Gas produced and put into bottles for bump testing and calibrating gas monitors has a finite shelf life, and paying attention to that expiration date might save your life. Using expired gas can lead to incorrect calibration of your gas monitor. Gas concentrations have been known to drop over time. Reactive gases such as hydrogen sulfide react with the material of the cylinder container, and tests have shown some levels to slip from 20 parts per million (ppm) to 6 ppm over a 12-month period. Since calibration gas can change so rapidly, it's important to buy only from a reputable source. Saving money on gas is simply not worth the danger posed to workers who rely on correct readings for their safety. Remember, you only need one accident to make that $100 gas savings seems like the worst decision you ever made. Because cal gas is regulated as a hazardous material when it is shipped, it will take longer and probably cost a bit more than most packages. Be sure to order a few weeks or even a month early if you see your gas supply is about to expire.