When preparing Hubei standard gas, we usually use 4L/8L aluminum alloy steel cylinders with a pressure of 10MPa. But there are also some Hubei standard gases whose pressure cannot reach 10MPa, why is this? This is influenced by the saturation vapor pressure of the substance.

Firstly, let's follow the editor to understand several concepts.

Critical temperature: The highest temperature at which a substance changes from a gaseous state to a liquid state is called the critical temperature. Each substance has a specific temperature above which, no matter how the pressure is increased, gaseous substances will not liquefy, and this temperature is the critical temperature.

Critical state: The state at critical temperature and pressure is called the critical state.

Saturated vapor pressure: The pressure at which a vapor in equilibrium with a solid or liquid in a closed environment at a certain temperature is called saturated vapor pressure. The same substance has different saturation vapor pressures at different temperatures and increases with increasing temperature.

Therefore, assuming that the preparation temperature of Hubei standard gas is 20 ℃, when a component is subjected to a pressure greater than its saturated vapor pressure, it will liquefy. In addition, since Hubei standard gas contains at least two components, the pressure borne by each component is also affected by the proportion of that component.

P partial pressure=P total x volume ratio Taking 10% butane Hubei standard gas (equilibrium gas is nitrogen) as an example, ◎ P1: butane 20 ℃ saturated vapor pressure of 0.2MPa ◎ P2: assuming P total=10MPa, P1=10MPa x 10%=1MPa>0.2MPa (P saturation)

Therefore, P total=10MPa does not hold, and butane will be liquefied

Actual calculation method:

P total=P saturation ÷ 10%=0.2MPa ÷ 10%=2MPa Verification: P1=P total x 10%=0.2MPa

0.2MPa is the critical preparation pressure of 10% butane Hubei standard gas (equilibrium gas is nitrogen) at 20 ℃.

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