A method for monitoring gas in a wafer processing system is provided. The method includes producing an exhaust flow in an exhausting conduit from a processing chamber. The method further includes placing a gas sensor in fluid communication with a detection point located in the exhausting conduit via a sampling tube that passes through a through hole formed on the exhausting conduit. The detection point is located away from the through hole. The method also includes detecting a gas condition at the detection point with the gas sensor. In addition, the method also includes analyzing the gas condition detected by the gas sensor to determine if the gas condition in the exhausting conduit is in a range of values.