2 Terms and Abbreviations
2.0.1 Combustible gas
It means flammable gases or flammable vapors formed by vaporization of class Ⅰ and class Ⅱ flammable and combustible liquids.
2.0.2 Toxic gas
It means gases causing acute or chronic health damages via body exposure during occupational activities of workers.Scope of toxic gases in the specification includes all toxic vapors or toxic gases listed in"List of Highly Toxic Materials"（Health Law Supervision Issue No.〔2003〕142），e.g.，nitrogen dioxide，hydrogen sulfide，benzene，hydrogen cyanide，ammonia，chlorine，carbon monoxide，acrylonitrile，chloroethylene and phosgene（carbonyl chloride），etc.
2.0.3 Source of release
Point or location from which a flammable gas，vapour or liquid may be released into the atmosphere such that an explosive gas atmosphere could be formed.
2.0.4 Detector
It means an electronic unit，consisting of sensors and transducers to convert concentration signal of combustible gas and toxic gas into electrical signal.
2.0.5 Indication and alarm device
It means electronic device that receives output signals from detectors to give out indication，alarm and control signals.
2.0.6 Detective range
It means the range of gas concentration that can be detected by detectors under test conditions.
2.0.7 Alarm set point
It means alarming concentration values preset in alarm unit.
2.0.8 Response time
It means time interval，from the first detection to the stable indication of a gas by a detector under test conditions.Normally，the time to 90% stable indication is referred to as response time and the time returning to 10% stable indication as recovery time.
2.0.9 Detector mounting height
It means the vertical distance from detection port of a detector to specified reference.
2.0.10 Lower Explosion Limit（LEL）
It means lower explosion limit concentration（V%）of a combustible gas.
2.0.11 Upper Explosion Limit（UEL）
It means upper explosion limit concentration（V%）of a combustible gas.
2.0.12 Maximum Allowable Concentration（MAC）
It means the concentration of any toxic chemical substance possibly exposed to that shall not be exceeded at any time and at any place within one working day.
2.0.13 Permissible Concentration Short Term Exposure Limit（PC-STEL）
It means the permissible concentration of exposure of 15-minute time weighted average that shall not be exceeded during one exposure.
2.0.14 Permissible Concentration Time Weighted Average（PC-TWA）
It means permissible average concentration of exposure time weighted for any are 8-hour working day.
2.0.15 Immediately Dangerous to Life or Health concentration（IDLH）
It means the hazardous concentration of pollutants in ambient air，e.g.，resulting in death or permanent health damages，or causing immediate loss of the capability of escape.