What is an Explosive Limit?

G. Wiesen

An explosive limit is a measurement that indicates the amount of a particular gas that must be present in air for that mixture to be flammable or combustible. This measurement is typically given as a percentage and will usually be accompanied by an indicator of the temperature and pressure in which this measurement is accurate. Both temperature and pressure can alter this number, so standardization is important when comparing the limits of various chemicals. An explosive limit is often indicated in two different forms: the lower explosive or flammable limit that indicates the minimum amount necessary for combustion and the upper explosive or flammable limit that indicates the maximum amount that can be present for a mixture to be flammable.

Scientist with beakers
Scientist with beakers

Also called the flammability limit, an explosive limit typically indicates the quantity of a particular gas or chemical that must be present in air for that mixture to be flammable. Different gases have different explosive limits, and understanding these various limits is important for proper ventilation in any environment where such gases are being used. The explosive limit for a gas is indicated as either two different values, the upper and lower limits, or as an expression of the range between these two limits.

A lower explosive limit for a gas is the percentage of a gas that must be present for it to become combustible. Any amount of the gas below this percentage is considered “too poor” to become combustible. This is important for storage of potentially combustible gases, and most regulations require that proper ventilation ensures an amount no greater than 25% of this lower explosive limit be allowed.

The upper explosive limit for a gas is the highest amount of that gas that can be present for the gas to remain combustible. If this limit is surpassed, then the mixture is referred to as “too rich” and will no longer combust properly. This value is important for mechanical engineering and other work that utilizes combustible gas, to ensure that mixtures react properly for various uses.

An explosive limit for a gas will typically depend on the pressure and temperature in which that gas is contained. Greater temperatures reduce the lower limit and increase the upper limit, creating a wider range in which gases are explosive. Higher pressures, however, increase both values, but flammable gases contained at higher pressures may result in explosive combustion if ignited. This is why the explosive limit for a gas will usually be indicated with the temperature and pressure for that given limit, to allow proper comparison of different gases.

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