Improper air velocity rates can put a cleanroom at risk.
Managing Editor A ccurate measurement of airflow in a cleanroom is vital to making sure that the facility is up to code and func- tioning properly. Precise air velocity measurement can be used to reveal proper or improper performance of an
airflow system, and steps can be taken from there to correct any
Air velocity measurement is usually accomplished by taking
velocity (linear) readings on a grid not to exceed 12 inches
apart, starting 6 inches in from the edges of the filter (per IST-
RP-CC002). Although a single point thermoanemometer (hot
wire) reading can be done at each point, the instrument of
choice is usually the square pitot array read by a micromanom-
eter. This instrument covers an area of 1 square foot, taking
multiple readings and averaging them into one. Gastner notes
that, “Unfortunately, measuring velocity this way is not the
most accurate measure of airflow from the filters. The velocity
measurement grid referenced in the standard is 12 inches off
the filter face, or as agreed upon. Most square pitot arrays come
with an assortment of ‘stand-offs,’ allowing measurement at 2
inches, 6 inches, or 12 inches off the filter face, giving you dif-
ferent velocities at different distances. The air exiting a filter in a
cleanroom ceiling will pull air from around it off the ceiling and
entrain it into the airflow, skewing your reading.”
“The most accurate way to measure airflow in a clean-
room is with a balometer and shroud, taking a volumetric
measure. The frame of the shroud has gasketing on it to
ensure a complete seal around the perimeter of the filter for
accurate measure,” says Gastner.
There are new velocity measuring instruments commer-
cially available using infrared technology, and other multiple
sensor arrays with either thermistor beads or mini hotwires,
but they can be extremely expensive. Gastner notes, “Until
such time as the standards that apply to the industry require
a new technology, or even recognize the new instruments as
legitimate and valid, the best approach is to verify with the
owner of the facility what they want used for the testing, and
have them sign off on that choice.”
Gastner cautions against quick inspections, however.
Gastner suggests that velocity or volume be checked periodically—normally, every six months—to help gauge the performance of the room. However, a drop in either area will be
accompanied and evidenced by a corresponding drop in room
pressure relative to the outer area or next room.
“The way room performance is monitored depends on
several factors—size of the room or facility, capacity of
the AHU (air handling unit), length of duct runs, etc.,”
Doing what’s right for your cleanroom
Gastner adds a disclaimer that this is a simplified overview of
air velocity measurement, and not specific enough to warrant
changing an approach to cleanroom performance monitoring.
Consulting with an expert is advisable when evaluating the
process and options.