Smoke testing refers to various classes of tests of systems, usually intended to determine whether they are ready for more robust testing. The expression probably was first used in plumbing in referring to tests for the detection of cracks, leaks or breaks in closed systems of pipes. Pre-dating the term itself, smoke tests were performed to detect leaks in wooden sailing vessels at least as early as 1836. After making a slow fire in the bottom of the hold, Richard Henry Dana writes, "Wherever smoke was seen coming out we calked and pasted and, so far as we could, make the ship smoke tight." History of the term The plumbing industry started using the smoke test in 1875. Smoke testing in various industries Plumbing In plumbing a smoke test forces non-toxic, artificially created smoke through waste and drain pipes under a slight pressure to find leaks. Plumes of smoke form where there are defects. This test can be performed when the plumbing is brand new, but more often it is used to find sewer gas leaks that may plague a building or an area. Any sign of smoke escaping can be considered a possible site for sewer gas to escape. Sewer gas typically has a rotten egg smell and can contain methane gas, which is explosive, or hydrogen sulfide gas, which is deadly. Plumbing smoke tests are also used to find places where pipes will spill fluid, and to check sanitary sewer systems for places where ground water and storm runoff can enter. Smoke testing is particularly useful in places such as ventilated sanitary sewer systems, where completely sealing the system is not practical. When smoke testing a sanitary sewer system it is helpful to partially block off the section of sewer to be tested. This can be done by using a sand bag on the end of a rope. The sand bag is lowered into the manhole and swung into position to partially block lines. Completely blocking the line can cause water to back up and prevent smoke from escaping through defects. Smoke testing may not be done after rain or when ground water is unusually high as this may also prevent detection of defects. Large downdraft fans, usually powered by gasoline engines, are placed on top of open manholes at either end of the section to be tested. If possible all lines in the manholes except for the