What should an oxygen sensor of a hand-held gas detector read at the surface?

The correct reading for an oxygen sensor of a hand-held gas detector at the surface should typically be around 20.9%. This value corresponds to the standard atmospheric concentration of oxygen at sea level under normal conditions.

When measuring oxygen levels in the atmosphere, it's crucial to recognize that while slight variations may occur due to factors like altitude or weather changes, the average concentration for fresh air is approximately 20.9%. This makes it the expected baseline reading for calibrated gas detectors.

Understanding this level helps professionals to assess whether the environment is safe or if there is a potential oxygen deficiency or enrichment, which can have serious safety implications in confined spaces or during industrial applications. Therefore, a reading of 20.9% accurately indicates a normal oxygen level in ambient air, making it the correct choice in this context.