Glossary / Physical properties
Upper explosive limit (UEL)
Upper explosive limit (UEL) is the highest concentration of a flammable gas or vapor in air, usually expressed as percent by volume, at which the mixture can still ignite. Above the UEL there is too little oxygen for combustion, so the mixture is too rich to burn.
Explanation
The UEL is the top of the Flammable range, with the Lower explosive limit (LEL) at the bottom. Only concentrations between the two can burn or explode. A tank or drum full of vapor may sit above its UEL and therefore not be ignitable inside, yet the situation changes the moment air mixes in.
This is why rich atmospheres are still dangerous. When vapor leaks out of a container or is vented, it dilutes as it mixes with air and passes through the flammable range on the way. A person who sees a cloud and thinks it is too concentrated to burn can be wrong, because the edges of that cloud are in range.
Gases and vapors with a very wide range between LEL and UEL are generally harder to control, because there are more ways for a flammable mixture to form. The figures are measured under test conditions, so temperature, pressure and the presence of oxygen-enriched air can widen or narrow them.
Fuel-rich storage is sometimes used on purpose, for example when a vessel is kept above the UEL as a control measure, but this relies on the system staying sealed and needs expert design. It is not a shortcut for everyday handling.
What to do and watch for
- Do not assume a heavy vapor cloud is safe from ignition; its edges are likely within the flammable range.
- Treat empty containers that held flammable liquids as hazardous, since residual vapor can sit anywhere in the range.
- Keep ignition sources out of any area where vapor can escape.
Where it appears on an SDS
- Section 9: Physical and chemical properties. Provides the upper explosion or flammability limit, typically as percent by volume in air, next to the lower limit.
SDS content and wording vary by supplier. This page is general reference and does not replace the current SDS for your product.