A change which takes place in an instrument @[email protected] over a stated period of unattended operation for a given value of the air quality characteristic. It can be characterized by the variation with time of its mean, specifying the @[email protected] and by the dispersion. Span instability is the change which takes place in instrument span over a stated period of unattended operation. Zero instability is the change in instrument @[email protected] in response to a zero sample over a stated period of unattended operation.