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Where does the danger lie? When measuring dangerous gases prior to enclosed space entry, we need to take into account the relative weight of the gas when compared to air. For example, we need to be aware that methane is lighter than air, carbon monoxide is the same weight and hydrogen sulphide is heavier than air. This difference in molecular weight requires gas measurements to be taken at different heights of the enclosed space to ensure thorough gas measurement prior to man entry into enclosed space. 

You can learn more from

Gard awareness campaign on Enclosed space entry training
Alert The silent and invisible killer onboard vessels
Loss prevention poster Dangerous gases

IACS Confined Space Safe Practice