About Gas – Terminology

 

Lower Explosive Limit (LEL)

The lowest concentration of combustible gas or vapour in air below which an explosive mixture will not occur. For most combustible gases or vapours this is less than 5% v/v. A list of the LEL values for common substances appears in the “Cross Sensitivity” section

Upper Explosive Limit (UEL)

The maximum concentration of combustible gas or vapour in air above which an explosion will not occur.

Vapour Density

Vapour density is a measure of the density of a gas or vapour relative to air. Gases or vapours with a vapour density of less than one (‹ 0.8) are lighter than air and tend to rise from the point of escape. Gases or vapours with a density heavier than air (› 1.2) tend to sink to lower levels and can behave like a liquid.

 

Flash Point

This is the lowest temperature at which vapour is given off at a sufficient rate to form an explosive mixture with air. Flammable liquids generally have a low Flash Point.

Ignition Temperature

When an explosive mixture of gas or vapour and air has developed it can be ignited either by a spark of sufficient energy or if it has been exposed to a surface at a sufficiently high temperature. The lowest temperature that will cause such a mixture to burn or explode is called the Ignition Temperature.

MAC/TLV/TWA

The MAC (Maximum Acceptable Concentration), TLV (Threshold Limit Value) or TWA (Time Weighted Average) relate to the time-weighted average concentration for a normal 8-hour working day or 40-hour working week to which most workers may be repeatedly exposed, day after day, without adverse effect.