A calibration gas is a reference gas or gas mixture used as comparative standard in the calibration of analytical instruments, like gas analyzers or gas detectors. Therefore, a calibration gas has to be of a precisely defined nature or composition, like zero gas or span gas, for example 500 ppm carbon monoxide in nitrogen.
To be a calibration gas, the gas must be traceable to a national or international standard. Traceability is the unbroken chain of comparisons to an acceptable international standard. The calibration gas standard establishes a known analyzer response to a certified chemical component concentration. In the calibration gas, preparation tolerance (PT) and certification/analytical accuracy (CA) are of utmost importance.

Span Gases