An Explosimeter is used to test for the presence of flammable gases or vapors in spaces like pump rooms or cargo tanks.

• Consists of a Wheatstone bridge circuit powered by a battery.
• Contains three fixed resistances and one resistance that is a hot filament within the combustion chamber.
• A galvanometer measures the current flow in the circuit and is scaled to indicate gas concentration as a percentage of the Lower Flammable Limit (LFL) or in parts per million (ppm).
• An aspirator bulb and flexible tube are used to draw a gas sample into the combustion chamber.

Working:

1. A gas sample is drawn into the combustion chamber using the aspirator bulb.
2. If flammable gas is present, it burns on the hot filament, causing a rise in the filament’s temperature.
3. The rise in temperature increases the filament’s resistance, unbalancing the Wheatstone bridge circuit.
4. This unbalance generates a current, which is indicated on the galvanometer. The scale converts this reading into % LFL or ppm.

(ii) Reasons for False Readings by an Explosimeter:

1. If the oxygen content in the sampled space is insufficient or if inert gas is present, combustion on the filament may not occur, leading to inaccurate readings.
2. The explosimeter is designed to detect gas up to the Lower Flammable Limit (LFL). If the gas concentration exceeds the instrument’s range, false or misleading readings may occur.
3. If the instrument is not purged before use, residual gas from a previous reading may result in an inaccurate measurement.
4. A drained or insufficiently charged battery can affect the accuracy of the readings.

• IMO Resolution A.1050(27): Revised Recommendations for entering enclosed spaces aboard ships. This resolution provides detailed guidance on safe entry procedures into enclosed spaces.
• SOLAS Chapter III, Regulation 19 (as of January 1st, 2015): This regulation addresses enclosed space drills, requiring them to be conducted every two months.
• SOLAS Regulation XI-1/7 (as of July 1st, 2016): This regulation mandates that vessels carry portable instruments capable of measuring oxygen concentration, flammable gases, hydrogen sulfide (H2S), and carbon monoxide (CO).
• Code of Safe Working Practices (COSWP), Chapter 17 is dedicated to "Enclosed Spaces", providing detailed guidance on the identification, risks, and safe procedures associated with entering and working in such environments.