3
downward, to prevent raindrops from entering the inlet. In some sampling
trains a 1/8" or 1/4" nut at the entrance of the inlet keepswater droplets away
from the edge of the inlet,where they couldbe drawn into the sampling train
with the sample.
ParticleFilter
Installed in the sampling train prior to the flow-controlling device, the parti-
cle filter prevents airborne particles from entering the sample flow path.
Particles could partially obstruct the flow path and alter the flow rate during
sampling. In extreme cases, particles could plug the flow path and stop the
sample flow.The smallest orifice commonlyused inapassive sampling train is
0.0012" (approximately 30micrometers).Without a particle filter, dust parti-
cles could occlude this opening as they accumulate in the orifice fitting.
Particles alsocanaffect the leak integrityof thevalve, andpossiblycandamage
the valve. Two types of filters are used for this application, frit filters and in-
line filters (Figure 2).A variety of models of each type are available;most are
of sintered stainless steel and have 2-, 5-, or 7-micron pores. Obviously, the
smaller thepores, the less likelyarepotentialproblems fromairborneparticles.
EPA CompendiumMethod TO-14A/15 recommends using a particle filter
with2-micronpores.
CriticalOrifice
The critical orifice (Figure 3, page 4) restricts the flow to a specified range. In
conjunctionwith the flow controller, this allows the canister to fill at a speci-
fied rateover a specified timeperiod.Themost common critical orificedesign
is a series of interchangeable stainless steel 1/4" NPT to 1/4" compression
unions,each fittedwithapreciselybored sapphireorifice.Eachorificeprovides
a specific flow range (Table 1). Stability over a wide range of temperatures
makes sapphire the construction material of choice. Typically during field
sampling, the sampling train is subjected to temperature fluctuations that
would causemetals to contract or expand, affecting the diameter of the aper-
ture and thereby affecting flow. Sapphire will not expand or contract across
any ambient temperature extremes incurredduring sampling.
A critical orifice can be used as the sole flow-restricting device, but it cannot
ensureuniform flow.The sourcepressureof the flowchangesduring sampling,
and the flow rate through the orifice alsowould change, producing an invalid
time-integrated sample. It is important that a highly consistent flow rate be
maintained during passive sampling. This is accomplished by the flow con-
troller that incorporates the critical orifice.
FlowController
The flow controller (Figure 3, page 4)maintains a constant sample flow over
the integrated timeperiod,despite changes in thevacuum in the canisteror in
the environmental temperature (Figure 4, page 5). In the Veriflo™Model
SC423XLFlowController shown inFigure 3, the critical orifice acts as a flow
restrictor,upstreamof a constant backpressure.This constant backpressure is
established by the balance between the mechanical spring rate of the
diaphragm and the pressure differential across the diaphragm. The latter is
established by the pressure difference between the atmospheric pressure and
the vacuum in the canister and the flow through the critical orifice. The net
result is a constant flow.
Figure1
A complete sampling train is
needed for reliablepassive sampling.
sampling inlet
sample
canister
vacuum
gauge
filter
flow
controller
critical
orifice
rain cap
(1/8" or 1/4" nut)
Table1
Critical orificediameter vs flow rate.
OrificeDiameter
FlowRateRange
Canister Volume / Sampling Time
(in.)
(sccm)
1L
3L
6L
15L
0.0008
0.5-2
24 hr.
48 hr.
125 hr.
--
0.0012
2-4
4 hr.
12 hr.
24 hr.
60 hr.
0.0016
4-8
2 hr.
6 hr.
12 hr.
30 hr.
0.0020
8-20
1 hr.
4 hr.
8 hr.
20 hr.
0.0030
20-40
--
2 hr.
3 hr.
8 hr.
0.0060
40-80
--
--
1 hr.
3 hr.
Figure2
Filtersused in sampling trains.
in-line
filter
critical orifice
frit filter
