Standard Gas: A gas whose composition, concentration, and accuracy are known.
Zero gas (fresh air): clean air or nitrogen that does not contain the measured gas or other interfering gases.
Maximum Permissible Concentration (MAC): refers to the concentration of toxic chemical substances that should not be exceeded at any time during a working day at the workplace.
Short-term exposure tolerable concentration (STEL): refers to the 15-minute time-weighted average allowable exposure concentration within a working day, which cannot be exceeded by any single exposure.
Time-weighted average allowable concentration (TWA): refers to the average allowable exposure level of an 8-hour working day specified with time as the weight.
Direct Harmful Concentration (IDLH): refers to the concentration of air pollutants in the environment reaching a certain dangerous level, such as the concentration that can cause fatal or permanent damage to health, or make people lose their ability to escape immediately.
Explosion range In the mixture of combustible gas and air, the concentration range of explosion caused by fire. That is, the concentration range between the upper and lower explosion limits.
Lower Explosion Limit (LEL): The lowest concentration value at which a combustible gas mixes with air and ignites to cause an explosion.
Upper Explosion Limit (UEL): The highest concentration value at which a flammable gas mixes with air and ignites to cause an explosion.
