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24-hour integrated sample can be an economical and <br /> practical approach to determine residential exposure to . <br /> indoor or outdoor air sources. The most common hard- w <br /> ware configurations used to take an integrated sample are <br /> illustrated. ' <br /> Flow controllers are devices that regulate the flow of air <br /> during sampling into an evacuated canister. Also known <br /> as flow restrictors, these devices enable a sampler to achieve a desired flow rate and thus, a sampling <br /> interval. Air Toxics Ltd. provides two general types of flow controllers: mass flow controllers and <br /> critical orifice devices. Both devices are driven by differential pressure between ambient conditions <br /> and vacuum in the canister. <br /> 3.2.1 Mass Flow Controller <br /> A mass flow controller employs a diaphragm that actively compensates to maintain a constant mass <br /> flow rate. As the differential pressure decreases, the flow rate tends to decrease and the diaphragm <br /> responds by opening up to allow more air to pass through. Mass flow controllers can be adjustable or <br /> fixed and can provide integrated samples with intervals ranging from hours to days. Air Toxics Ltd. <br /> provides a fixed :mass flow controller that is calibrated at the laboratory for 24-hour sampling. <br /> Adjustable mass flow controllers have a knob that can be adjusted in the field to provide integrated <br /> samples with intervals ranging from one to 24 hours. The rugged conditions of field sampling are not <br /> usually compatible with adjustable mass flow controllers and Air Toxics Ltd. designed a more reliable <br /> flow controller based.on a critical orifice design. <br /> 3.2.2 Critical Orifice Device <br /> Air Toxics Ltd. designed a critical orifice flow restrictor to provide (time weighted) samples with <br /> intervals from 0.5 to 8 hours. The device restricts air flow by forcing the sample to enter a capillary <br /> column of minute radius. This device is passive compared to an actively compensating diaphragm <br /> and the flow rate decreases as the driving force (differential pressure) decreases. For sampling <br /> intervals from 0.5 to 8 hours, however, the flow rate is time weighted. The main advantages of the <br /> Air Toxics Ltd. flow restrictors are improved ruggedness and cleanliness. With no moving or <br /> adjustable parts, the Air Toxics Ltd. design is unlikely to lose its flow setting. In addition, a vacuum <br /> gauge is built in to the device to monitor sampling progress. To ensure there are no contamination <br /> issues from previous use, the capillary column is replaced before shipping to the field. <br /> 3.2.3 Sampling Interval and Flow Controller Setting <br /> When you request canisters and flow controllers from Air Toxics Ltd., you will be asked for the <br /> sampling interval, and the flow controllers will be pre-set prior to shipment according to the table <br /> AIR TO\ICS LTD. <br />