(ii) Description and operation: The explosimeter works on the catalytic combustion principle. A known volume of the tank atmosphere is drawn by the aspirator through the probe into the analyser, where it passes over a heated catalytic sensing element (a platinum/platinum-rhodium coil, often coated with a catalyst). When a flammable gas is present, the gas burns catalytically on the hot filament, raising the filament temperature relative to a reference element; this change in resistance upsets a Wheatstone bridge, producing a current proportional to the amount of flammable vapour, which is displayed on the meter as % of the Lower Flammable Limit (LFL). The scale is usually calibrated for a specific gas (e.g. n-hexane/methane) and indicates the percentage of LFL reached (e.g. 10%, 50%). It is used to check whether a tank/pump room atmosphere is within the flammable range and safe/entry-appropriate (an atmosphere is considered unsafe for entry when above 10% LFL and gas-free when below ~1% LFL).

(iii) Reason for a false reading: A common cause is that the meter is calibrated for a particular gas (e.g. methane or hexane) but the actual vapour is a different hydrocarbon with different calorific value (e.g. gasoline/mixture), giving an incorrect reading; conversely, high oxygen/high temperature, the presence of other gases, poisoning of the catalytic element (by silicones/halogenated compounds/sulfur), or a depleted/over-heated filament, or a low battery, or a wrong zero/calibration can cause an erroneous (often low) reading. Also in an oxygen-deficient atmosphere the catalytic sensor will read too low (fails to respond), so the "safe" reading may be misleading.

• LFL (lower flammable/explosive limit): the lowest concentration (volume %) of flammable vapour in air at which the mixture can be ignited (propagate flame) in the presence of an ignition source. Below the LFL the mixture is too lean to ignite ("too little fuel").
• UFL (upper flammable/explosive limit): the highest concentration of vapour in air at which the mixture can ignite; above the UFL the mixture is too rich to burn (insufficient oxygen to support flame). Between LFL and UFL lies the flammable/explosive range. For most petroleum, the LFL is around 1-4% and UFL around 6-10% (by volume in air). A tank atmosphere outside this range (below LFL or above UFL) is not flammable at ambient, but a mixture within the range is hazardous; hence monitoring is essential before entry/gas-free.