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In the cylinder with a compressed gas, not only mixtures in the gaseous phase may be contained; also mixtures which have been liquefied under pressure with a quantitatively small amount of the gaseous phase in relation to the liquid phase (approximately 1:1000) may be contained.
If components have a very low vapour density, the calibration gas mixtures have to be filled under the correspondingly low pressure. This would mean that only a small amount of mixture will be filled into the cylinder. For this reason, we recommend to prepare mixtures in a liquid form, if higher amounts of mixture are required.
The following three methods of sampling of the mixture from the cylinder with the liquid phase are possible:
- If the cylinder with a compressed gas is equipped with a normal valve, the gas can be taken from the vessel in the upside-down position;
- If the cylinder valve is equipped with a dip-tube the vapour pressure in the cylinder expels the liquid mixture through the valve from the vessel in the normal position;
- If the cylinder is equipped with the double-output valve with a dip tube the liquid may be taken by bringing a neutral gas under pressure (e.g. helium).
The mixture expelling by the neutral gas pressure may also be applied in the first two options; it is advisable that the gases that represent the buffer were supplied by the gas mixture producer.
Should the ingredients have different vapour tensions, the lighter and more volatile ingredients are saturated in the gaseous phase and the heavier ones are preferentially contained in the liquid phase. The outcome is that the ingredient concentration will not be homogeneous in the whole gas volume during the entire gas mixture consumption; mixture composition varies during the consumption – it depends on the takings from the gaseous and liquid phase. Should such changes be minimised during the consumption, it is necessary to use one of the above-mentioned procedures.
The liquid phase that is taken from the vessel may be used immediately or after its complete vaporisation. The quantitative composition changes might be calculated if the consumption profile and procedures are completely known.