ISO/TC 158/WG 4 - Comparison methods and certificates

This document specifies the general provisions and gives the basic definitions of terms relating to sampling for gas analysis, including sampling devices, sampling methods, sampling technical considerations, and sampling safety. This document applies to both direct and indirect sampling of gas in pressure receptacles and pipelines, including pure gases and gas mixtures. Compressed and liquefied gases are both considered. This document applies to the sampling of processed gases and does not involve gas treatment processes. The sampling procedures specified are not intended for the sampling of special products which are the subject of other International Standards, such as liquefied petroleum gases (see ISO 4257) and gaseous natural gases (see ISO 10715).

ISO 16664:2017 describes factors that may influence the composition of pure gases and homogeneous gas mixtures used for calibration purposes. This document only applies to gases or gas mixtures that are within the "utilization period". It provides the following guidelines for the handling and use of calibration gas mixtures: - storage of calibration gas cylinders; - calibration gas withdrawal from cylinders; - transfer of calibration gas from cylinders to the point of calibration. It also outlines a method of assessing the stability of a gas mixture, taking into account the gas composition uncertainty given on the certificate and the user's measurement uncertainty.

ISO 12963:2017 provides methods for - calibrating an instrument with one or two calibration gas mixtures, - determining the composition of a gas sample, and - evaluating the uncertainty of the composition of the gas sample in relation to the uncertainty of the composition of the calibration gases used and the contribution of the measurement process. ISO 12963:2017 sets requirements to, and acceptance criteria for, the utilization of different measurement calibration designs with a limited (i.e. minimum) number of calibration gas mixtures used in calibration. The methods in this document are described for amount-of-substance fractions, but are also applicable for other composition quantities (such as mass fractions, volume fractions or concentrations).

ISO 6141:2015 specifies minimum requirements for the contents of certificates for homogeneous gas mixtures in gas cylinders to be used as calibration gas mixtures. Pure gases, when used as calibration gas mixtures, are also covered by this International Standard. Gases and gas mixtures produced for other purposes are not considered. The requirements in ISO 6141:2015 deal with the metrological aspects of calibration gas mixtures. Other aspects, such as safety and legislative aspects, are not covered. Furthermore, it specifies additional information (optional data) recommended for describing a homogeneous gas mixture, supplied under pressure in a cylinder or other container. It does not cover the field of safety-relevant data and related labelling.

This International Standard provides methods for _ determining the composition of a calibration gas mixture by comparison with appropriate reference gas mixtures, _ calculating the uncertainty of the composition of a calibration gas mixture in relation to the known uncertainty of the composition of the reference gas mixtures with which it was compared, _ checking the composition attributed to a calibration gas mixture by comparison with appropriate reference gas mixtures, _ comparing the composition of several calibration gas mixtures, e.g. for the purpose of comparing different methods of gas mixture preparation, or for testing consistency among gas mixtures of closely related composition. NOTE In principle, the method described in this document is also applicable to the analysis of (largely) unknown samples instead of prospective calibration gas mixtures (i.e. gas mixtures which are intended for use as calibration gas mixtures). Such applications, however, require appropriate care and consideration of additional uncertainty components, for example concerning the effect of matrix differences between the reference gases used for calibration and the analysed sample.