How are particle counters calibrated?
Primary particle counter calibration is carried out by the manufacturer and is usually conducted following manufacturer procedures and guidelines from ASTM and JIS B 9921 where a Condensation Particle Counter (CPC) is used as a reference unit so with this standard the accuracy of the Particle Counter is set. Size calibration was checked using a PHA.
What happens after the calibration interval is up? For field calibrations the previous methods followed were not adequate. Particle Counter Service centers across the world used different methods and followed different guidelines.
Particle counter manufacturers went in two different directions with field calibrations. Some chose a calibration method using a calibrated reference unit and followed guidelines in ASTM F649-1 while other particle counter manufacturers used a pulse height analyser (PHA) and followed guidelines in ASTM(4) F328-98 and some parts of JIS(5) B 9921.
ISO 21501: The complete solution
With so much emphasis on particle counter importance and the various different guidelines, ISO 21501 has been developed to harmonise the current situation. ISO 21501 is at last a complete solution to the calibration of particle counters.
ISO 21501 addresses inherent issues within previous particle counter calibration guidelines where either sizing or accuracy calibrations were performed in the field.
Firstly, it requires that BOTH methods are followed (i.e. the PHA for sizing calibration and the Ref Unit for accuracy calibration).
Secondly, size resolution must be reported. This is the measure of the ability of the particle counter to distinguish between particles of different size and is the verification of size calibration.
Thirdly, and most importantly, Count Efficiency must be reported. This is the ratio of counts between the reference unit and the particle counter. The reference unit must be more sensitive than the particle counter with its counting efficiency a known value and itself be calibrated by a higher sensitive instrument like a CPC.
Counting efficiency is the verification of the accuracy of the calibration.
Summary of ISO 21501 requirements
- Size calibration
- Verification of size setting
- Counting efficiency
- Size resolution
- False count rate; maximum particle number concentration
- Sampling flow rate
- Sampling time
- Sampling volume for liquid particle counters
- Calibration interval to be defined