Calibration


How to check the calibration and linearity of gas analysers C


alibration is the process of establishing the relationship between the output of a measurement and a


known input. In common with most measuring instruments, gas analysers require calibration following installation, and then at regular intervals to check that they are performing to specification. Regular zero and span checks (known inputs) are necessary, but it is also necessary to check the linearity of the analyser regularly. Analyser linearity is unlikely to change after initial commissioning but if it does, errors can be substantial, so analysers should be checked regularly and/or following every major service.


Some instruments employ sealed gas cells or


reference spectra for self-calibration, but a number of gases of known concentration are required for a linearity check. So, what is the best way to conduct accurate, reliable calibration and linearity checks, without incurring excessive cost? First, where analysers are being used for


compliance purposes, it is necessary to be familiar with the regulatory requirements. For example, the European standard EN14181 describes the quality assurance procedures needed to assure that Automated Measurement Systems (AMS) installed to measure emissions to air are capable of


meeting the uncertainty requirements. Under EN14181, the linearity of an


analyser’s response must be checked using five different reference concentrations, including zero, all of which should be traceable. The reference concentrations shall be selected such that the measured values are at


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September 2021 Instrumentation Monthly


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