line between the manholes are partially blocked. Smoke is created using either a smoke bomb or liquid smoke. Smoke bombs are lit and placed on a grate or in a holder on top of each fan, while liquid smoke is injected into the fan via a heating chamber. The fans create a pressure differential that forces the smoke into the sewer at a pressure just above atmospheric. With properly installed plumbing, the traps will prevent the smoke from entering the house and redirect it out the plumbing vents. Defective plumbing systems or dry traps will allow smoke to enter the inside of the house. The area around the section being tested is searched for smoke plumes. Plumes coming from plumbing vents or the interface between the fan shroud and manhole rim are normal; however, smoke plumes outside of the manhole rim are not. Plumes are marked, usually with flags, and defects are noted using measurements from stationary landmarks like the corners of houses. The plumes or markers may also be photographed. Woodwind instrument repair In woodwind instrument repair, a smoke test involved plugging one end of an instrument and blowing tobacco smoke from the tester's mouth into the other to test for leaks. Escaping smoke revealed improperly seated pads and faulty joints (i.e. leaks). After this test the instrument was cleaned to remove nicotine and other deposits left by the smoke. Described in a repair manual written in the 1930s, smoke testing is considered obsolete, and is no longer used by reputable technicians. The usual alternative to smoke is to place a bright light inside the instrument then check for light appearing around pads and joints. Automotive repair In the same way that plumbing and woodwind instruments are tested, the vacuum systems of automobiles may be tested in order to locate difficult-to-find vacuum leaks. Artificial smoke is deliberately introduced into the system under slight pressure and any leaks are indicated by the escaping smoke. Smoke can also be used to locate difficult-to-find leaks in the fuel evaporative emissions control (EVAP) system. Infectious disease control In infectious disease control a smoke test is done to see whether a room is under negative pressure. A tube containing smoke is held near the bottom of the negative pressure room door, about two inches in front of the door. The smoke tube is held parallel to the door, and a small amount of smoke