Composition of Gas Mixtures
The composition of a gas mixture is defined by the amount of the various components in the mixture. Different units may be used for specifying concentrations. The amount of substance (% or ppm) is often used, as this unit is pressure and temperature independent. Also widespread are volume content and mass concentration. These pressure and temperature dependent units are defined at standard conditions (0°C and 1013 mbar). For the conversion from one unit into another Messer uses software based on the standard EN ISO 14912 (Gas analysis – Conversion of gas mixture composition data).
The feasibility of a gas mixture is limited by chemical, physical and safety restrictions. For instance, gas mixtures containing both oxidizing and flammable components can only be produced under limited conditions. A team of Messer's experienced experts checks every single gas mixture which is produced for the first time and defines all relevant process parameters in detail. A specially developed thermodynamic software package is used for the mixture's calculation.
Tolerance, Uncertainty and Stability Period of a Gas Mixture
The (production) tolerance describes the permitted deviation of the actual concentration (actual value) of a component from the required concentration (rated value). Depending on the production process, the concentration range, and the type and number of components, the tolerance is normally about 5% to 10% (relative).
The actual value of a component can only be stated with a degree of uncertainty. Messer's certificates always state the expanded uncertainty with the coverage factor k=2, meaning that the true value lies with a probability of 95% in the specified interval.
When gas mixtures are used for calibrating measurement instruments, the content of a gas cylinder often lasts for many months. The stability period specifies the time from the date of manufacture for which the actual value in the certificate applies. Usually this period is 12 months, although longer stability periods for many gas mixtures are possible (Longlife Option). In this context, the internal treatment of the gas cylinders plays a crucial role. The production of stable gas mixtures is only possible through thorough and consistent cylinder pre-treatment with extensive purging and evacuation cycles at high temperatures, as well as appropriate conditioning procedures.