When selecting combustible gas detector and toxic gas detector, you need to know the following knowledge.

Significance of explosion limit:

Combustible substances (combustible gas, steam and dust) and air (or oxygen) must be uniformly mixed within a certain concentration range to form premixed gas. Explosion will occur only when encountering fire source. This concentration range is called explosion limit or explosion concentration limit. For example, the explosion limit of carbon monoxide and air mixture is 12.5% - 80%. The lower and higher concentrations of combustible mixtures can explode are called the lower explosion limit and the upper limit of explosion respectively, which are sometimes referred to as the lower limit of ignition and the upper limit of ignition. When the explosion is lower than the lower limit and the concentration is higher than the upper limit of explosion, neither explosion nor fire is allowed. This is because the former is not enough combustible concentration, excessive air cooling, preventing the spread of flame; The latter is the lack of air, which leads to the flame not spreading. When the concentration of combustible is approximately equal to the reaction equivalent concentration, it has a greater explosive power (i.e. the concentration ratio calculated according to the complete combustion reaction equation).

Factors affecting explosion limit:

The explosive limit of the mixed system is different with the composition. The explosion limit of the same mixing system can be changed due to the initial temperature, system pressure, inert medium content, the space of the mixing system, the material of the wall and the size of ignition energy. The general rule is that the original temperature of the mixing system increases, the explosion limit range increases, namely, the lower limit and the upper limit increase. Because of the increase of system temperature and the increase of internal energy in the molecule, the original unburned mixture becomes a combustible and explosive system. The system pressure increases and the explosion limit range is also expanded. This is because the system pressure increases, the distance between molecules is closer, the collision probability is increased, and the combustion reaction is easier to carry out. The explosion limit range is reduced when the pressure is reduced; When the pressure drops to a certain value, the upper limit coincides with the lower limit, and the corresponding pressure is called the critical pressure of the mixing system. When the pressure drops below the critical pressure, the system will not become an explosive system (abnormal phenomena in some gases). The amount of inert gas in the mixing system increases, the explosion limit is reduced, and the concentration of inert gas increases to a certain value, so the mixture system cannot explode. The smaller the diameter of the vessel and pipe, the smaller the explosion range. When the diameter of the pipe (flame channel) is small to a certain extent, the heat released from the solid cooling surface corresponding to the unit volume flame will be greater than the heat generated, and the flame will be interrupted and extinguished. The larger diameter of the flame which cannot be transmitted is called the critical diameter of the mixing system. The explosion limit will be expanded due to the high intensity of ignition energy, large area of hot surface and contact time between ignition source and mixture. In addition to the above factors, the materials, mechanical impurities, illumination, surfactant and so on of the closed shell which the mixing system contacts may affect the explosion limit range.

Explosion limit and the harm of combustible:

The wider the explosion limit range, lower the lower the explosion limit and the higher the upper explosive limit, the greater the explosion risk of the combustible mixture. This is because the wider the explosion limit, there are more chances of explosion conditions; The lower the lower the explosion limit, the explosive condition will be formed if the combustible material leaks slightly; The higher the explosion limit, a small amount of air will penetrate into the container, which can be mixed with the combustible in the container to form the explosion conditions. It should be noted that when the concentration of combustible mixture is higher than the upper limit of explosion, although it will not ignite and explode, it can be burned when it escapes from the container or pipe and contacts air again, and there is still a risk of fire.

Explosion limit and the harm of combustible:

The unit of explosion limit of a unit gas or vapor, expressed as a percentage (%) of the volume in the mixture, such as the explosion limit of a mixture of hydrogen and air of 4% - 75%. The explosion limit of combustible dust is expressed by the mass ratio of the volume in the mixture, such as the explosion limit of aluminum powder is 40g/m3. The explosion limit of combustible vapor is determined by the vapor concentration produced on the surface of combustible liquid. For combustible liquid, the flash point temperature corresponding to the lower explosion concentration can be called the lower explosion temperature; The liquid temperature corresponding to the upper explosive concentration can also be called the upper explosive temperature.

Combustible gas or vapor	Explosion Limit (%)
	UEL	LE
H2	4.0	75
NH3	15.5	27
CO	12.5	74.2
CH4	5.3	14
C2H6	3.0	12.5
C2H4	3.1	32
C2H2	2.2	81
C6H6	1.4	7.1
C7H8	1.4	6.70
C2H4O	3.0	80.0
(C2H5)O	1.9	48.0
CH3CHO	4.1	55.0
(CH3)2CO	3.0	11.0
C2H5OH	4.3	19.0
CH3OH	5.5	36
C4H8O2	2.5	9