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To classify gases and vapours as lighter or heavier than air derives from the specific weight of the pure components. Hydrogen, methane
and ammonia are mainly lighter than air. All other components, especially higher hydrocarbons, are generally heavier than air.
The extension mechanisms derived from the rule of thumb mentioned above are only valid for undiluted components - this means, a gas
concentration of 100 Vol.-% supposed.
The use of gas detection systems serves for monitoring of undesired components at very low concentrations. On the field of explosion
protection the ranges of concentrations are below 5 % (v/v), whereas the ranges for toxic gases are in ppm and much lower.
There is no real difference in specific weight between gas mixtures with such low concentrations and ambient air. Therefore their
extension does not depend on the up- or down-draft of the gas, but on other streaming influences, such as wind or thermal lift.
A mixture of gas and air is not split into its fractions – as wrongly supposed sometimes- in case a mixture has already taken place
(Fig. 1). This effect – known for example from water / oil mixtures – does not appear on gases.
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