4
The critical orificedetermines the flow range.The adjustablepiston is used to
set a specific, fixed flow ratewithin the flow range.An adjustment to theposi-
tion of the piston changes the back pressure, which changes the pressure dif-
ferential across the critical orifice. If the piston is lowered away from the
diaphragm, the flow rate will increase. If the piston is raised toward the
diaphragm, the flow rate will decrease. This flow controller will accurately
maintain a constant flowdespite changes invacuumover a rangeof 30"Hg to
7"Hg.Flow is constantuntil thevacuum rangeof thedevice is exceeded,mak-
ing the flowcontrollerunable tomaintain theconstantpressuredifferential. In
Figure 5, for example, the flow rate is constant from29.9"Hg to approximate-
ly 7"Hg, at which point the flow rate decreases because the flow controller is
unable tomaintain theproperpressuredifferential.This controlwill allow the
user to collect approximately 5 liters of sample in a 6-liter canister. This is an
extremely important factor in obtaining valid time-integrated samples
through passive sampling. We will discuss this point further in the Field
Sampling sectionof this guide.
VacuumGauge
A vacuum gauge enables sampling personnel to visuallymonitor changes in
the vacuum in the canister during sampling. If the flow rate changes unex-
pectedly (e.g., due to a leak or an incorrect setting), the vacuum gauge will
indicate a disproportionately high or low vacuum in the canister, and correc-
tive action can be taken (i.e., flow adjusted) in time to ensure a valid sample.
This type of vacuum gauge is attached to the sampling train, for use in the
field. The vacuum gauge should be of high quality, to ensure that it does not
introducecontaminants into the sample.Allwettedparts in thevacuumgauge
shown inFigure6 (page7) are constructedof stainless steel; thegauge is accu-
rate towithin1%of full scale.
Tomonitorpressure in thecanisterbeforeandafter sampling,useamoreaccu-
ratemeasuring device. Test gauges built byAshcroft are accurate to 0.25% of
full scale.These sensitive gauges shouldnot beused in the field - they typical-
ly are wall mounted in the lab. Once used for sampling, a gauge must be
cleaned, and should be certified clean. Procedures are described later in this
guide.
Canister
The canister is a stainless steel vessel designed tohold vacuum to less than 10
mTorrorpressure to40psig.Canisters areavailable ina rangeof volumes: 850
mL, 1.0 liter, 1.8 liter, 3.0 liter, 6.0 liter, 15 liter, and 35 liter. The size of canis-
ter used usually depends on the concentration of the analytes in the sample,
the sampling time, the flow rate, and the sample volume required for the sam-
pling period (Table 1, page 3). Typically, smaller canisters are used for more
concentrated samples, such as soil gas collection, 3-liter and 6-liter canisters
areused toobtain integrated (TWA) ambient air samples at sampling timesof
up to 24 hours, and large 15-liter and 35-liter canisters are used for reference
standards. Sampling time will be limited by the combination of canister size
and the flow rate atwhich the sample is tobe collected.
A well-designed canister is essential to the success of the sampling project.
First, thecanister shouldbemadeof stainless steel, so thecollected samplewill
not permeate through the vessel wall or degradedue to exposure to light dur-
ing shipment to the analytical laboratory. Second, the interior surface of the
canister shouldbe inert, to reduce the potential for interactionswith the ana-
lytes in the sample. Third, all canisters involved in a particular application
should be of consistent volume, to simplify calculating sample volumes.
Finally, the canister should have a high quality valve that resists abuse in the
field (e.g., overtightening that potentially could cause leaks).An inferior valve
can fail, causing sample loss and incurring replacement costs. It can bemore
expensive to sample again than to replace a valve.
Two types of canisters are available, the difference being the interior surface.
The traditional canister is the stainless steel SUMMA®canister.The interiorof
Figure3
Flow controller & critical orifice.
atmospheric
reference
inlet
outlet
critical
orifice
adjustable
piston
diaphragm
Figure 3 courtesy of Veriflo Corp.,
a division of Parker Hannifin Corp.
