To measure explosive limits you need a gas detector capable of measuring this type of vapour and outputting, normally, in % of LEL. Hence a reading of 50% means you're 50% towards getting to the lower limit. Often work stops when you get to 5% of LEL.
HEL is normally just used when you're justifying something based on the concentration of gasoline is well above the HEL and hence a hazard won't exist. However usually at some point the atmosphere needs to move from 0% ( empty, new condition) to above the HEL, so it's really a bit academeic.
No idea what AIFR is (Aluminium Internal Floating Roof?). No idea why you want to measure gasoline vapour in this space - everyone just assumes it is a Zone 0 and acts accordingly. Hence there are no "permissible levels" other than the 5% of LEL I was talking about which is the limit for doing work in a potentially explosive atmosphere. No one does work inside a storage tank which contains gasoline, even with an internal roof. Equally no one actually bothers to measure gasoline vapour inside such a space either - there's no point.
You might need to measure the vapour coming out of the tank or drifting somewhere else on site, but not in the tank itself, IMHO.
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