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i • <br /> valve. It is important to prevent liquids from entering the canister.A sample port <br /> oriented downward may have liquid standing in the valve. Purge the sample port <br /> adequately before connecting the sampling train. <br /> • Consider the effects of sampling a process under vacuum or pressure.When <br /> collecting a grab sample from a stationary source such as an SVE system or LFG <br /> collection system,some sample ports may be under vacuum or pressure relative to <br /> ambient conditions.When the sample port is under vacuum, such as the header pipe <br /> from the extraction well network, it may be difficult to fill the canister with the desired <br /> volume of sample.A vacuum pump may be used to collect a canister grab sample <br /> from a sample port under considerable vacuum. See the related discussion on sam- <br /> pling at altitude in Section 5.2.When the sample port is under pressure,such as the <br /> effluent stack downstream of the blower and treatment system,you may inadvertently <br /> pressurize the canister. Only a DOT-approved sample cylinder should be used to <br /> transport pressurized air samples. Under no circumstances should an Air Toxics Ltd. <br /> canister be pressurized more than 5 psig for a 6 L canister and 15 psig for a 1 L <br /> canister. Bleed off excess pressure by opening the valve temporarily while monitoring <br /> the canister with a pressure gauge. <br /> tr. <br /> 26 <br />