instability (with reference to instrumentation)

https://doi.org/10.1351/goldbook.I03061
A change which takes place in an instrument
reading
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
drift
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
reading
in response to a zero sample over a stated period of unattended operation.
Source:
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2197 [Terms] [Paper]