It is not reasonable to use vacuum to indicate that vacuum is historically used to measure vacuum using a U-type pressure gauge.
In a general vacuum system, the degree of vacuum is usually expressed by the hydrostatic physical quantity of the isotropic neutral gas pressure. Therefore, the measurement of the degree of vacuum is only due to the measurement of the pressure. However, special attention should be paid to the measurement conditions, which are measured by stationary (random motion), steady-state, isotropic, neutral gases in a limited container. In this case, Maxwell's velocity distribution, cosine scattering law and hydrostatic pressure concept (p = nkT, v = 1/4nc, p = ρgh) are all in line with objective reality, and the measurement of vacuum is relatively simple. .
According to the definition of the degree of vacuum, the degree of vacuum is preferably expressed by the molecular density n, and the degree of vacuum by pressure does not contradict this. When measuring pressure, the general gas is in equilibrium and satisfies the Maxwell velocity distribution law, ie p = nkT holds. The gas temperature T is constant at the time of measurement, so the gas pressure p is proportional to the molecular density n. That is to say, the pressure at this time is a measure of the molecular density, so the degree of vacuum can be expressed by pressure.
In space research, the research object is the motion of infinite space (1 ~ 10kms-1 or higher) and the complex atmosphere under the action of unsteady and integrated environment. Maxwell's law of velocity distribution and cosine scattering law are not necessarily Established, so the pressure also lost the original physical meaning, the measurement of vacuum is more complicated and difficult.
In general, the degree of vacuum expressed by pressure is popular and used, but it is not unique. The following parameters can also be used to indicate the degree of vacuum:
When the degree of vacuum is very high, that is, the molecular density is very small, the statistical fluctuation is very obvious. For example, when the pressure p = 10-12Pa, the statistical fluctuation is greater than 5 × 10-2, and the pressure has lost its true meaning. From this point of view, in some cases, stress is only a relative indication of other quantities.