What does zero adjustment accomplish in calibration gas use?

• A. Establishes a baseline signal corresponding to zero concentration.
• B. Sets the span of the instrument.
• C. Calibrates the sample flow.
• D. Measures ambient humidity.

Answer: (A)

Zero adjustment sets the instrument’s baseline to zero when no analyte is present. By using a calibration gas with zero concentration, any background electronic offset or baseline drift is corrected, so the detector reads zero for zero gas and provides an accurate starting point for all subsequent measurements. This is about the intercept of the response curve, not its slope.

Setting the span, on the other hand, determines how much the signal changes per unit of concentration and defines the full-scale range. Zero does not involve adjusting flow or measuring humidity; those are separate aspects of calibration or instrument operation.