Over the past three decades, many ATI customers have asked questions regarding the correct amount of aerosol concentration required to accurately evaluate and test the integrity of a high-efficiency particulate air (HEPA) filtration system.
To answer these questions, David W. Crosby of ATI wrote an article regarding concentration for Performance Review, the technical journal of the Controlled Environment Testing Association (CETA). The article appeared in Volume 1, #5, Spring of 1994. A copy of the publication can be obtained from CETA.
In summary, the article critiques the subject of concentration and today’s trend to use less concentration to challenge filtration systems. As indicated in the article, ATI concurs with the National Environmental Balancing Bureau (NEBB) guidelines for cleanroom certification which dictate the use of a challenge concentration between 20 and 60 micrograms per liter.
While ATI photometer QA test procedures require that they be capable of obtaining a 100% setting from 10 micrograms per liter while maintaining a stable 0% reading, ATI does not advocate using such a low challenge unless necessary. Challenge concentrations of less than 10 micrograms per liter, though possible, are not recommended because of the resulting increases in instrument display instability.
June 2005 Update
Since the above article was written, customers have questioned whether ATI photometers are accurate using concentrations below 20 micrograms per liter of upstream concentration. The answer to this question is yes. The reference to 20 micrograms per liter was intended to maximize stability in the photometer response while operating at a level considered “low” at that time.
The solid-state amplifier circuitry used in ATI photometers since the 1970’s was designed to accurately operate using 10 micrograms per liter. That basic design criteria remains true to this day. Newer digital units incorporate design improvements to the scattering chamber and operational controls to further enhance photometer stability using 10 micrograms per liter as a base at 100% setting.