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Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures.Liquid helium may show superfluidity.. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temperature of −269 °C (−452.20 °F; 4.15 K).
Once two of the three reduced properties are found, the compressibility chart can be used. In a compressibility chart, reduced pressure is on the x-axis and Z is on the y-axis. When given the reduced pressure and temperature, find the given pressure on the x-axis. From there, move up on the chart until the given reduced temperature is found.
The lowest pressure at which He-I and He-II can coexist is the vapor−He-I−He-II triple point at 2.1768 K (−270.9732 °C) and 5.0418 kPa (0.049759 atm), which is the "saturated vapor pressure" at that temperature (pure helium gas in thermal equilibrium over the liquid surface, in a hermetic container). [1]
Values are given in terms of temperature necessary to reach the specified pressure. Valid results within the quoted ranges from most equations are included in the table for comparison. A conversion factor is included into the original first coefficients of the equations to provide the pressure in pascals (CR2: 5.006, SMI: -0.875).
Helium remains liquid down to absolute zero at atmospheric pressure, but it freezes at high pressure. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) at about 25 bar (2.5 MPa) of pressure. [107] It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same
ref T c (K) T c (°C) P c (MPa) P c (other) V c (cm 3 /mol) ρ c (g/cm 3) ; 1 H hydrogen; use: 32.97: −240.18: 1.293: CRC.a: 32.97: −240.18: 1.293: 65: KAL: 33.2: 1.297: 65.0: SMI: −239.9: 13.2 kgf/cm 2: 0.0310 1 H hydrogen (equilibrium)
With a constant pressure, the voltage needed to cause an arc reduced as the gap size was reduced but only to a point. As the gap was reduced further, the voltage required to cause an arc began to rise and again exceeded its original value. For a given gas, the voltage is a function only of the product of the pressure and gap length.
Where p is the pressure, T is the temperature, ... where ρ cr is the density of the substance at its critical point. ... Helium, He 2.1886: 0.05984: 0.01400: 0.0: 40