4. Operation
4.1. Iso-kinetic
Sampling
The GT-321 comes with an iso-kinetic probe that attaches to its inlet nozzle with the
short piece of Tygon tubing provided. Attach the other end of the tubing to the GT-321
inlet nozzle (See section 2.2 Familiarization).
The iso-kinetic probe helps reduce count errors related to the sample flow velocity and
the aerodynamics of small particles. The iso-kinetic probe should be used for most
sampling applications. When taking a sample of typical indoor or outdoor aerosols the
opening of the iso-kinetic probe should always face upward. The GT-321 can be held in
your hand or placed on a flat surface with its display facing towards you.
When sampling in an area having constant airflow, such as a clean room, duct, vent or
the downstream side of a filter, always face the opening of the iso-kinetic probe into the
air flow. The length of the connecting tubing going from the inlet on the GT-321 to the
iso-kinetic probe can be increased if necessary but count losses, especially for larger
particles, will become a problem after approximately a four-foot length. Try to keep the
tubing length short.
The sampling height will affect the GT-321 particle count reading. Taking a sample near
the floor can give results several times higher than a sample taken at eye level.
When using the GT-321 to find the source of a contamination problem, be aware that
not all contamination problems are continuous, some are the result of a short-term
event or burst of particles. Locating the source will require taking a number of samples
in the same spot.
4.2.
Taking a Sample
CAUTION
:
Never sample smoke, paint spray, oil mist, reactive, or pressurized
gasses to avoid damaging the sensor optics.
Remove the rubber cap from the GT-321 aerosol inlet nozzle and attach the Iso-kinetic
probe. Turn the GT-321 power switch on. Press the
<SELECT>
key and select the
particle size range of interest.
When you select a particle size, you are setting a lower size limit for the counter in the
GT-321. The industry standard setting for a particle counters size selection is 50%
counting efficiency. This means that the GT-321 will count ½ of the particles of the
selected size when a sample is taken. Counting efficiency increases rapidly to 100% for
particles larger than the selected size. All particles larger than the selected size are
counted. This type of counting is defined as cumulative counting.
After selecting a particle size, press the
<START>
key. You will hear the pump inside
the GT-321 activate. After 9 seconds the first sample result in particles per cubic foot is
displayed. The GT-321 will automatically take a series of 10 samples; each sample after
the initial sample that includes 3 seconds of warm-up time for the internal pump takes 6
seconds. The display will update every 6 seconds with the new result. After all 10
GT-321-9800 Rev D.doc
GT-321 Operation Manual
Page 11 of 18
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