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Homogeneity of standard calibration gases
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Homogeneity of Standard Calibration Gases Standard calibration gases are classified as gaseous reference materials, and like other solid and liquid reference materials, their homogeneity is one of the most critical and fundamental characteristics. Although various gases exhibit high mobility and diffusivity, when preparing standard calibration gases, several component gases are sequentially introduced into a high-pressure cylinder. Coupled with differences in molecular weights among the components, this process often leads to an uneven distribution of gas components within the cylinder, and in some cases may even result in layering. Moreover, there exists an equilibrium between adsorption and desorption of gas components on the inner walls of the cylinder, which can adversely affect the homogeneity of the standard gas components.
Homogeneity of standard calibration gases
Standard calibration gases are classified as gaseous reference materials, and like other solid and liquid reference materials, their homogeneity is one of the most critical and fundamental characteristics. Although various gases exhibit high mobility and diffusivity, when preparing standard calibration gases, several component gases are sequentially introduced into a high-pressure cylinder. Coupled with differences in molecular weights among the components, this process often leads to an uneven distribution of gas components within the cylinder, and in some cases may even result in layering. Moreover, there exists an equilibrium between adsorption and desorption of gas components on the inner walls of the cylinder, which can adversely affect the homogeneity of the gas composition. For these reasons, special mixing and homogenization procedures must be employed during the preparation of standard calibration gases to ensure uniform distribution of all component gases within the cylinder. The cylinder-rotating rolling method is a quick, simple, and highly effective approach for achieving thorough homogenization of standard calibration gases. Alternatively, the natural diffusion method can also produce uniformly mixed standard calibration gases; however, it typically requires a longer time to reach a state of complete homogeneity.
Stability and Shelf Life of Standard Calibration Gases
Standard calibration gases are classified as gas reference materials, and their stability is a crucial fundamental characteristic. The stability of standard gases encompasses two key aspects: the change in the concentration of the standard calibration gas components over time, and the change in the concentration of these components as a function of pressure. Generally, stability studies for primary gas reference materials require more than one year, while those for secondary gas reference materials need at least six months. Only on the basis of the results from these stability studies can the expiration date—i.e., the validity period—of the standard calibration gas be determined. The length of the validity period for standard calibration gases cannot be arbitrarily set; it must be established according to the findings of the stability studies.
It is critically important to maintain the long-term stability of component concentrations in standard calibration gases. The material of the packaging containers for standard calibration gases and the treatment of their inner surfaces are key factors that influence the stability of these gases.
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