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Table 3.2.3 Flow Rates for Selected SamplingIntervals (mL/min) Sampling Interval (hrs) 0.5 1 2 4 8 12 24 6 L Canister 167 83.3 41.7 20.8 11.5 7.6 3.5 1 L Canister 26.6 13.3 6.7 - Note: Target fill volumes for 6 L and 1 L canisters are 5,000 mL and 800 mL, respectively. below. The flow controller is set to collect 5 L of sample over the sample interval. Final canister vacuum is targeted at 5 in. Hg. The flow rate is set at standard atmospheric conditions (approxi- mately sea level). If the air sample is a process (pressurized or under vacuum) or is collected at elevation, the canister will fill faster or slower depending on sample conditions. If you specify the source at project set-up, we can set the flow controller accordingly. See Section 5.2 for a discussion of collecting a sample at elevation. The 24-hr flow controllers should not be used for process or source samples. 3.2.4 Final Canister Vacuum and Flow Controller Performance Ideally the final vacuum of a 6 L canister should be 5 in. Hg or greater. As long as the differential pressure is greater than 4 in. Hg ambient pressure, then the flow through the device will remain approximately constant as the canister fills. If there is insufficient differential pressure, the flow through the controller will decrease as the canister pressure approaches ambient. Because of the normal fluctuations in the flow rate (due to changes in ambient temperature, pressure, and diaphragm instabilities) during sampling, the final vacuum will range between 2 and 10 in. Hg. • If the residual canister vacuum is greater than 5 in. Hg (i.e., more vacuum), the flow rate was low and less than 5 L of sample was collected. When.the canister is pressurized to 5 psig prior to analysis, sample dilution will be greater than normal. This will result in elevated reporting limits. • If the residual canister vacuum is leas than 5 in. Hg (i.e., less vacuum), the initial flow rate AIR TOXICS LTD. 12