According to the basic physical characteristics of sulfur hexafluoride gas, it is relatively easy to liquefy compared with air. The state of sulfur hexafluoride depends on its pressure, temperature and other parameters.

The critical temperature of sulfur hexafluoride gas is the high temperature limit indicating that the gas can be liquefied. The lower the critical temperature, the less likely the gas is to be liquefied. The critical temperature of the gas is 45.6 ℃. When the temperature is higher than 45.6 ℃, the gas can be kept in a constant state. Below this temperature, it can be liquefied as long as the pressure is high enough (for example, liquefaction starts at about 1.2MPa at 0 ℃). However, for N2, there is no liquefaction problem at room temperature, and it can be liquefied only when the temperature is lower than - 146.8 ℃.

The critical pressure of sulfur hexafluoride gas indicates the pressure required for liquefaction at the critical temperature, that is, the saturated steam pressure at that temperature. At the critical temperature, the density of saturated steam is the same as that of liquid, and the boundary between them disappears. The heat of vaporization becomes zero, and there is no difference in physical properties.

High purity sulfur hexafluoride is a non-toxic inert gas. But the mass of gas is about five times that of air. In the high-voltage distribution room or cable trench, once there is a large amount of gas overflowing from the sulfur hexafluoride circuit breaker, sulfur hexafluoride first deposits on the bottom, and then continuously accumulates from the low to the high to drive away the corresponding air. When workers enter the room or cable trench, they will inhale the gas deposited at the bottom, thus causing anoxia or suffocation.

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