Question 53 GLI05 - Master or Mate of LT 200 GRT

The Correct Answer is D **Why option D ("fresh air") is correct:** An oxygen indicator (or oxygen analyzer) measures the concentration of oxygen. After taking a reading, the instrument's sensing cell and sampling line will contain the sample gas being tested. If the sample was oxygen-deficient (e.g., nitrogen or CO2 inerted atmosphere) or oxygen-rich, the sensor needs to be brought back to a known baseline state to ensure accuracy for the next measurement. Purging with "fresh air" (ambient air, typically containing about 20.9% oxygen) establishes a clear baseline and allows the instrument to recover quickly and verify its calibration against a known standard atmosphere before the next test is performed. **Why the other options are incorrect:** * **A) CO2:** Purging with CO2 (or any inert gas) would rapidly drive the oxygen reading to 0%. While this verifies the lower limit of the sensor, it does not reset the instrument to a readily achievable, standard baseline (like ambient air) that is used for typical calibration checks and stabilization. Furthermore, excessive exposure to high concentrations of CO2 can sometimes affect certain sensor types. * **B) water:** Oxygen indicators are typically electronic devices, and introducing liquid water into the sampling system or the sensor cell would cause damage and make the instrument inoperable. * **C) the tested compartment's air:** The purpose of purging is to remove the previous sample. If the instrument is purged with the tested compartment's air, it means the sample is never truly cleared, and the sensor would remain saturated with the last measured gas concentration, making subsequent readings unreliable or slow to respond.