Gas detectors are designed for the detection and measurement of potentially life threatening atmospheric conditions. Hazards such as oxygen deficiencies, explosive atmospheres, and toxic gases and vapors regularly kill as many as three-hundred workers per year in the United Sates, many of them during confined space entry procedures. The atmospheric conditions which lead to these accidents and fatalities are usually invisible to the workers who are involved. The only way to insure conditions are safe is to use an accurate atmospheric monitor. The only way to know that the readings are accurate is to expose the instrument to known concentration test gas; in other words, to check the accuracy of the instrument on a regular basis.