The long term drift in calibration of a Guardian® NG NDIR gas monitors is due to a variety of ageing factors most of which result in a small change in the relative strength of the IR signals reaching the detectors.
This produces a very slow drift in the zero reading.
Due to the nature of the detection process this zero offset produces a much greater calibration error at higher concentrations than at zero.
Both offsets are generally fixed by correcting the zero indication.
The CO2 Zero Calibration Kit for the Guardian® NG wall mounted monitor provides a convenient alternative to bottled gas for the checking of zero offsets and if necessary, zero calibration.