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1E4VM <br /> j S <br /> FIGURE 3.SUB-SLAB DIFFERENTIAL PRESSURE READING USING MAGNEHELIc GAUGE <br /> 6.3.1 Purging <br /> Attach a Nalgene hand pump or equivalent device. Two to three strokes of the hand pump should purge out the system. <br /> • If the hand pump has a built-in gauge, note whether a rise in vacuum occurs during purging. <br /> • If the sub-slab material is dry and porous,the gauge will quickly drop back to zero after each pump. <br /> Note: If the sampling point will hold a 15"Hg vacuum for 5 minutes, the sampling location is not suitable for canister sampling. <br /> • Unplug the probe by inserting a wire the length of the probe to ream out any fines or other occluding material. <br /> • If this does not work, install a sampling probe at another location. <br />