Class 4 (4th Engineer) SSEP 📅 Feb 2025

Exam Question

Petroleum vapours are a dangerous substance and when mixed with air can be ignited. With this in mind, briefly describe,

(a) An explosimeter or combustion gas indicator which can be used to check the atmosphere of a tank or pump room (10)

(b) Describe the explosimeter and its operation. (10)

Reference Answer

### (a) & (b) Description and Operation of an Explosimeter
An explosimeter, also known as a combustible gas indicator, is a portable instrument designed to measure the concentration of flammable gases or vapours in the air. It provides a reading as a percentage of the Lower Explosive Limit (% LEL). Its primary use on board ships is to test the atmosphere in enclosed spaces like cargo tanks, pump rooms, and cofferdams before entry or before commencing hot work, ensuring the environment is safe from the risk of explosion.
#### 1. Principle of Operation
The most common type of explosimeter operates on the principle of **catalytic combustion** within a **balanced Wheatstone bridge circuit**.

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