Explosimeter Working Principle
The atmosphere of a tank or pump room can be tested with a combustible gas indicator which is calibrated for hydrocarbons. Frequently the scale is in terms of the lower explosive or lower flammable limit (LFL) and marked as a percentage of the lower limit. Alternatively, the scale may be marked in parts per million (ppm). 
A lean mixture will burn in the combustion chamber, because of the filament. False readings are likely when oxygen content of the sample is low or when inert gas is present. The instrument is designed for detecting vapour in a range up to the lower flammable limit and with large percentages of gas (rich mixture) a false zero reading may also be obtained.
The instrument and batteries must be tested before use and samples are taken from as many places as possible particularly from the tank bottom. It is possible to obtain a reading for any hydrocarbon but not for other combustible gases on an instrument which is scaled for hydrocarbons. Detection of other vapours must be by devices intended for the purpose.
The explosimeter is primarily a combustible gas detector but will also give guidance with regard to safety of a space for entry by personnel. If a space has been ventilated to remove vapours, the remaining concentration can be measured with explosimeter, provided that it is below the lower flammable range. Generally any needle deflection above zero is taken as indicating a toxic condition.
Crude oils contain all of the hydrocarbon products extracted in the refinery and many of the products are highly toxic. Benzene (C2H6) is an example and its low threshold limit value (TLV) of 10 ppm indicates this. Sour Crude oils carry highly toxic hydrogen sulphide (H2S) with a TLV also of 10 ppm. Petrol (Gasoline) has a TLV of 300 ppm. Entry to the cargo tanks and pump-rooms of a crude oil carrier exposes personnel to these risks. These are additional risks involved in the tank entry, where inert gas has been used. The inert gas adds the risk of Carbon monoxide (CO) which has a TLV of 50 ppm; Nitrogen dioxide (NO2) with 3 ppm; nitric oxide (NO) with 25 ppm; and sulphur dioxide (SO2) with 2 ppm. Trace amounts of the hydrocarbon products which are very dangerous, and other toxic gases which may be present, require special means of detection. Threshold limit values are updated annually and given in references available from health and safety authorities.
REFERENCES
“General Engineering Knowledge” by H.D. McGeorge