The Explosimeter is the name normally associated with the instrument for measuring hydrocarbon gas in air at concentrations below the Lower Flammable Limit. Its full name is a Catalytic Filament Combustible Gas Indicator.
A full understanding of the construction and principle of an Explosimeter is essential for its safe and efficient use and it is essential that any person using this instrument carefully studies the operating manual. There is also a detailed explanation in the ISGOTT carried on tankers.
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The Explosimeter measures from 0 to 100% of the Lower Explosive Limit (1.4% by volume). If the gas to air mixture is above the upper explosive limit (6% by volume) the meter reading will initially rise to give a reading of 100% or above, but will rapidly fall towards zero because the mixture of gas and air in the combustion chamber is too `rich' to sustain combustion.
The meter must therefore be constantly observed for this phenomenon, as an apparently safe reading may be obtained when the atmosphere is in fact highly dangerous.
Calibration checks must be carried out at two monthly intervals and when a filament has been changed in accordance with manufacturers’ instructions. Note that, in general, an explosimeter may be calibrated by different gases. It is essential that the correct gas is used otherwise an error may result. Explosimeters will not read hydrocarbon levels in an inert atmosphere.
Oxygen Analyzer Equipment's
According to the new law, all ships are to be supplied with a portable oxygen analyser. This equipment is supplied for use in checking that spaces to be entered have been properly ventilated. It is also to be used on Tankers, Gas Ships and Chemical Carriers to check that the atmosphere of inerted tanks remains below 7%.
Two tests should be carried out on the instrument prior to use and a permanent record of readings kept on board.
(i) Zero Adjustment
This is done by using an oxygen-free gas, such as Nitrogen or Carbon Dioxide. Equipment is supplied for this test. Note that CO2 is paramagnetic and therefore may not give a zero reading on certain instruments.
(ii) Span Adjustment
This must be done in FRESH AIR and the instrument carefully checked that the reading has stabilised at 21% before the atmosphere of any space is tested.