Lower / Upper Explosive Limits
Lower and Upper Explosive Limits for Flammable Gases and Vapors. Before a fire or explosion can occur, three conditions must be met simultaneously. A fuel i.e. combustible gas and oxygen (air) must exist in certain proportions, along with an ignition source, such as a spark or flame. The ratio of fuel and oxygen that is required varies with each combustible gas or vapor.
The minimum concentration of a particular combustible gas or vapor necessary to support its combustion in air is defined as the Lower Explosive Limit (LEL) for that gas. Below this level, the mixture is too “lean” to burn.
The maximum concentration of a gas or vapor that will burn in air is defined as the Upper Explosive Limit (UEL). Above this level, the mixture is too “rich” to burn. The range between the LEL and UEL is known as the flammable range for that gas or vapor.
Eg: Methane
LEL: 5% by volume in Air.
UEL: 17% by volume in Air.
Lower and Upper Explosive Limits
The values shown in the table below are valid only for the conditions under which they were determined (usually room temperature and atmospheric pressure using a 2-inch tube with spark ignition). The flammability range of most materials expands as temperature, pressure and container diameter increase. All concentrations in percent by volume.
Gas LEL UEL
Acetone 2.6 13
Acetylene 2.5 100
Acrylonitrile 3 17
Allene 1.5 11.5
Ammonia 15 28
Benzene 1.3 7.9
Butadiene 2 12
Butane 1.8 8.4
n Butanol 1.7 12
Butene 1.6 10
Cis 2 Butene 1.7 9.7
Trans 2 Butene 1.7 9.7
Butyl Acetate 1.4 8
Carbon Monoxide 12.5 74
Carbonyl Sulfide 12 29
Chlorotrifluoro ethylene 8.4 38.7
Cumene 0.9 6.5
Cyanogen 6.6 32
Cyclohexane 1.3 7.8
Cyclopropane 2.4 10.4
Deuterium 4.9 75
Diborane 0.8 88
Dichlorosilane 4.1 98.8
Diethylbenzene 0.8
Difluoro Chloroethane 9 14.8
Difluoroethane 5.1 17.1
Difluoro ethylene 5.5 21.3
Dimethylamine 2.8 14.4
Dimethyl Ether 3.4 27
Dimethyl propane 1.4 7.5
Ethane 3 12.4
Ethanol 3.3 19
Ethyl Acetate 2.2 11
Ethyl Benzene 1 6.7
Ethyl Chloride 3.8 15.4
Ethylene 2.7 36
Ethylene Oxide 3.6 100
Gasoline 1.2 7.1
Heptane 1.1 6.7
Hexane 1.2 7.4
Hydrogen 4 75
Hydrogen Cyanide 5.6 40
Hydrogen Sulfide 4 44
Isobutane 1.8 8.4
Isobutylene 1.8 9.6
Isopropanol 2.2
Methane 5 17
Methanol 6.7 36
Methylac etylene 1.7 11.7
Methyl Bromide 10 15
3 Methyl 1 Butene 1.5 9.1
Methyl Cellosolve 2.5 20
Methyl Chloride 7 17.4
Methyl Ethyl Ketone 1.9 10
Methyl Mercaptan 3.9 21.8
Methyl Vinyl Ether 2.6 39
Monoethy lamine 3.5 14
Monomethy lamine 4.9 20.7
Nickel Carbonyl 2
Pentane 1.4 7.8
Picoline 1.4
Propane 2.1 9.5
Propylene 2.4 11
Propylene Oxide 2.8 37
Styrene 1.1
Tetrafluoro ethylene 4 43
Tetrahydrofuran 2
Toluene 1.2 7.1
Trichloro ethylene 12 40
Trimethylamine 2 12
Turpentine 0.7
Vinyl Acetate 2.6
Vinyl Bromide 9 14
Vinyl Chloride 4 22
Vinyl Fluoride 2.6 21.7
Xylene 1.1 6.6
Principles of Gas Detection
One of the many requirements for entering confined spaces is the measurement for flammable gases. Prior to entry of a confined space, the level of flammable gases must be below 10% of LEL.
The most common sensor used for measuring LEL is the Wheatstone bridge/catalytic bead/Pallister sensor (“Wheatstone bridge”).
LEL Sensors Explained
A Wheatstone bridge LEL sensor is simply a tiny electric stove with two burner elements. One element has a catalyst (such as platinum) and one doesn’t. Both elements are heated to a temperature that normally would not support combustion.
However, the element with the catalyst “burns” gas at a low level and heats up relative to the element without the catalyst. The hotter element has more resistance and the Wheatstone bridge measures the difference in resistance between the two elements, which correlates to LEL.
LEL Detector
Unfortunately, Wheatstone bridge sensors fail to an unsafe state; when they fail, they indicate safe levels of flammable gases. Failure and/or poisoning of Wheatstone bridge LEL sensor can only be determined through challenging Wheatstone bridge sensors with calibration gas.
LEL Sensors Limitations
Two mechanisms affect the performance of Wheatstone bridge LEL sensors and reduce their effectiveness when applied to all but methane:
PID
A Photo-Ionization Detector measures VOCs and other toxic gases in low concentrations from ppb (parts per billion) up to 10,000 ppm (parts per million or 1% by volume).
A PID is a very sensitive broad-spectrum monitor, like a “low-level LEL monitor. A Photo-Ionization Detector measures VOCs and other toxic gases in low concentrations from ppb (parts per billion) up to 10,000 ppm (parts per million or 1% by volume). A PID is a very sensitive broad-spectrum monitor, like a “low-level LEL monitor.
How does a PID Work?
A Photo Ionization Detector (PID) uses an Ultraviolet (UV) light source (Photo-light) to break down chemicals to positive and negative ions (Ionization) that can easily be counted with a Detector. Ionization occurs when a molecule absorbs the high energy UV light, which excites the molecule and results in the temporary loss of a negatively charged electron and the formation of positively charged ion.
PID – Photo-Ionization Detector
The gas becomes electrically charged. In the Detector these charged particles produce a current that is then amplified and displayed on the meter as “ppm” (parts per million) or even in “ppb” (parts per billion).
The ions quickly recombine after the electrodes in the detector to “reform” their original molecule.
PIDs are non-destructive; they do not “burn” or permanently alter the sample gas, which allows them to be used for sample gathering.