3
800-356-1688
•
Sampling Inlet
The sampling inlet—the entrance to the sampling train—typically is cleaned
stainless steel tubing, either
1
/
4
"ODor
1
/
8
"OD.USEPACompendiumMethod
TO-14A/15 recommends samplingat aheight of 2meters above theground. In
ahighly traffickedarea,thiswouldminimize theproblemofdustparticlesenter-
ing thesampling train.Thisheight isnotmandatory,however,and it iscommon
practice touse an inlet that is 12" (approximately
1
/
3
meter) high.TheEPA also
recommends having the entrance of the sampling inlet face downward topre-
vent raindrops from entering the inlet. In some sampling trains, a
1
/
8
"or
1
/
4
"nut
at theentranceof the inlet keepswaterdroplets away from theedgeof the inlet,
where they couldbedrawn into the sampling trainwith the sample.
ParticleFilter
Theparticle filter is installed in the sampling trainprior to the flow-controlling
device to prevent airborne particles from entering the sample flow path.
Particles could partially obstruct the flow path and alter the flow rate during
sampling. Inextremecases,particles couldplug the flowpathand stop the sam-
ple flow. The smallest orifice commonly used in a passive sampling train is
0.0012" (approximately30micrometers).Without aparticle filter,dustparticles
couldocclude thisopeningas theyaccumulate in theorifice fitting.Particlesalso
canaffect the leak integrityof thevalve, andpossiblycausedamage to thevalve.
Two types of filters are used for this application, frit filters and in-line filters
(Figure 3).A varietyofmodels of each type are available;most are of sintered
stainless steel and have 2-, 5-, or 7-micron pores. Use of smaller pore filters
reduces the likelihoodof problems fromairborneparticles.EPACompendium
MethodTO-14A/15 recommends using aparticle filterwith2-micronpores.
CriticalOrifice
The critical orifice (Figure 4) restricts the flow to a specified range (Table II). In
conjunctionwith the flowcontroller, thisallows thecanister to fill atacertain rate
overaspecified timeperiod.Themostcommoncriticalorificedesign isaseriesof
interchangeablestainlesssteel
1
/
4
"NPT to
1
/
4
"compressionunions,each fittedwith
apreciselyboredrubyorifice.Eachorificeprovidesaspecific flowrange(TableII).
Stabilityoverawide rangeof temperaturesmakes ruby theconstructionmaterial
of choice.Typicallyduring field sampling, the sampling train is subjected to tem-
perature fluctuations thatwouldcausemetals tocontractorexpand,affecting the
diameterof theapertureand therebyaffecting flow.Rubywillnot expandorcon-
tract across ambient temperatureextremes incurredduring sampling.
A critical orifice can be used as the sole flow-restricting device, but it cannot
ensure uniform flow. Since the source pressure of the flow changes during
sampling, the flow rate through the orifice can also change, resulting in an
invalid time-integrated sample. It is important that a highly consistent flow
rate bemaintained during passive sampling, and this is accomplished by the
flow controller.
FlowController
The flow controller (Figure 4)maintains a constant sample flowover the inte-
grated timeperiod,despitechanges in thevacuum in thecanister,or in theenvi-
ronmental temperature (Figure 5). In the Veriflo® Model SC423 XL Flow
Controller shown in Figure 4, the critical orifice acts as a flow restrictor,
Table II
Critical orificediameter vs. flow rate.
OrificeDiameter
FlowRateRange
Canister Volume / Sampling Time
(in.)
(mL/min.)
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
8–15
1 hr.
4 hr.
8 hr.
20 hr.
0.0030
15–30
— 2 hr.
3 hr.
8 hr.
0.0060
30–80
— — 1.5 hr.
4 hr.
0.0090
80–340
— — 0.5 hr.
1 hr.
Figure3
Filtersused in sampling trains.
critical
orifice
frit filter
stand
alone in-line filter
Frit filter inside
passive samplingkit
Figure4
Flow controller & critical orifice.
atmospheric
reference
inlet
outlet
critical
orifice
adjustable
piston
diaphragm
Drawing courtesy of Veriflo Corp.,
a division of Parker Hannifin Corp.
spring
washer
in-line
filter
Website :
E-mail :
TelNo : 03 9762 2034 . . . inAUSTRALIA
