In what type of atmosphere must the zero calibration of a gas detector be made?

Zero calibration of a gas detector must be conducted in fresh air because fresh air provides a baseline where the concentration of the gases being measured is known to be at zero levels. This ensures that when the detector is calibrated, it accurately reflects the presence of hazardous gases when exposed to environments where those gases may be present. When conducted in fresh air, any reading detected after calibration will indicate the presence of gas due to changes from the baseline.

Calibrating in a vacuum, confined space, or oxygen-enriched setting would introduce variables that can lead to inaccurate readings and unreliable gas detection. In those scenarios, the calibration would not reflect true atmospheric conditions, resulting in potential safety hazards.