Sentence:  Another qualitative test which should be performed during the geometric model study is the injection of neutral buoyancy smoke into the roof and hopper regions of the collection chamber, particularly near the outlet, to check that significant amounts of gas are not exiting the precipitator without passing through the collecting plates (' ' sneakage' ' ) and that there are not strong flows in the hopper regions (hopper <WSD>sweepage</WSD>) which could cause excessive reentrainment. 
Meaning of the word marked with <WSD> tags:  The process by which gases passing beneath the electrodes of an electrostatic precipitator pick up dust from the hoppers, removing it from the precipitator exit.