Question 8 TK02 - Tank Vessel Assistant - Liquefied Gas

The Correct Answer is D **Explanation for Option D (Correct):** Combustible gas indicators (CGIs), often called explosimeters, typically operate based on the principle of catalytic combustion. They measure the percentage of the Lower Explosive Limit (\%LEL) of the atmosphere. For the catalytic sensor to function correctly, there must be enough oxygen available to combust the gas sample. When the concentration of the hydrocarbon (combustible gas) in the atmosphere exceeds the Upper Explosive Limit (UEL), the gas mixture is too rich (i.e., there is not enough oxygen to burn all the fuel). This lack of oxygen results in the sensor filament not heating up or burning the gas effectively, leading to: 1. **"Drowning" or "Quenching" of the sensor:** The sensor is saturated with gas, displacing the necessary oxygen. 2. **False Reading:** The instrument will either fail to register the dangerously high gas concentration (often reading zero or a very low \%LEL) or will become temporarily or permanently inoperative until recalibrated or the sensor is replaced. Therefore, an atmosphere above the UEL causes the instrument to give an erroneous (typically low or zero) reading, despite the high gas concentration. **Explanation for Incorrect Options:** **A) Inerted content of the atmosphere is less than the lower explosive limit:** This phrasing is slightly confusing, but it likely refers to the presence of an inert gas (like nitrogen or CO2) being relatively low. If the atmosphere is truly below the LEL (regardless of inert content), the CGI will accurately read a low \%LEL, meaning it is operating correctly. If the atmosphere is inerted to the point where oxygen is low, that **would** cause an error (oxygen deficiency), but "less than the LEL" describes a condition where the gas concentration is safe and the meter works fine. **B) Compartment to be tested is free of CO2:** Carbon dioxide ($\text{CO}_2$) is an inert gas that does not participate in the combustion process measured by the CGI. Its absence (being "free of $\text{CO}_2$") is an ideal condition and will not cause the instrument to become inoperative or give erroneous readings, assuming sufficient oxygen is present. (Note: High concentrations of $\text{CO}_2$ would displace oxygen and cause an erroneous reading, but its *absence* is not a problem.) **C) Atmosphere has sufficient oxygen:** A sufficient concentration of oxygen (typically $\ge 10\%-12\%$ depending on the instrument) is **required** for the catalytic sensor to operate correctly. If the oxygen is sufficient, the instrument will operate as intended and give an accurate reading of the \%LEL. This condition is necessary for proper operation, not a cause for error.