CLIMET INSTRUMENTS CI-154, User Manual, Page: 82 / 98

CI-154 User’s Manual
Page 72
The particle counter requires a minimum flow through the diffuser and if the valve
drops the airflow below this requirement, the particle counter may suck in ambient
air, contaminating the gas sample.
The diffuser will reduce the pressure to atmospheric level, and an adequate flow will
enable the particle counter to sample the gas sample without outside contamination.
Refer to the High Pressure Diffuser manual for additional information.
5.2 Monitoring
5.2.1 The where, why, and how of monitoring
When establishing sampling protocols, the standards that should be consulted for
guidance are Fed. Std. 209E (which has been retired), ISO 14644-1 and -2, and
ASTM-F50. These standards are not always as clear-cut as one would like when
looking for a recipe book approach to cleanroom monitoring. Cleanroom standards
focus more on validation than monitoring, so a basic understanding of the purpose
of monitoring and of risk assessment are needed to develop a good monitoring plan.
Sample Probe placement
A starting point in developing a monitoring plan is to understand the dynamics of
the areas to be monitored. In some cleanroom designs, clean air flows vertically
through the room, from HEPA banks in the ceiling to exhaust holes in the floor.
Other designs have air flowing horizontally through the room from filter banks near
the ceiling on one side of the room to exhaust ports near the floor on the opposite
side of the room.
If you are monitoring an activity in a vertical flow room, placement is normally
below the critical area of interest and the probe is facing up. Any point around the
process will be bathed in air with about the same level of particle content. However,
if there is an activity in close proximity to the area monitored, it could cross
contaminate the process being monitored, and be detected by the counter.
In rooms with horizontal flow, placement is normally downstream of the critical
process with the probe facing into the direction of the flow. However, the process
itself may inherently produce particles, that would be detected by the counter. You
may be interested in the contribution of the process to the overall particle burden of
the room, but your primary interest is in the cleanliness of the air that envelops the
process, so you will want to place the sample probe slightly upstream of the
process, but not so close as to sample particles produced by the process, itself.
One other consideration in determining where to place the sample probe is the type
of air flow at the monitoring point. When close to a HEPA filter, air is laminar, but at
some distance from the filter, the flow becomes nonlaminar or turbulent. In laminar
flow, the air flows uniformly in one direction; a smoke ring will hold its shape
without dissipating. In nonlaminar areas the air flow is not uniform, and a smoke
ring will quickly dissipate. When sampling in laminar areas the standards specify
that an isokinetic probe be used to sample, and they further specify that the probe
be pointed towards the air source. The isokinetic probe maintains laminar flow as
the air sample accelerates from the slower velocity coming out of the filter to the
higher velocity it will travel while flowing through the narrow transport tubing.
Where to sample
When validating a room, the standards specify laying out a grid, and sampling at its
grid point. But the objectives of validation are not the same for monitoring, because