Recent underground coal mine incidents have again raised the issue of: under what circumstance is it safe for personnel to re-enter the mine? Mine rescue guidelines make recommendations about the limits in terms of the circumstances, lives at risk, no lives at risk and training exercises. These are based upon percentages of the upper and lower explosive limits. Many factors influence setting limits for safe re-entry. These include: the representativeness of the sampling location(s), the rapidity of getting an underground sample analysed and the rate of change of the atmosphere being monitored. Value judgements based upon these parameters will then influence the setting of a trigger point based upon per cent explosibility. This raises the question of how the percentage of the lower or upper explosive limit is established. It is easy to define this when the sampling point data sit on the fresh air limit line. However - what value is used when the sample sits somewhere in the body of an explosibility diagram? This paper explores the various options and illustrates various pitfalls and problems that are faced in making such a determination. These include how the data are represented and the assumed knowledge of those doing the interpretation.
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