Flammable Limits (LEL/UEL) Testing

Our test method adheres to ASTM E681 / ASHRAE 34, and used the same quality system as our accredited tests.

This test is conducted to determine the lower and upper flammable concentration limits of chemicals that have sufficient vapour pressure to form flammable mixtures at atmospheric pressures at specific temperatures.

This test can be conducted on vaporised liquids and gaseous materials. Refrigerant gases are tested to ASHRAE 34, using the method detailed within ASTM E681, with specific ignition criteria and air moisture content.

The Lower Explosive Limit (LEL) or lower flammable limit (LFL) is the minimum concentration of a combustible substance that can propagate a flame in a homogeneous mixture of the combustible material and the air, under the specified conditions of the test. Generally, the material is added to a glass vessel in 1% v/v steps under a vacuum.

Air is allowed back into the system and the material and air are mixed before an electrical source of ignition is activated, and observations for flame propagation are observed. This test can be performed at any test temperature up to 300 °C, to allow for vaporisation of material and match industrial process conditions.  

Upper explosive limit (UEL) or upper flammable limit (UFL) is the maximum concentration of a combustible substance that can propagate a flame in a homogeneous mixture of the combustible material and air, under the specified conditions of the test. Once the Upper Explosive Limit has been established, a theoretical Upper Explosive Limit is calculated, and testing is started at a concentration above.

The material is added to a glass vessel under a vacuum, air is allowed back into the system and the material and air are mixed before an electrical source of ignition is activated, and observations for flame propagation are observed. The material is then incrementally decreased in 1% steps until ignition is observed and the upper explosive limit is defined.

To ensure that avoiding a flammable atmosphere as a ‘basis of safety’ is accurately conducted and monitored and to make sure that flammable limit alarms and detection systems are correctly set. A mixture of materials cannot simply be based upon worst-case constituent values, therefore testing of the product is essential.