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If the components of calibration gases required contains heavy components in such high concentrations that a gas mixtures would not provide a practically applicable gas volume, it may be preferable to produce the mixture in the liquid phase.
This situation is most relevant for process control, where pressure and temperature in the process ensure that the compound is in the gaseous phase, while a calibration gas must be stored in completely different conditions.
There are , in general, three types of liquid mixtures:
- Liquid phase in a gas cylinder where the mixture’s own vapour pressure is high enough to force liquid out of the cylinder.
- Liquid phase in a gas cylinder where excess pressure of helium (or other inert gases) is added to force liquid out of the cylinder.
- Liquid phase in a glass vial, where tests are taken with a needle through a septum.
For mixture in gas cylinders, the cylinder valve with normally be equipped with a dip tube to enable outlet of the liquid.
For mixtures which contain light components (such as hydrocarbons <= C4) , the content of the cylinder. The phase will decrease, while the concentration of the heavy components will increase.
If such problems are likely to arise, Vadilal supplies calculations which show how the concentration which show how the concentration changes when drawing liquid from the cylinder.