Explosimeter is device that is used to determine the content of hydrocarbon in the atmosphere of pump room or tank spaces on ships. The scale used in the explosimeter is marked in terms of lower explosive or flammable limit and as a percentage of the lower limit (LEL). The scale may also be marked in parts per million (p.p.m).
Explisometer works on the principle of Wheatstone bridge. The Wheatstone bridge is supplied with a battery and there is no flow of current through the meter when the bridge resistance is balanced. One of the four resistances in the Wheatstone bridge is a hot filament. This resistance is enclosed in a chamber wherein a sample is drawn with the help of flexible tube and aspirator.
The combustion of the atmospheric sample takes place in the chamber in the presence of hot filament. The combustion of the gas drawn from the atmosphere causes an increase in the temperature, which further causes changes in resistance and imbalance in the Wheatstone bridge. Due to this imbalance, current flows through the meter and the reading is calibrated to indicate in percentage of LEL or P.P.M.
A false reading may be obtained if the percentage of oxygen in the sample is low or due to the presence of inert gas. This instrument is designed for detecting vapors in range up to lower flammable limit. False reading may be obtained if the percentage of gas is too high.
The instrument must be tested before use and should be purged enough so that there is no left over atmosphere in the chamber of last test. The sample should be taken from as many place as possible particularly from tank bottom to provide an accurate result.
Marine Auxillary Machinery By H.D Mc George