ISO 16664:2004 describes factors that may influence the composition of pure gases and homogeneous gas mixtures used for calibration purposes. ISO 16664:2004 only applies to gases or gas mixtures that are within the "utilization period", and it pays special attention to
- storage of calibration gas cylinders;
- calibration gas withdrawal from cylinders;
- transfer of calibration gas from cylinders to the point of calibration.
ISO 16664:2004 also outlines a method of assessing the stability for a gas mixture, and takes into account the gas composition uncertainty given on the certificate and the users measurement uncertainty.