Gas monitoring is widely used to detect the presence of a gas, which may cause a hazard, or the absence of a desirable gas. Gas monitoring is broadly split into three categories:
- Combustible gases and vapours
- Toxic gases
- Oxygen deficiency or excess
Combustible gases and vapours
A wide range of gases can form a combustible mixture with air. Modern combustible gas detectors are designed to detect for the presence of a build up of a combustible gas long before it presents a hazard to personnel or plant and equipment. Combustible gases can be normally present due to a range of industrial or domestic processes.
For a gas to burn in air its concentration in air must fall within certain well-defined limits. If the concentration is too small then it cannot sustain a flame and will not ignite. This low limit for combustion is termed the Lower Explosive Limit. Similarly if the concentration is too high then there will be insufficient oxygen for the gas to combine with for combustion. This upper limit is termed the Upper Explosive Limit.
- The concentration of the gas is between the LEL and UEL
- A sufficient amount of Oxygen exists
- There is a source of ignition
However, under certain circumstances, gas can explode even if not mixed with air. This is possible either because the gas molecules contain oxygen which must be liberate to support combustion or because the molecular structure is unstable and may decompose explosively if for example it is stored under pressure or comes into contact with a triggering agent.