B - 4
HAPSITE ER Operating Manual
The excess vent flow (overflow) from this sampling tee
(in the gas flow direction) should exit through stainless
steel fittings of at least 1/4 in. size and a short vent line to
a fume hood or other exhaust system.
The smaller “leg” of the sampling tee is coupled to the HAPSITE ER. The total flow
to the sampling tee should be approximately 1 liter/min. to allow sufficient excess
over the HAPSITE sampling flow rate which is approximately 100 cc/min. and to
prevent external air from being drawn back into the vent “leg” of the sampling tee
which would alter the concentrations delivered from the cylinder or mixer.
B.1.2.2 Inert Sample Bag
Ultra clean Tedlar sample bags, dedicated to a given VOC compound
mix/concentration level, will be the most economic option for regular calibration
(more than once a week) and eliminates the waste of certified gas mix out of the
sampling tee vent. The dedicated Tedlar bag can be filled directly from the
associated gas cylinder or gas mixing system effluent.
Regulate the gas delivery to avoid overfilling the bag. The
bags are not designed to be pressurized.
Alternatively, a bag can be filled by delivery of a set volume of the diluent gas (via
a mass flow meter), then adding a set volume of the certified cylinder VOC gas mix,
followed by mixing to homogeneity in the bag to obtain the proper dilution. A 12-liter
Tedlar bag will allow about 60 HAPSITE samplings of the contents between refills.
The use of properly filled Tedlar bags inherently assures that the gaseous contents
are at atmospheric pressure for sampling. The bag should not be filled to the point
where the bag appears like a firm “air pillow”, as the bag would then be at above
atmospheric pressure, and could not be sampled accurately by the HAPSITE. In
addition, this would lead to eventual leakage along the bag seams, destroying the
integrity.
Clean Tedlar bags to be filled with a certified gas mix should be filled once with the
gas mix and allowed to stand several minutes for preconditioning, then evacuated
with a transfer line and a diaphragm vacuum pump and refilled again with the mix.
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