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 keeps water droplets away
from the edge of the inlet, where they could be drawn into the sampling train
with the sample.
Particle Filter
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 commonly used in a passive sampling train is
0.0012" (approximately 30 micrometers). Without a particle filter, dust parti-
cles could occlude this opening as they accumulate in the orifice fitting.
Particles also can affect the leak integrity of the valve, and possibly can damage
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 the pores, the less likely are potential problems from airborne particles.
EPA Compendium Method TO-14A/15 recommends using a particle filter
with 2-micron pores.
Critical Orifice
The critical orifice (Figure 3, page 4) restricts the flow to a specified range. In
conjunction with the flow controller, this allows the canister to fill at a speci-
fied rate over a specified time period. The most common critical orifice design
is a series of interchangeable stainless steel 1/4" NPT to 1/4" compression
unions, each fitted with a precisely bored sapphire orifice. Each orifice provides
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 cause metals 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 incurred during sampling.
A critical orifice can be used as the sole flow-restricting device, but it cannot
ensure uniform flow. The source pressure of the flow changes during sampling,
and the flow rate through the orifice also would 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.
Flow Controller
The flow controller (Figure 3, page 4) maintains a constant sample flow over
the integrated time period, despite changes in the vacuum in the canister or in
the environmental temperature (Figure 4, page 5). In the Veriflo™ Model
SC423 XL Flow Controller shown in Figure 3, the critical orifice acts as a flow
restrictor, upstream of a constant back pressure. This constant back pressure 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.
Figure 1 A complete sampling train is
needed for reliable passive sampling.
sampling inlet
sample
canister
vacuum
gauge
filter
flow
controller
critical
orifice
rain cap
(1/8" or 1/4" nut)
Table 1 Critical orifice diameter vs flow rate.
Orifice Diameter Flow Rate Range
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.
Figure 2 Filters used in sampling trains.
in-line
filter
critical orifice
frit filter