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No. 5
Aerosol Concentration
Over the past three decades, many ATI customers
have asked questions regarding the amount of concentration
required to accurately evaluate and test the integrity of
a high efficiency particulate air 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). This article appears in Volume
1, #5, Spring of 1994. A copy of this publication can be obtained
from CETA, phone #202-737-0204.
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 photometers are not allowed to leave
the factory unless they can obtain a 100% reading with 10
micrograms per liter concentration and still maintain a stable
0.000% reading, ATI does not advocate using such a low challenge
unless necessary. Challenge concentrations of less than 10
micrograms per liter should not be used because of the resulting
instrument display instability.
June 2005 -
Note: 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 has always been 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
circuit in ATI photometers, since the 1970's, was designed
to accurately operate using a minimum of 10 micrograms per
liter. That basic design criteria remains true to this
day. ATI's newer "digital" photometer designs
carry over the same operational amplifier used in the older
analog instruments. These newer digital units incorporate
design improvements to the scattering chamber and operational
controls to further enhance photometer stability at 100% using
10 micrograms per liter as a base.
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