Inficon HAPSITE ER - Correct Delivery of the Mix to the Inlet of the HAPSITE

B - 3
HAPSITE ER Operating Manual
B.1.1.2 Diluting the Gas On-site
Guidelines to verify acceptable performance of suitable dynamic gas
mixing/dilution systems are suggested in the Federal Register (vol. 59, No. 148,
Aug. 3, 1994 Proposed Rules, 40 CFR Part 51 Method 205).
Systems conforming to the Method 205 suggestions are available commercially
from Environics
3
(Series 2014 Computerized VOC Gas Dilution System) and
Alltech
4
(GB-2 Gas Blender).
The materials in the flow stream must be inert to the VOC compounds of interest,
and heat-traced to prevent condensation and accumulation of any VOC's in the
flow channels. A well performing gas mixing system minimizes future outlay in
certified cylinder gas standard mixes. In this system, only the cylinders at the
highest calibration concentration levels are required. 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 samples at atmospheric pressure. Internal
standard 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.
WARNING
Connecting the inlet of the probe 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 in. stainless steel Swagelok fittings.
3.Environics: (203) 429-5040
4.Alltech: (800) 255-8324