B - 3
IPN 074-397-P1G
HAPSITE Smart Operating Manual
The materials in the flow stream must be inert to the VOC compounds to be used,
and heat-traced to prevent condensation and accumulation of any VOC's in the
flow channels. A good gas mixing system minimizes future outlay in certified
cylinder gas standard mixes. This allows a lab to require only the cylinders at the
highest calibration concentration levels. Lower concentrations (by as much as a
factor of 1000) can be prepared by serial dilution (with VOC-free Nitrogen or air) of
these cylinder mixes to the desired calibration levels with the gas mixing system.
This is probably the most economic route for labs which must frequently do multi
concentration re-calibrations for known VOC mixtures.
B.1.2 Correct Delivery of the Mix to the Inlet of the HAPSITE
The HAPSITE is designed to draw in samples which are at atmospheric pressure.
Internal standards gas is mixed with the sample in a ratio which is dependent on
the flow rate of the sample gas and the suction of the pump.
Connecting the inlet of the Hand Control Unit to a sample
at a pressure above or below atmospheric will cause the
mixing ratio of the internal standards to be incorrect, so
the resultant calibration will be invalid.
There are two basic approaches to assuring that the calibration mix is at
atmospheric pressure: a free flow of gas or capture of the gas mixture in an inert
sample bag.
B.1.2.1 Free Flow of Gas
The free flow of gas from the regulator of a pressure cylinder is reduced to
atmospheric pressure when the impedance to flow is small. This can be achieved
by placing a sampling tee at the point where the line becomes large in diameter.
The connection of the HAPSITE sample probe inlet should be at right angles to the
direction of gas flow with 1/8" stainless steel Swagelok fittings.
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" size and a short vent line to a
fume hood or other exhaust system.
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