CLIMET INSTRUMENTS CI-154, Руководство пользователя, Страница: 81 / 98

CI-154 User’s Manual
Page 71
5. Application guide
5.1 Using standard and optional accessories
5.1.1 Using the Isokinetic probe
The Isokinetic probe is the cone-shaped sample probe that is provided as a standard
accessory for the CI-154. Often referred to simply as the
isoprobe , it is used when
sampling laminar airflow, such as the airflow from a HEPA filter. When used to
sample laminar flow, the probe should be pointed towards the source of the flow.
The isoprobe maintains laminar flow into the sample tube by having a wide entry
diameter that tapers down to the tubing ID. The wide entry diameter prevents
turbulence from developing at the entry point of the probe as the velocity of air from
the HEPA filter is accelerated and the air sample is compressed into the smaller
diameter of the inlet and transport tubing.
Air that is not laminar is considered turbulent . Since there is no consistent direction
of airflow in turbulent air, it makes little difference which direction the probe is
pointed, or even whether an isokinetic probe is placed on the sample tube. The now
discontinued Fed. Std. 209E specified that the sample tube should be placed about
30 inches above the floor. ISO 14644 recommends that the sample probe be placed
at work height, pointed upwards.
5.1.2 Using the zero count filter
A zero count filter is included with the CI-154 as a standard accessory. Placing a
zero count filter over the inlet allows the user to determine the background counts
that are produced in the absence of external contaminants.
Counts could be produced from a variety of sources. A few counts per minute may
be the sloughing off of particles that have adhered to the sides of the inlet while
sampling relatively contaminated areas or processes. Extremely large counts limited
to the smallest channel point to an electronic problem.
ASTM F-50, which is an important standard for cleanroom monitoring procedures,
recommends the use of a zero count filter to verify that background counts will not
affect actual sampling. Based on this standard, some companies have adopted an
SOP to perform a zero count test before starting the daily round of sampling. It is
important, when adopting such procedures, to understand that absolute zero
counting is not what is required, but rather that the counts are small enough,
relative to the counts in the areas being monitored, that they have no statistically
significant affect on the counts obtained.
5.1.3 Using high pressure diffusers
The CI-301 and CI-302 High Pressure Diffusers are optional accessories used for
sampling high purity gases from high-pressure gas lines. The pressure from a high-
pressure line is usually too high to apply to a particle counter, and trying to apply
the pressurized gas directly to the CI-154 would, at the very least, create turbulence
that would cause false counts. Trying to reduce the pressure by using a valve to
reduce the pressure will also reduce airflow, which could cause erroneous counts.