The explosimeter is an instrument which is specifically designed for measuring the lower flammable limit (LFL). It will only function correctly if the filament has an explosive mixture in contact with it.
When in use, the sample tube is lowered into the tank and a sample of the atmosphere is drawn up into the instrument by several depressions of the rubber aspirator bulb. If the sample contains an explosive mixture the resistance of the catalytic filament will change due to the generated heat.
An imbalance of the wheat-stone bridge is detected by the ohm meter which tells the operator that hydrocarbon gas is present in the tank in sufficient quantity to support combustion.
The graduations on the meter are a per cent of the lower explosive limit (LEL) reading between 0% and 100%. A deflection of the meter between 0% and 100% shows how close the atmosphere being tested approaches the minimum concentration required for explosion. When a test is made with the explosimeter, and a deflection to the extreme right-hand side of the scale is noted and remains there, then the atmosphere under test is explosive.
Limitations of explosimeters:
The explosimeter has been designed to detect the presence of flammable gases and vapours. The instrument will indicate in a general way whether or not the atmosphere is dangerous from a flammability point of view. It is important to realize that such information obtained from the instrument is appraised by a person skilled in the interpretation of the reading, bearing in mind the environment.
The oxygen analyser
The oxygen analyser is an instrument that measures the oxygen content of an atmosphere to establish whether entry is possible, but it is also employed for inerted spaces which must be retained under 5% oxygen to affect a safe atmosphere within the tank.
The oxygen sensor will be either an electromagnetic heated filament or an electrochemical resistor cell. The instrument was designed to measure the oxygen content only and will not detect the presence of any other gases. Oxygen analysers are portable instruments which draw a sample of the atmosphere for testing through a sampling hose by means of a rubber aspirator bulb. The principle of operation is a self-generating electrolytic cell in which the electric current is directly proportional to the percentage oxygen in a salt solution connecting to the electrodes.
The electrodes are connected to a micro-ammeter, so that the current read by the meter can be calibrated to indicate directly the percentage oxygen of the sample.